THE GENETICS OF COTTON

ALLELES OF CI4 XXED UT_4_ T OF GO~SY+PfU~JI BARBADENSN L. DIF2FEP~ING IN DOSf]2N~ANCE 0xn cY EXPE mENTAL VE mATIO OF

:r rEO ¥ OF O0 f[ ASCCE

. B S [ S Y D N E Y CROSS H A ~ L A N D Ah'D OLGA M. ATTECK

Institute: qf Cotton Genetics (Sociedad Naciona~ Agraria), Lima, Peru

C O N ~ E N T S PAGE

Xnflr oductiOn . , ~2 . .

M~teriM' and methods 23 {1) Types o[ Upl~nd oo%ton (g. hirs~tu.m L.). Types 8, 9, 34, 57, 459 23 (2) TyDes of Bedrbon co t ton (G. 2urpurasce'ns Poh'.), Types 19, 196 23 -(a).:.~alt~se- P~d- . . . . . . . 24

(4) Da.rw.inii W ~ t t ,. 24 ( 5 ) iamentosum Nixtt, 24 ,(8) Crinkle_z!s used ~s bs.ckgrounds . 24

The .e~.'pm:j~:ne~L~l resnlts " ' . , . 06 . • . . o

(I} Transferences of norma.ls to ba.rbadenss cr ink led t sTe 4 26 A. "Trsmsferenees of hirsutu~n no rma l s 26

• A L h~rsutum t y p e 8 2G A 2, h b ' su~s i l, ype 34 27 A 3.. hlr.sut)~m ~yl)e 57 • . 27 A 4. hir~ut~m ~ype 459 28

B. Transferences of 2u.riJu,'asce.~s noxuimls to barb~,,dc~.~e cri~dded t ~ d ~ 2~ _18 L ~m'pu, ra.~ce~s t.}q)e 12 , . . , 29 B 2. })ur))£rasccna t y p e t96 (va.r. 2Jo~'rilgi Cook a:, E%bbard) ' 30

C. Transference of T~itenss n o r m a l to i~m, rtoaffcn~e erinld.ed type 4 31 C 1, g. Tugtense P a d . t.yIJe 77 . . . . . . 31

D. ' Tra.nsf~rencs of DarwiM, i normM to barbage.n#e cr inkled type 4 39, • D I. g. Darwb~ii Wa.~ t y p e BI37 . . . 32

(2) Tr,'msferenees of norm-ds to hirsulum m'kdded t y p e O 33 A . " barbads~vse %ype 15A . 33 ]3. tome~tosu~it. , 35 C . hirsetUv~ t ype fi7 3g ~). .Darwf'nii- 37

(3) Trz, nsference of normals t,o hlr.sut~n critflded type 57 37 • A.:: Da,'rw gnii . . . . 37

(4) Trausfgrdne~ of A0rmo,|s go cr ink led h.irs.~&ltm gyl)e 250 38 Z~-. 5arbagenss . . . . ~,ype 18 . . 38 ~B: hi'rsUfi~.m ~ y p g 9 a " "

" • " " 38 [S j .-%>an{terence of normals ~o super-m:inkled_ g rade 1 39

D D{sCussion .. 4] 8~ 8ammiwy

~efe reddes 40 47"

22 T/~e Ge~ae~,ica of Cotto,~,

INTI%OI)IICTION

Tn~ Crinkled Dwarf muta.t~, of Sea, Island. cotton (~oss,~j~J.h~m, b~.~'&~- de'~se L.) has been ~,he subject o.f much. genedcM investiga~io~l by the writer (].916, 1918, 193~, 1933, 1935, 1936) hnd by IItltehinson & (-1.hose (i987). Cert~dn conclusions rdgardinod the existence of normal alleles at the crinkled loeu.s d.iffering in dominance potency were preset~ted in preliminary form (1936), ehas: " t~ has .been sb.own tha~ barbade'~se possesses one allele of crinkle& which may be ~ermed G ~ , and tha~ /ag-ra,z,t,,~#,~~ h.as ~wo more di:ffecent alleles, namely, C TM (in. Agrs;~.~gea,~ rat. L~ieacl.e T 9) and C m~ (in Mrs,~,a~,,m vat. Triumph T 57). These three alldes are disbing~dshable only* by their dominance reactions on various m'inkled genetical backgrounds, and from ineompI¢,ed emperiments it is s,h'eady clear tha t ft~ther different normal alleles ~ are present in the other four species." In the same paper the writer's views 6n dozninanee in. relation to the crk~kled muta,~ were summarized in ~ single sentence: "The dominance reIation is due to the interaction of a normal allele d specific potei.ley with a modifier comple_~ to wldch it is precisely adjusted, artcl tha~ ~ke genes constit~.tiag the dominance modif~e~ complex have been preserved not because of t h e i r function as noel]tiers of initially dis- advaniageous heterozygotes but because of sdective value on their own account." These views have now been modifmd.

As early as 1939_. it was established tha t ~he c£nkled heterozygote was indistingmshable h'om the normal homozygote not only its barbaden.se, where it occurs natm:MIy, but aiso in three different types of Mrse~t~m. Since that date it has been f~trth.er shown ~hat transference ~o a wild type of~owrp~asce~.s (vat. Morri~g Cook & Kubbard), to whiehi~was transferred by repeated backcrosses, also resubs in complete dominance of normal. I t was fl~rther established tha t the modifiers of dominance were ~zot ide~ticaa in the three species, since crosses between any two of the three species eri~kteds gavein/<, a series ofirapereeptib[ygraded erinideds ranging from semiqethal (supercrinkled) to phenof,.?Tieally normal (pseudo-normal).

The f~rther question then arose: are the ~mrv~al alleles of eri~lded alike ha different species of New World Go~sgi~{w~ ? Using various t2laes of homozygous or nearly homozygous crinkled as bases, it became possfbte to transfer various norm.als on to them by repeated baekcrossing. The erinkled bases upon which work was beg~n comprise~[ barbade~se, four different types of 7drs~vmL ~oz~.lO~.rc~see~zs vat. Morrigi, and four cliff~ren~ homozygoas grades of crinkled extracted from the sg~ of ba~'bade,~zse crinkled by/~ir~twm crinkled. The experiment, s here reported are only a

small par~ of ~hoses~arted in 1932, bu,t, nevertheless, place o~ record the de~Mled or/deuce for the existence of normal alleles of differing dominance po}ency in New World cottons and serve as a basis for discussion of the .dOminance mechanism in so far as it concerns the crinkled mutant.

. . . . . . . . (1% t), adhering to the Fisher theory of dominance. havh :d~sntissed my previous findings (t936) of alleles with differing domin}~nC e potency as inconclusive, aI~hough presented only in the form idf ai~oi!ef . part of the present da%a, and pu% forward their abstrac~ of :yie~ in ~hese words: '.'It is clear that the main cause of dominance is g~oups.of modifiers pr@sen~ in the regf of fhe genotype and is independent of cEfferendes at the crinkled locus."

.The object, of these experiments, then, is to present the results of t~'ansference .by backcrossing of the normal allele of some species of NeW..W0)ld"--co~%ons to various crinkied backgrounds, in order %0 demonstrate the ex{stenee of different degrees of dominance potency m snell atleles, and in Order to evaluate current theories of dominance.

~IATEI~IAL AND N]~THODS

~IkTEEIALS

(i) Types of Upla~zd eo~to~z (~. hirsutum Z.)

Type 8~ ~ variety of naked-seeded Upland collected by the writer in 1929 in a commercial :field in North Carolina. Ig has been self-fertilized every yebz since 1928.

Type 9./An especially long-linted selection from the variety Neade, self-i%rtfiized since !925, i.e. for more than thirteen generations, since at ~ime s two generations were grown in a single year. Suspecte~l to contain ba'H)~genSe mocl~ffers.

Type 3{[. A. dwarf Upland with khaki lint grown by native Indians in ~uatetriala. It. was obtained fi:om the United Sta~es Department of Agrimdturein 1930 th]:ough the courtesy of Dr T. Ii. I(earney.

Type 57. ;.A type selected for high tint index fz'om the commercial variety Triumph in i928 and afterwards self~fereifized for no~ less than SLX generations: .

Type ~!59. A seleetion from the Upland variety King, collected in S o ~ h O~rolina by: %h e Writer and setf-fertiliied for four generations.

(2) T~{Te~of Bourbo~z eo~on (G. purpurasesns Poi.r.) Type 12. A.typ~eal pub.pub.asserts with red leaves, found as a single

~Iant in a.garden in!Trhfidad an J 923. Bred true on being introduced flute pedigree,cMtttre, sO fai as eye judgement could determine.

24 T,',~e Ge,~,et@s o f d'o~to~,'~

T?~e 196. Th.is type was desc.'ribed by Cook & I{ubbard (t926) as ~. Mo~'~'.iZ~.i. It was coil.coted from the west coast of Ne~eo. Compaz'ison wi~;h an ex~ensive collection of.pu.'rp)'urasee~'~s types led to i~s inclusion iu ~his species.

(3) Taiten.se Pc~r~.

~'ype 77. The standard tYu?e of 6< Taite.nss P~r]_., the endemic species of d~e Fiji Islands, Seeds ~vere ob~Mzted from Nr ].~. A. Anson in. i929, a.nd a single phm~ was subsequently mMntained hy occssio.nM selling.

(4) Darwimii Watt

Type 2127. Seeds of (;. Dc~,rwi,m:i were collected i£ ~he O~lapagos Islands, where ~ke species is el~.demic, and a few were give~. 5o bhe ~rrite:r by the Umited States Department of Agrlcntture. The particular i.)]ant used in this series of experimer~ts possessed ~wo genes not known to exist in any other New World species, viz. tl D (fed plant body) and yn (yellow corolla).

(5) tomentosum f.u~t.

T?q)e 22. The endemic species of ~he ItawMian Islands. Seeds were co!leered by tlie writer in 1925 ia the Island of Oshu, and {he type maintained by grafting anvil 1933, When experiments were begun -wi[h this species.

(6) 6'ri,n]@ds used ax bac]cp~'o~ds

The following crinldeds were used:

(~) Type {, ~. bc~,rbz,,de~~s'e (Sea Island). TMs is the origigM crinkled mutant wlieh has been 11sod in all expedmen?s since i932. The strain used in the present experiment was self-fertilized for ten or more generations.

(5) Type 9, crinkled. This erinkl.ed ~,as obtained by ten repeated backcrossings to hirsut.~mz ~ype 9 with sabsequeat extraction by selfing.

(e) T}q0e 57, crinkled. Obtained by eight repeated backcrossi~gs to hi'rsut~m type 57, and then extracted b!¢ sek'mg.

(d) Type 250, crinkled. The crinkled corresponcHng to hirsut'um type 250 (vireseent ydlow). Ogtgined by a similar process.

(e) Grade t, super-crinkled. Obtained from the interspeci~e cross berbadense type '~ (crinkled) x h.ira~.~um type 9 (orimkled). This was the extremes~ and least improved cri~lded which occurred in the N~ of the above cross. It was m~inta,ined by selLfert~lization for eleven generatimts, add preserved its uniformity.-

SyDs~Y C~oss 2L~Las~) ~-D OL~a M. AT~]~c~ 25

G~'ac~in 9 of c~'i~dded. The above five,~Xpes of crinkled can be arranged ~eo0rding 60 the series of grades menbioned in a previous paper (1935}.

Grade 1: the extreme form of crinkled (super-crinkled). B: more improved. 3: ghe grade of type 9 hirsutu,m crinkled.

: 6he grade of barbade.nse type 4 crinkled. 5: crinkled modera~eIy developed. 5-5 : ~he grade of the he~erozygo~e of hirsu~um, ~ype 9. 6: crinklgd fah'ly well developed. 6-5: fain~ trace of crinkled. 7 :. normal..

hdrsut~m crinkled T57 was about 5.5-6. ~irsug~m crinkled T250 was also about 5-5-6.

Passing from grade 7 to grade 1, the most prono~mced changes are :that ghe leaves become progressively more crinkled or rugose and develop eMorophyll-de~_eien?~ areas giving a mosaic appearance; the edges of ~he leaves become more andmore ragged, a~cl the size of the leaf (and pla.n]b) smaller. The growth and rigger of the plant is also roach reduced Un~il grade t is a dwarf plant a few inches high with minute flowers and bolls with only ~woor three seeds instead of ~he normal eomplement of 18240.

~{ETIIODS o

A s ~ star~ing-point a single 791 plant was selected at random from an ie 1 9ulture grown from a sillgle crossed boll (ci~'ea thirty i?].ants).

_For each successive baclcc]:oss usually one boll was used, giving rise e01 l~weat y or more plaJ.lts. If segregation into a normM (hegerozygous) g-reap and recessive cz'bJkled group was visible when ~he plants were a few ihches higb.i a preliminary separation was made aboag a moRth after . . ' . .

S0wi~g, and ~wo ~o four random hete.rozygotes were retMned for back- crossing, tn o~her cases ~he separation could be made only at a much la}e~ period Th.e eb.oice of the het~rozygotes was always m~de ~o obviate a,ny. ])ias in i'~vo~Tr of any particular grade of crinkled or of any special vlgom}-, s61eci;ion JJei_tig serictly random. As regards vigonr, tl~'e plants were. gr6wj~..iu 6 in. pegs in a special uniform compos~, producing wetl~ devel.op.ed plambs. In. one or ' two isolated instances lack of material restricted the 'choice ~o one or two plants, but this was rec6ifed in. sue- Ceeding generations.

After the ~i'th b'ackcross the plants were gro~l, either ou.tdoors or in

26 The -' " ~,e'r~ehcs of Cotton

beds in the greenhouse, bat the same pre.eautimxs were taken to ensure 2

random sample of heterozygotes for baokcrossing.

THE EXPEI%IMENTAL ~ESULTS

(I) T[~ANSI~ERENC',E8 OF NOgNIALS TO A' .LI~I3ADENS~ CI~INIKLND TYPE {

A.. Tr~,~,~fere.,~ces of hirsntu.m ,,~.orr,~als

A 1. hi-rsutum ~,ype 8.

The results of this e-vpedment are given .in Table I.

Table 1. G.ro, d,e of heterozyyote i~ re?e~ted badco.rosses of type 8 hirsugum ,norm~d to orin/cZed barbadense (ty~)e ~)

G r a d e o f h e t e r o z y g o t e s ,<

F ~ m H y ' 5 5 .5 ,-3 6.5 7' I % e m a r k a

] g a . u k e r o s s I - - - - - - 4 - -

g . . . . . . . 3 ' - - 3 - - - - - - 5 1

3 . . . . . 3 - - - 4: ~ . . . . O l d plangs 4 4 . . . . . O t d p l a n ~ s 5 . . . . . 4: 6 . . . . g 7 - - - - - - i - - 8* - - - - 10 30 2 2 m o ~ t h s o l d p I ~ n t s 8* 42 . . . . 5 m o n t h s o td p l a n t s

* T h e r e w e r e i 8 h e g e r o z y g o t e s a n d 49 t y p e 4= c r i n k l e d i~ ~Ns b a c k e r o s s , a d o s e ~ioproxi- N a l i o n to ~ i : 1 ra~io. S fx p l a n t s o f ~ h e h e l ; e r o z y g o u s g r o u p w e r e s ~ e d t h r o u g h b a d soi l eoncEt ions a n d t r e r e n o t e x a m i n e d .

From ~he results of Table 1, and general-ly spe~ --tdng from the remdts of all the baekerossing experiments, ig appears t o be tumecessary to carry the b~mkcrossing beyond the fourth or at most the fifth baekoross, as the heterozygote is by then stabilized on the ~lew genedcaI background. To meet any possible criticism, however, this ex-periment was continued to the eighth baekeross. The heterozygotes of this baekoross were ezamin.ed a~ two stages in the life history in order to trace variation in the grade of crinkled with age, Exan~nation of the plants at 9 months old, .i.e. when jus~ beginning to flower, gave forty plants perceptibly arimkled and two plants phenotyFiealIy normal. A later exam'mation at 5 months old showed that all the pla~ts were extremely crinkled, about grade 5 (just above ~he grade of ba~'bagd}~se mutant-). It thus appears tha~ while there is a deeinite f~ilu~e of dominance of the h#suta~m normal allele in young plants, the failure becomes exSremely pronounced with age.

S Y D N E Y C t ~ O S S H A I ~ L ~ N D A N D O L G A M . A T T E . C X 2 7

To s u m up: The normal allele ~f hi~'sutum t}-pe 8 cannot produce .c.o.mplet e ctdmin~ce on a ba~'badense background, and therefore san~o~ be identical Wi~h ~he barbadsnse allele Ces, which does produce complete

, . . , .

domig~nd e. I t may be considered as a weaker allele at the same locus, and .foil0~hgShe-method of indicating a species allele by a superscript (1989). may be termed C ~ .

A/2.: hirsgtUm type 3~.

The results of experiments in which normal of ~?qoe 34 was brans- ferred ~o the barbadense crinkled background are given in Table 2.

Table 2. Grads of ]w~erosygote i~ ~'epsatsg basics.tosses of ty2e 3-~ hirsutum ~ofma~ to ca'iuk~le~ b~rbademse (tyl)e i)

Grade Of- he~erozygo~ss

F~tmily ~ 5 5-5 6 6.5 7

B~ekeross I --- ~ ' - - _ _ _ _

2 1 2 1 - - - - 3 - - ' - - 4 - - - -

4 I - - - - 6 - -

o - - - - __ 2 --~ 5 . . . . . 2 - - 6 --- - - :[6* I8 - - 6 All of gra.da~ 5-5-5 I"

* E x a m i n e d ~t 2 mom-,hs old.

~ e m a r k s

6 plan~s recorded as in ter- er iskled

2 plan~s~rec, orded as inter- crinkled

On old pIangs On old pla.ngs 1 p lan t grade g - - o l d p l an t in field

J" Ex,~mined at~ 5 mo~ ths old.

Essentiall.y. tjzey are similar 5o those of the previous sectioxL It ~dll be ngted tha~ ghe.gr<n de of crinkled in mature heterezygotes (5 months old) is !about ~-5.5, thus closely resembliJag barbads~s.e eriukled., being, ho~%ger; super{or t o the latter in heigh~ a, nd vigour. The normal allele of.type 3~, tJaerefore, is similar in d.omi~ance potencyto tha,t of type 8. A..:. careful eomparis0D, of the laeterozygotes of type 8, with those of typ-e 34, g~ve the imtzression ~b@,t ~h.e latter were de§.rdtely more eNmkled. It earmqt .yet be concluded that the normal alleles of types 8 and 34: are diff'eren% since the expressio~t of crinkled in heterozy~otes is sensitive ~o 5thor. iAt~.aer/ees besides age, and tb ; techmique for distingui~16ng slight ctfl~'ereuces in domins,n.ce p0te~tcy have not yet been fully worked o u t :

A 3. hirsut~m.ty2s 57.

The ~'ess.t~s of experiments with type 57 are given in Table 3.

28 The C/e.~z.et.ics of Cotton

%~ble 3.. G'~'ade of heterozy~7ote i~, ,re~)ea.ted bact;c'rosses of tjpe 57 hk~su~um ,~,o.rma2 to o.ri-~fclec~ 5arb~d.qnse (type 4)

* Ex ghird backcxoas selfed (two GmiIies}.

The resales of experim.en.~s wigh tyioe 57 were curried only as fa,r as the fourth buckeross. The fonrf, h backcross wa, s no~ m~de direcely from third backcross m~teri~d, as bhe crosses failed to germinu~e. ~orgunal)ely, three selfed fi~mifies were ~vailabte from selfi~g of ~hird baelkcross hers: and ~wo plants were selected ~o cross with b~,rbadense crinkled for the fourth backcross. 01~e plang w~s recorded ~s erinkled,(but behaved as a typical he~ wh.en crossed with barbadense crinkled, w~hile the other wa, s recorded ~s normal (grade 7) and was evidently a homozygote, since it produced a fa, mily of th i rVdwo pla,nts, ~II distinctly crinkled of grades 6 and 6-5, when crossed wiOh ba.rbadense eriakled. I~ general, the resukgs from ~his series of crosses fall into line wi~h those of the awe previous eXl?e~iments , a~d i5 may be concluded ~hat ~ype 57 also possesses the weak normal allele C ~ ,

A 4. hirsatum type -t59.

The results of experiments with type ~59 are giren in Table ~.

Table '~. Grade q/ heXe'ra~ygote i'~ 'repeated 5ackerosse.~ of type ~59 hirsu~nm to crin~led barbadense (tyy)e ~)

Family 6

Baekerosa 1 2 3

S 6 5 6

.6

Grad~ of hc~erozygo~es

8-5. 7

1 - - 1 3 - -

- - - - ] [

g* 36 i If* ,'32 I

* Plants examined ~t 2 montks old, ~'~%en 5 months old %he grade of ericLkled of ~he hers diminished to 5-5,5, simil~r to the grade of the ~hree hi~ut,i~m hers jus% discussed.

(~r~de of hegerozygotes

.I]~m[ly 5" 5 6 6,5 7

Backcross I --- I -- 2 1 -- 1 2, --

2 1 I 1 --

3 ] -~ o - - zs

3 :~ p[~m~s recorded, only as "l le~s" 4:* • o 1 5 :' o

-I* -- I5 17 --

SYDNEY Ogees HAI~LA.XVD AND OLgA 1~{. ATTECX 29

From the results of Table 4 it is .eviden~ that beginning with only a, sligk~ failure of dominance in )71, the failure is repeated in successive baekerosses ungil in the sixth backcross eighty-five plants out of eighty- Seven exhibited ~he typbal slightly crinkled heterozygoge at 8 weeks old. As.in .previous experim.ent s the grade at 5 months old approximated to the grade of ~he recessive ba,rbage~ss crinkled, or a little above it (grade 5.-5-5).

To Sum up~ This iseries of experiments, in which the normal allele of crinkled in for~ differellt types of hirsu~um was transferred by up to eigh~ backcrosses ~o {he baskgz'mmd of ba~'bade~zse crinkled, has given a ebmpie~e demonstrs;tion of the faints of dominance in the heterozygo~e. .O~ffnlding is clearly visible in young plants, bu~ is so strongly marked in 01d pianos as to approximate to the grade of the barbs&has crinkled itsetf. The normal allele of hi~'sutum, ~bh,ts established ~o be of weaker dominance pot~ency than ~hat of ba~'bads~zse, has been termed C ~ , )hgngh i~ is believed ~hat differences in dominance potency exdst among the four types, of hirs~tum worked with. In the writer's p~evious com- zrmnieatio~ i% was staged that ~he normal alleles of hirsu~um ~ypes 57 and9 were different. ~Further evidence regarding the rdativs potency of the alleles of ~ypes 57 and 9, whbh substantiates this belief, will be fo~Jad later in this paper.

B ! Tq'a~,~'ersnces of purpm'ascells normab to barbadense c.ri,n;£ed ty))e 4

In tiffs section ~dli be considered the results of experiments in which the normal allele of crinkled, in & l~uUjurasce.~ Yoir. wa, s transferred t,o the ba.ck~ound of barbagense crinkled.

]3 I.. 1)urpurasce.us ~yTe 12.

The rdsults of uncomplei~ed experiment are presen%d in Table 5,

Table 5. @fade of heterozygote 4'~, 'rcpea&ct bac/co~'dases of ~ype 12 (pu'epm'aseens) ~o crinlcled barbadense

Grade of ];et, erozygo~es

F~mi/y 3 ,l 5 8,5 6 6.5 7 2"1 ' Crhlkled jtLs~ visibIe ]Sa.eker0s~ i -- __

s _ - - - - - - g . t - - - - - - 1 . t _ _

. 3 _ _ " , 3 - - - , - - _ _

4 - - , 15 (10 ~aI.[ ~ild g short}

From these restdts it appears thag as far as %he second backm'oss this Cype of puspUrasccs, S behaves similarly to ~he hirsutum types, i.e. showing

30 The ge.n.etic,~ of Cottov,

a slight though well-m.arked failure of dominance in the heterozygote. The ~kird bsekeross plan~s, howevea:, showed, uniform and st-rent crink- ling of the same grade as barbadense crinkled. The three plants of grade 4 in ~Ne thh'd backcross were recorded ss heterozygo~es ~hen yomig, and were selected 8o continue the baekerossing. ~arther, ~he pare~.~ of the third be, choreas was a typical heterozygote of grade 6-5. On.e probable 1)u6 unconllrmed exptana~io~ of the results is that the normM alleIe of type ].~ is so weakened i:a pote~xcy when assoeia~ed w:[th the herbage,rise modit~er oomplex, that it is pke~otypioally erinkl£~L Te was observed. tha t the fourth baekcross ]?Ian:ts ~,ere el[ of the same grade of crinkled (grade -'1-), but that there were ten t~tl crinkled and five short, It seems likely that the ten. tells were heterozygoees, but without repeating ~he whole experimen} 2g is not possible to ~,rri.ve at ~,he correct explanation. The normal allele of tyR~e ' ].2 is, however, demonstrated to be of weaker dominai~ee potency than that of barha~le~se, and is coluparable in this respect with the ~.ormal allele of hirs~tum..

B 2. ptu:puraseens ty.j)e 196 (.vat. Zorrilli Coofc & ffubbc~rd).

The results of esperiments involving type .196 are give~l in Table 6.

Table 6. G~'ade of heteroz$c/otes i~ ,repeated badco~'osses of type 196 (G, pur- t~urasceus Poi~'. = @. Norrilli Cook & Hubba.rg) to barbadense o~Oz]cleg ty2e 4

O r a d e of h e g e r o z y g o t e s

F a m i l y 5-5 6 6-5 7 l ~ e m a r k s

f~ -- --- ¢ --

Baekeross 1 -- -- 3 -- o -i a ~ o

8 -- I -- .-- 3 3 2 - - - - - O l d p l a n t s 4 . . . . . No~ r e c o r d e d 4 . . . . Not r e c o r d e d 5* -- -- 4 --- 61t - - 15 27

One plant began ~s a 6.8 at the ~o~ering aS~ge aud the gro~ing poba6 subseq~entIy b e c a m e e x t r e m e l y c r i n k l e d . . .

"~ Ex fifth b~okcross selled which gave fihree erinldcd grade 7 and one ez'inkled grade 6"5. $ The sixth baokcross plants were first recorded a~ the flowering stageJ When examined

at 5 men,ha old, all showed extreme crinkling at the ~op of the plant, aboa~ grade 5.5.

The resalts of Table 6 indicate that in fy~.~pu.rasce,ns (Mor.riZLi) type 196 the normal allele is similar in potenoy to those found in fg.rs~tun~, being of weaker potency than the normal G ~B of ba~'bade,use.

Some restdts from selling ~fth backoross plants for two genera.tions

S Y D N E Y C ~ o s s H ~ : m A ~ ASD O L ~ M. ATT~aK 31

are als6 available. First, two selfed i~termediates of the fifth backcross

gate: Normal Intm~edia~e Crinkled

3 1 0 4 6 4

In t he n e w generation, progenies of all the norma]s and intermediates y~ere growl .. Fivenormals thI'ew only normal; one (04278) produced 11 intermediates i l l crinkled, and another (04956) 35 normats: S inter- m e d i a t e s .

Now 04278 was classified as normal until it was 6 months old, when th e g r o ~ g point apparently muta ted to a strong grade of crinkled (abon~ 5). I t is therefore highly probable tha~ the octopiere absence of fl0rma]s in ~ts progeny c a n be accounted for by assuming mutat ion from .nq~mat to crinkled in the parental germ cells, The three intermediates of famfly C4256 mus~ have resuRed from the union of mutated gemn cells With normal, sifice ~hLs plant when c r o s s e d with crinkled behaved as a normal, giving fifty-two %ypical intermediates.

~u tab i l i ty of genes as a consequence of transference to the genetical backgrmind of another species has previously bee~ described (Harland, • 1937e), and the possibility of its occurrence ia interspeeific crosses has always to bebo rne in miud.

C. "2ra~sference of Ta,itense ~w.rma~.s to baz'ha, de~lse cri~dcled tgj)e 4

(3 1, Q- Tait:ense Y)a.rl ty2)e 77.

The results are set forth in Table 7.

Table. 7. Grade of heterozyyote i~, ~'el)eated b~clcc.rosses q/ty2e 77 (Tai~,ense) to crinlged barbadense ty2)e ¢

Grade of hetet'ozygo~es r

Family . 5-5 6 6'5 7 l~em~rks 21 6 sli¢,hLly- crin]ded -gaekeross ] 4 sligh~l) erh)kled

o

--- 2 1 3 - - ¢~ -- - - Old plant, s 4 . 1 2 4 - - 5* ,-- 8 9 .-- 5*f - . - - 19 ~ 6 - -

• tg~ fonrth, ba.ekcross selfed. :~" 3{uch more erinlded m older plan.re (o~bou~ 5 " 5 - 6 ) .

:£~-om":Ta,ble 7 i t n~ay. be concluded "~hag the normal aItele in G. T'aite~zse.t;ype. 77.is a weaker allele as regards domin.ance potency than the normal al leleof bathe&nee, resembling the ]d'rsutum normal in this respect.

32 T h e Genet'ic~ of Co~to~

D. T.re~,~sj'e~encs of Darwinii .~t.o,rmaZ to barbsdense cr.inlded type 4-

D 1. (4. Darwin.ii ['JZagt ty2)e 2127.

The resu.Rs of experiments wireh G. Da~wi'tdi are give~t in Tab]e~ 8, and 9.

%¢ble 8. g,ra& of heterozygote i,)~ ,r~,i)ea, Zec~ &zelcc'rosses oj' ty2e 2127 ({L Darwinii) ~o c.ri~&~ed barbadense ~y')~'6

Qr~de of heteeozygotes

l~;~mity 6 6-5 7

~a - - 4 - - .'.~ekm:oss i - - - - 4,

2 - - 2 7 3 - - - - $ 4- - - i 3 5 - - - - - 2 5 " - - - - - ~7 6 - - - - 25 6 - - - - 35

* E x .[om'$h bael~eross soi led,

Table 9. Cerade af heterozygote .i.,z f~.~rth badccros~' ~e~fed (e:e -~ aoZe 8)

G r a d e a f n o r m s [ g r o u p C r h g d e d 3 . _ _ _ _

F a m i l y 6-5 7

0 281 - - . 3 C 286 1 47 C 1993 - - 6 C 1990 - - 15

To~a l 1 71

72 E_,zpeetecl 3 : i 69 23

0 1 t

4 5

20

Table 8. contains the backeross results as far as ~he sixth baekeross, while Table 9 contains results from sslfings of the fourth backcross. l~rom the res~dts it will be seen that after the rough backcross the

• * . t )

normal allele of Da,r~s@Ai behaves sirmlarly to the normal allele of ba,,'b~- dense, and may thns be elassi:ffed as an allele of grea~ dominsnce potency. The resu.Rs of Table 9 are con£rmatory. The resuRs, in fact, form a complete contrast to those obtained with the weak noJ:mals of hirs,~tv,~,, pu,rpurasos~s, and. 2aits~se. Tt~e appearande of sligh$ crinlding in the F1 indicates that although ~he normal allele of Da~'u@zii may be of similar dominance po~ney to that of ba,rba~ense, the two species possess differen~ modifier, background.s.

SYDN?~Y CI~OSS HARL_dND AND OLGA ~ , ATT:BCK 33

(2}-.Tt~A:BTSF]gRE2~CE OF NORI~£ALS TO }ZII?SUTUz],I CRINKLE]) T-~-PE 9

i~ ha~previonsly been established that when the ba~'bad, snse crinkled mutan~ is transfm'ed to t,he backgroand of hi~'s~tum t)~2e 9 (a. pare 15ze %f the" cOmmsrciM variety Meade), t,he het,srozygote is intermediat% and setting o~ het6rozygotes gives 1 normal" 2 intermediate" 1 crinkled. In contrast .wit,h. thJ.s result, t,ransference of crinkled to otther 7g~-su~m backgrounds has always given complete dominance of t~irsutum normal. This has 5sen proved to be the ease wRh three different hirsutums, n~meiy,Tex~s Virsseeng Yellow, a large-boiled selection from Triumph Jin0wi~ as type aT, and a variety of Upland Dora the Punjab (type 19} .which ~!as. selected from the commercial variety 285~. in addition, .crinkled transferred ~0 ~he background of Tu~purcrscens vat. Morr~&.~ (Cook & IIubbard) was manifested as a complete recessive. There was no siglt o~ crinkling in the heterozygote, ttahehinson & Ghose (1937} dis- covered, the crinkled mat,ant in an Indian t}q?e of hirsutum. Nero also normal .was completely dominant,. I t seems therefore that h~fs~.tu~ type- 9 const,itutes an. exception to the generaI condition of complete dominance; m~d an explal~atio n has been put forward for Slits. The variety 3!ends from which type 9 was selected has mack longer staple t,h.an, any or,her Upland variety, and ~his has been ascribed t,o its supposed 0ngm Dora S cross wit}3 ba.-rbadense Sea island, which possesses extremely longstaple. If t,ype 9 contains a mixture of hi.r~ug.z~-and ba,.rb~ge~se genes, and. if, as 5as been demonstrated, /g~'su~,~m and ba'rbaden~e eae]~ have don~inance mechanisms geodetically constru.eted in differsn~ ways, type 9 should exhibit failure of dominance through disharmonv~ of inodifiers, k'Iost eombh:a,tions of modifiers, on this theory, will be less efficient than the p~re t~'i~'~zg~m, or ba~'bc~gs~se const,ell~tions.

one feature of raarked importance in type 9 crinkled is t,he degree of crinkledness. I t is more crinkled and weaker t,han t,he ba~'badense e~inMed, whereas the erin]deds corresponding to ordinary hi~'sut~n~ are s~.ore ~mproved than the barbads~sc crinkled.

The ~2xlheriments nsin~ type 9 cri~,kled as a t)ackgronnd fox ~ransferenes a.re ~he~:eNreimp0rt,ant i.£ elucidating the part played:by ~he normal allele andl~hs modi£er ;ore.pier., respectively, in proilucing domiJ~anee.

A. barbs, dense ¢.y/pe 1hA The expsrim.en~s may be considered in three sections: (~3.. ~es~db of s~ccessive baekc;osses (1-8). (2) ~esnRs frm:n selflng plants of the seventh ba.ekcross. .

.. Crosses of sevent,h baekcro,is heterozygotes to type 9 normal. Joura: Of Gene~ics 42

3

34: Tlae ge.,ad.:gca of Oo~~,o~

h~. Table 10 sviil be i%und th'e restdts of eight successive backerosses. I t wi].l be seen that four plan.ts only i= the whole series of experiments are .r4oorded with a fM~t de f t e ro f crinkling (b,%ckorosses 2 and 4). _:-kfter t~he fourth backcross eomplete stability was a,rrived at: the plants showed o~ trace of 5a~rbcde,~~se characters, and the dm.~inanoe of the bc~'bade,m'~e normal C: ~u was complete. I t i.s eviden%, th.en, that the subst i tu tbn of the barbc~de.~e aormM, allele for the type 9 ~orm;.d allele on the ba.ek~

Table 10. Grade ~(/" heterozy.qote i% re2~ec~ted bc~,oT,:.crosses ~<f barbadense .~mr,maa ~o Mrsutmn c.;'i%~:Zed, tyloe 9. ~es'~dts oj" &'e,)~.~%.re,~zce ea~perime'~z~ barba, dense ,~o,nnaZ ~o c.r.i,~dJe,~ ty2e 9

~ r a d e o f h e t e r o z y g o t e s k

S-fi* 6 6-5 r

. / T r ~ . . . . . . $ 3 pianos r e c o r d e d as ' ~ z ~ o r mM " ; o b M r s no~ examh~ed

[ g a e k c r o s s 1 . . . . . . . t i r a d e n o t r e c o r d e d 2 . . . . 2 - . - g l i g h t l y c r i n k l e d whm~ o ld a . . . . . . 6 4 ~ - - - - i 3 4 . . . . 1 3 4 . . . . . J_

5 . . . . . I2, 5 . . . . . . . 2 6 . . . . I 0

6 . . . . . 8 T . . . . . S 7 . . . . . 1 S . . . . 5:2 S . . . . . . . . 47

* I n d i e ~ t e s posit~ion o f g r a d e o f hM, e rozygo~¢ t y p e 9 (Afr~ut'~v~) n o r m M ~c t, y p e 9 ( ldrsv , - ha.m) c r i n k l e d .

ground of type 9 co%~e.rts tan lwterozygo~e f r o ~ {,%$eT')~l.ec[{cff;e, to f 'l~I~ %o.rmaI. The c]Jfferenoe in dominance potshey of the bc~'btde,re.se and t~rs~t'~tm aormals has thus been es~,ablished from yet another angle.

The results of seI~ng seventh baekeross heterozygotes are given fn T~ble II. The norm~Is were edl of grade 7, with no trace of crinkled at

any stage of growth, and the l~ropgr~ions of normal and crinkled were dose to expectation.

The h.ypothesis that the ilormM alleles of lag~'~z#tzar~ &nd be~rbcde%se differ in dominance potency was finally subjected to the crucial test of crossing the heterozygotes of the seventh bad~oross of composition CR~c~ with normal hi.,'s,e~t,e,~m t)qpe 91 of composition C ~x. This,. cross

fshould give 1 C ~B C ~ : 1 C m~ o e'. z T h e n o r m a l a l le le o f %vt~e 9 is d e s i g n a t e d a s G ~ a n d n o t ' G~t~ s i n c e i t is r e g a r d e d a s

d J E e r g n t ~v-eaker a l le le .

S Y D N E Y C t ~ O S S . [ ~ A I % L A N D A N D 0 L G A ~ { , A T m ~ O ~ C 3 5

The results of the experiment are given in Table 12. It will be seen that the two kinds of expected plants appear in appro~maSely equal numbers, and ~he hypothesis of alleles of different dominance potency in hi~'sutum and barbade~se is thus fully construed.

Table 11. ~ Results of sel fi~g seventh bac]ce,'oss he~erozygotes ex Table 10 F a m i l y NormM Crinkled

39/(4 702 23 703 43 14

(4 70~ 40 9. g 705 24 5 G 706 22 9 G 707 42 13 (.¢, 708 48 23 O 709 24 Ii

Tota l 266 89 :Expected 3 : 1 265.5 88.5

Table 12. Results of c~'osses of seve~th backeross heterozygotes (2, Rs c ~. with hirs-a~nm ~wrmals g RM C, R~

Family

39 /~ %54 G 755 G 756 G 757 g 758 G 759

{~ t~1~'{ C ~

I n te r - C R~ G ~ media tes

G "R'~ c R C ~ C-, R~ NormM~ {.grade 5-5) O 4044: x 9 - 0 5502 26 27 C 4 0 i 5 x 9 - 0 550] 15 14 C 4045 ": 9-C 5502 35 37 C 4049 x 9 - 0 5501 12 I! C '10J9 x 9-C 550.2 iX 9 C 4:052 x 9-C 5502 9 6

To~M 108 iO,:t E x p e e % d 106 !06

t3. b o m e n t o s u n ~

These experime:.d;s were ta, ken a.s fax a.s ~,.1.te :5.fl;l~ backcross, and t,l: e resales are. presented in. T~l:,le 13. The chief point, worChy of note is that

Ta,ble 13. Grate of heterozygotc in ,'spec~tcg ba.&crosses of iy2e 2:2 (g. %omentosum) to ty2s 9 hirs atum c rio~.a;lc(~

(4rude of het,e.rozygo~es _ _ _ j .

r

Fami Iy 5~5 6 6.5 7 . lq i -- i -- Bad<cress 1 -- ] 1

2, - - 2 2 3 No~ recorded

4 - - - - - ~ 8

5 - - .-- 44: 5 - - - - - - 50

the ,F~ sh.owed dis~inc~ crJ.1:ddi]lg, whbh disgp]?~a,red i:.~ ,subsequent generations, uatiI in the fourth and fifth backcrosses atl heterozygotes

3-2

36 T h e Genetics o f C.ott, o~

are completely normal of grade 7.,, The norma! allele of to~me~tos~m thus resembles the ba,rbaden6.e allele in being'of s~rong dominance potency, and differs from the hir~'utu,~z normal.

C. hirsutrtm t?72e 57

le wiII be recollected that the normM allele of type 57 h.irsutv:m, is completely dominant to crinkled on its own. backgrocmd. Tlxe interest in tNs experiment is to see whether tMs dominance will be maintained o~ the type 9 backgroun.d. The results are set forward in Table 14.

Table 14. C:~'~'ade of hete.rozygote 'i~. refea.ted ba~kc'ro.~.s'es q/" tyfe 57 (hirsute.urn) to tyf)e 9 hirsutum eri.n/cled

G r a d e o f h e t e r o z y g o g e s t" A

FamiIy 5.5 6 6-5 7 r~ -- -- 2 -- l ~ a . e k e r o s s 1 - - - - 1 2

2 ~ - - I 5

3 * - - l-i • 2 2 - -

3" l 23 1 6 - - -

* .Nx s e c o n d b a e k e r o s s , s e l f e d .

Here it will be seen ~rst that the results were carried only as far as

the third backcross, and so are no~ absolutely oonclasive, but the following points are significant. The P~ shows sHgh~ impairment of eleminence, the ~_rsg and second baclzeresses a rather less impairment, and the third baekoross a flat,her modification in the directio~ of crin!ded,. there being one plant of grade 5"5, the grade of the type 9 heterozygote. Now whether fr, rther backerossing wend result in the heterozygote being stabilized at the same pc>inS as the type 9. heterozygote, or whether, as the writer believes from other uncompleted experiments, the normal Mtele of type .57 differs in potency from those of barbade.nse and ~ype 9, cannot yet be staged with eertMnty: What is, however, clear, is that ~ypes 9 and 57 differ ir~ their modifier complexes, so that while type 57 is completely dominant on its ow~ bac~<grourld, it is not dominant on the t}q)e 9 background. Type 57 crinkled is much nearer normaI )hen type 9 crinkled, and the behaviour of type 57 normal on the two backgrounds is related to the phenotypic appearance and vigonr of ~he latter. On the bad crinkled background both type 57 a~d type 9 exhibit intermediacy in the heterozygote, but when the crimkled recessive is improved to %he condition of type 57 crinkled, the intermediate is cor'respondi'~~gZ:z t ira.proved, and becomes phenoe~ypioMly normal.

SYDNEY CROSS HA~LA~r~) riND OLGA M. ATTACK 37

D. O. Ds,rwinii These experiments were carried oul, only as far as ~he first backeross,

as the.material was lost when the work was transferred from Trinidad to Brazil The results may be summarized for wj~at they are worth:

2'I. Twoplgn~s both grade 7 (full dominance). F2/i~ backcross. TWO plan~s both grade 7. 80 far the"resalts agree with the hypothesis tha~ the zmrmal allele

o£G. Dctr~vi~gi is equal.in pobeney ~o that of g, barbadense, and grea~er than ~ha~ of ~he normal alleles of tdrsutu~, pur2~zrascens , or Tc~itense.

(3) T R A N S F E R . E N C E oF NOItBIAL8 TO Ht~OUTUiZ5 CR, IiCXLED T Y P E 5 7

A. Da~winii These experiments, were taken only as far as ~he ~hird backcross.

The. reshl~s are given i~ Table 15.

Table 15. Grade of hsgerozyqote in ~repec, ged bad~erosses of O, Darwinii type 2127 to ty2e 57 hirsu~um c'rin£led

G r a d e o f he~erozygo~es

~mily 6 6-5 7

]3ackcross 1 - - 2 2 2 2 - - 8 3 . - - I# 8 3 " - - J ~ I I

* E x r e c o r d ba, d~e, ross r.effed. J" D o n b t f u t 6:5.

From the resrflts of Table I5 it will b6 seen that the 1~ 1 begins w ~ . c0!~plet½ dominanc.e. There is impa.irme~t in Lwo pIan.% of the second. b~gkcross and complete do'minance in the third backcross with two doubtful exeep~im~s. Ifi seems that by ~he tbird baekeross equilibrium h~s been re~¢hed, and th.at ehe ztomnal allele, of Darwi~ii exhibits com- plefie dOlhin~nce on the backgronnd of type 57 m'i~dded.

~. hirsutum t~pc 9 P~eguhs as Jar as the third baekeross are giveax in Table t6. From

~hese results, it is seen tha.t typ., 9 n.orma[ hi~'s'u, tu'm does not exhibit

TM)le. 16.. @rage. of hei.e, rozygotc i'~, rel~eal-eg baolccrosse,s of O. hirsu~u.m type 9 to. tylJe 57 hirsa~,lnn o'/nl,:Zed

Clrade o f hO;erozygotes

~Family 8 6"5 7

~s~ckaross 1 - ~ 2 1 2 - - - - 5 3 7 5 3. S 16 __

38 The Ge~edcs of Cot,~on

complete dominance" on the background of type 5'7 eriut.:let~ in the third backcross, while type 57 itself" does so. It is therefore almost certain that type 9 has a slightly weaker normal allele.

(![-) ~I~ANSFERENCE OF NORS~[ALS TO CI{IN'KLED HIA,WUTU2I TYPE ~],'J0

A. ba,rb~dense t?jl)e 18

The results are presen.ted in Table t7. eUnfertunatety, it was possible to carry gl~e resuits only a,s far ~s the second backeross, b a~ in. view of the f a c t t h a t t h e f i r s t a n d s e c o n d l ? a c l c c r o s s e s showed com]?lebe dominance

T0Jaie 17. G,~'ade ~"/~e~o'o::y.qote i*~ fe.~)ee~tec~ ba.c/~cro,sses vj' (~r. } ) a r l J a d e n s e

tyfpe 18 Zo [drsugnm crin/cled ~y))e 250

Grade of he~erozygotes

Family 6 6.5 7

]3a,ekcross I . . . . 3

it may be assumed l;ha,~ ~he barbadznse norm.al allele is oomple~e!y dominant on a background of type 250 crinkled. The results are therefore. according to expectation.

B. hi_feature tyfge 9

, This expel-iment was expected to give similar results to t, hose in which type 9 normal ,was transferred to ~he background of type 57 cr i~led , sbxce type 57 crinlded and type 2j50 crinkled show very little morphological Nfference.

Table 18. (&'a~e of heterozygote i¢~ .re,peated ha@crosses of G. lfirsutum tyT)e 9 to hi~sutum c.ri~z.~led ~y2ze 250

Grade of hetarozygo~ea

f a m i l y 6 6-.5 7 F l - - - - 2 ]3ackeross I -- ~' --~ 4~

2 - - I 3 3 -- --- 12

~i - - l 27

The results are given in Table ]8. As will be seen, the ~esuIts as far as f, he fourth baclrcross show that complete dominance ensued when the type 9 normal was substituted for the type 250 normal, althongh"as will be recollected type 9 is incomple~ely dominant on its o-vra back- ground. This is iz~ marked contrasfi to the results previously obtained

SYDNEY C~oss H~_~LA~D A~I) OLGA M. ATTECX 39

waere the substitution of type 9 for type 57 on a hi'raut~on crinkled type 57 fMled ~b give complete dominance. I t is clear that the type 9 allele must }~e 6f similar potency to the type 250 normal, Tke attainment of dominance i~ tiere ~ qsCestion of modifiers, improvement of crinkled being reflected. ,i~g torte@ending improvement of heterozygous normal.

~5) TBAh-SFER.ENCE OF NOI~}IALS TO SUP]~P~-CR,INKLED GRADE l

Since the baekgrmmd of super-crinkled represents an association of • : - , - .

an.3m.fa~so~rabte group of modifiers combined with c ~, and is the least idable ~nd ieast improved of all crinkled forms, it was thought to be of [great. ~nte~est to transfer various normals to this background. It was believed that such transference would constitute a powerful method of }.evealing differences in dominance potency of normals. The results are oi~ly frag~mntary, but present some points of interest. They are given in T~ble 19.

Table ]9. Res~,~*.~ of ~r~n~e.re~ce of no~m~ M~e~ to s~i~sr~cri~led Grade o f c r i n k d e d of h e t e r o z y g o t e s

/ ,

' . T y p e . . of n o r m ~ Cross 3 ~ 5 6 6-5 ~7 t~.irsu~m, t y p e 9 ~ ~ 6 2 - - 2 - - hirsz~tum t y p e 5 7 • 7e~ - - - - - - 2 2 2 barSagenss t y p e l S ]i'~

. . . . . . o

h~rs~z~um ~ y p e 9 B a c k c r o s s 1 $ ~ -- -- ' - - _ _ ]drsu~m t y p e 5 7 : f d a c k e r o s s 1 - - 1 1 . . . . 5r*r~t3dense t y p e . 1 8 ] 3 a c k c r o s s I - - - - - - - i 2 3

The main.conclusions-to be drawn from the table are as follows: ]?he grade of ~z is as expected from previous experiments :if three normal Mtelesqf 3.iffermg dominance poteucy are concerned. It is barel? ~ possible ~hgt the .three parental types possess modifier complexes which may oven:iae differences ig potency of tits normal allele. That this is probably not so is shown by the results of the first backeross. These plants contain ~oughly[T5 ~/~ of super-erflxkled modifmrs, and it is dear that not only are the relative differences in dominance potency of the normats strongly brgught, out ;but tha~ in presence of an excessively weak modifier coin )!e~:.gven the:powerful b¢,rbadense ~llele is not able to produce complete dominlance :ifftin.ee plants, although it was able to do so on all the back- :grot£tt~{.g;pld,d;tmly sguclied, a artb.er signiaoant point is tTaat the grade i~ Crinkle& 0f.th6 het, erozygotes may be lower than that of the original

- - , ' - 2 . . . . .

ba{b~denSemiitaht, reeessive, w'hich was grade ~. ]?afore work on the d°rmnancePotenqY: of normal M!e}es in different species should be per- formed- using }.hper-crinkled as a background. To sum up: These experi- monks, .:although"-incomplet%i~xdicate once more that hir,s~ttu,m and

40 The gene~ics of Cdto~z

5a.,rbade,nse differ in.'the d.ominm)ce pol~ency of ~heir ~orm~l a~letes, a~d the conclusion, previously arrived a~ respecting the greater.i{otenc ~ of the. /~i,r~utum type 57 allele, as compared with ~hat of hi'rsitt.~m type 9, is strengthened.

In Table 20 the data of tb.e prececting tables is summarized. Thee diffcreimes in dominances potency exhibited by the va~:tous ~ormal alleles

Table 20. Sum.marized ~'esuZts o]'9,rage ~#' hezerozygote in (d~, the t'ra,~,sj~,'e,nce e.~;2)e,ri'me,nts

Type o f dr~J'ded background Type of nc)rmai ' 3

barbade~ss type 4 hirsutum type 8 /~b'sz~t~.~, type 34 Mrs'~.X'am type 57 hirs~.~i~m~ type 459 t~w(ln~.rascens t, ype 12 pl<F~ra.sce.ns type 19 6 Tai~ac~'e type 77 ,])a,'r~eLq.[~: type 2t27 barba~en~e

hirs'utum bype 9 bavbadense

hirsuh~.~ ~5Te :57 ~9..r w#t.i l

~drs'ttg~,.~ type 57 hirsutu~n type 9 barbaric'rise hi, rsuhtm, type 57

h~rSuh~)n type 2,50 5a.rbads.nse hirsutum type 9 hirsut'tt.~ type 250

Super-erirdded$ barbaden~e hirsutum Wpe 57 ]~i.r,s~ttu~ t)u?e 9

Grade of heterozygotes

X X

X - - - >;

b;

;< X

X - - ;<

N - - ;<

L

8 6-5 7

;<

:<

y '

- - - ;<

X

* Previons experiment recorded elsewhere (1916). ? Previous experiment recorded elsewhere (1933). $ ~esLd~s of firs~ baekeross only.

on different crinkled backgro-ands are clearly shown. The normals may be grouped as follows:

The normal of strongest dominance potency is barbadense, since it exhibits full dominance o~ all backgrounds excep~ that of snper-erinkled, where dominance is sligh.tly impaired'i n the first backcross. The norma]s of tome~tosu,m ~nct Da,~',wi~ii can be placed wi~h bc~'bade~vse, since both are completely 4ominan~ o~ the weak type 9 crinkled background.

Hirsutum, ,#~tfp,~.~a.soens, and Taitense form a ctJ.sdnct group with normals of weak dominance potency. ~['his grouping is in accorclance with wfla~ is lmown of their taxonomic relationships~ Within this gr'dup, it is almosf certain that differences in donztnance po~en.cy exist, although as previously stated, it is not easy to measure them. The ~ormal. of

SYDneY C~oss HAgnA,_'rD ~XD One<4 ~ . A m ~ o ~ 41

• ~ype 57 is more potent than that of types 9, 34, or 8, while 2t~r2u.rascens type:12 seems to be of weaker potency than any of ~hese. When the results are considered as a whole, it is likely that the number of normal aIleles at the crinkled Icons may be very large. The evidence for dis- Criminatiug between the alleles of types 9 and 57 is considered snf~cien~ ~gl assign different genetic symbols ~o them, C TM (type 9) and C m~

DISCUSSION

Two main schools of thought have put forward genetioal explanations for t]aee~stende of the phenomenon, known as dominance. According to Fisher (1928a, 19286; 1950, 1931), dominance is the resld% of an evolu- ~iona W process by which an i~itially disadvantageous and i~termediate mutant is elevated towards normal by the accumulation of a series of modif3dngfac~ors until it Becomes indistinguishable from it both physiologically and phenot3q~ieally. This theory has been criticized by Wright (1929, 193a.), Haldane (lS30, 1939), ~arland (1933, t936)~ Muller (1932), 8ilow (1939) and others. The main opposition may be expressed as the '°factor of safe~y" hypothesis, according to which alleles "having an actlvRy well above the necessary minimum will be of advantage to the organism" (Dobzha~sky, 1937). ~aldane (1930) made the [mportant suggestion that plus znntations a~ any ]ocus may be favoz, red by selection provided that t.he original gene is not completely dominant, dnce the original type of gene is at a disadvantage and ghe new gene is no~. Fisher (1981) accepted this as supplementing ~he .mechanism of dominance evolution which he originally s aggegted, thor~gh he stiI1 believed ~hat aecamcdation of modiiiers by the heter(~zygote played the most importaut push.

The experiments described in tb.is paper have been conducted almost continuously since 1929, wi.th the aim of throwing light upon the methods by which dominance over a mutaut is atta,in.ed. A survey of the evidence presented in this paper will show that this objeet lJas been largely achieved, that t~he views of the ~wo schools can be reconciled, and Lha% {hers'are in fact two methods by which dominance can be attailled. One of these is the Fisher :me~hod; the other is ~he I-Ia,ldane method.

An important reason for the writer's previous non-adceptance of the tcisher tlmory was that coml3J.ete dominance was 1:he rule in hi~sutu,m, where the mutant had ~lot been reemlded, and vd.mre it was not believed ~o occur. TNs objection ha,s been removed, by the discovery of the crinkled m~tant in/drs'~tum by 7:Iutchiason & @bose (1937). They found{

i t lmt rm.ly So be ful ly r~cessive, br~ ~ also 8o be eonsicleJ:;.d)]y s, meliora, ted. This ~ml.elioration they a,scvibc (~o the conditions l?revaili,:~g dm'h~g th.e domestic~don of this species, drouth, as will be explained late~, there are strong reasons for belLevhtg this view to be incorrect. The second main argumeztt against the Fisher theory,was the i'act thai; there were two processes involved. First, the aecrtm.nladon of modifiers by the heterozyg@es, and secondly, the spreading of these modi:~ers throughout d±e species so that it became homozygous :%r them. It appeared to be impossible to imagine any mechanism which could, accomplish this in self-fertilized plants (}]-aldane, 1939), oth.er titan, a strong selective advantage of the heterozygous p.h.ase over sigher of the homozygous phases. With the demo~sbr~tion here presell%d, theft in three species out of six examined0 cloud.trance has bee~ attained by the use of a strong normaJ allele (the ]:Iald.ane effecg), this objec~ion is less cogent. It will, however, have to tie seem:ned d~at the m_~dn, if not the only me~hod of attaining dominance in a self-ferdlized ,species, or one predomimzntly so, is the Hal&ua.e method..[is]dane (1939) has discussed the dominance mechanism in inl?red and. outcrossed species. Considering species which are predominan@ self-fertilized, he points out that the majori W of non-Iethal mutants are present in llomozygotes and few Jn heterozygotes. Consequently, the intensity of selection, for dominance must be less in inbred than in outbred species. He shows that dominance fs just ~s common, in inbred as in outbred species. The data which he presents, however, must have little bearing oiz whether the Fisher effect act-a~,lly occurs, si~ee he does not take into accotmt the dominance mechanism ~rst proposed by him, and here demonstrated to be of much more than casual importance.

THE HALIJAXE EFFECT

The e~ddence for the attainment of dominance by tlfis modred appears to be unshakeable. In the first place it has been shown that failure of dominance oec~trs m the genogyps of t~Te 9 }~i,rs~.u~, the heterozygote being a strict intermediate. ~'~en its nbrmal allele C m~ is displaced by the ba, rbage~se allele C a~, the interned.late is converted to a full dominant at one step. ]~urther evidence is give~ in Table 12, ~-here the intermediate heterozygotes containing £i ~ are clearly distinguishable from those con- gaining C~, about half of e~eh type being present according to ex- pectation. A final piece of evidence is the fact that C ~ can retain m.uch of its don~inance potency even on the background of the extreme super° crinkled form.

SYDNI~Y Ct¢oss Id[ARLAlVD AND OLg.4 ]~{. ATTEC]I{. 43

Considering the six species worked~vrRh, it, is concluded that 5~rba- de~se, tom~tosum., and Darwinii confain norma] alleles of the I:Ialdane ~ype, "and. this is consistent with what is know-a of their taxonomic relationships.

THE ~lS~[~R, I~FFEOT

This consi~utes the main type of domina.nce construction. Here a.n0rma.l allele of weak dominance potency interacts with an unknown bn~ prolsably large number of modifiers to produce don:dnanoe. This type O f weak normM allele has been demonstrated to exist not only in hirsutu',n but aIso in fourp~.,rasce~s and Taftm~.se. This grouping is again in accord- Once ~yith what is know-a of the taxonomic, affinities of these species. I t was ~ s t shown that the fully dominant Aa* phase of barbagense was donverted into an intermediate Aa phase by simply" substituting the normal alleles of hirsuam% 2~4"puraaes~s, or Taitenss. Nex% the mod.igers of hirsutum type 250, which conferred complete dominance in this type, were made to substitute those of type 9, which, as will be recollected, produced an intermediate heterozygote. The resuR w~s not only eon- versiml of the in%rmedi~te ]~eterozygobe to complete dominance, but a si'm.~Imneo.us mzd censigeraS~e gml)rovement of the recessive..Now in the case of barbadeuse, dominance bebg a~ta, i.~ed at one step, t,he relatively 3mimproved recessive must signify that direct selecgim~ pressure ~t;o improve R ha.s ~o~ been of any great consectuenee, and has had ]Rth, effect. This argumeat is the key ~o fl~e situs.rich, for if it, he assumed -tha-e seJee{,ion pressure on the recessive has not been. dissbzdlar i~: hirsutum ~nd 5m'badenss, ~he dissimilarity in the stage of im.]?rovemeat of the Id'rsuh~m~ recessive can be due mJy to selection pressure ae~dng on either the Aa or AA phases. Now since there is no ree~,son to suppose that {be AA pha,se is other than neutral towards modi~ers of crinkled, the 2,ragress of &e 4nterme&:e~e Aa 5phase towr~rds n, orma, litf ~)~ust have been the outcome of direc~ selection 2,ressw,.e v40o9~ the hete~'ozggote. This process :is that requ.:ired by the Fisher theoO% :Pad exl?e.rimenta,1 verification of this- theory may :now be eo~side~:ed complete.

Hutchinson & (4.hose (1937) have argued theft fhe attM~me~t of dominance in both barbagense and hirsut'~.m, has bem~. a recent consegaence of domesticatioa. They also n~ted that the modification of erinlded in th~ direction of normM had gone much fm.'the:t' in. #i~'s~t'~.m tha~ ill b~rbagense, and considered this to be due to the fact %hat hirs'~tu'm ihs, s

a A a :heterozygoge, A A =homozygous n o r m a l a a : h o m o z y g o u s recessive.

44 Else (/e~et.ic,s' o f Oo~to~

been longer i.~ cultivation as an a,nn.ual tha,n berbade~zse. Aps,rt from the explanation just given--that bhe ace umalation of modifiers by the weak Aa phase ~n l~:i,rs~.t,a~ neeessarily improves the recessive--it can be shown on other grounds that the conditions of domestication can have had nothing to do with the attahament of dominance. First, c@kled ~rans2 %fred to the background of (/. Lpa~,r.T'z~ta,s~e'~s va,r. Mo,r.rill.i, a truly wild type :from the sand dm~es of the west coast of Ne~co, ga~e not oNy a compb~ely dominant hetetozygote but also a recessive similar to thai; of ki~'s~t.u,,m, in its grade of improvement, t t has Mso bee~ established that Mo~'tilg has a weak normal a[le.le of sil~ila,r domiaance potency to that of the weak tz'i'~'st~m. A second point: J.t has been ~ound (].9~10) that two ~41d Nexiean 13-ch~'omosome diploids, (;. A~',~w~.t~'ianum and ~. Tku.rberi, eontain normal alleles at tl{e oriakIed locus, whbh e~n be transferred to be~bede~zse crinkh~d by repeated baekerossing. When transfem'ed, drmo~.,~',;a~e~z proved to have a normal allele of great potency, similar to barbade~zse, and TAwrberi one of weak potency, similar to that of hir- s~twm. The history of dominance at the crinkled locus may thus extend back for many millions of years to the origiaa.1 diploid component of the New World group of tetraploids.

It has b~en observed t.ka~, there may be ~ slight disturbance of dominance in some inter-barbade~.~e m'osses. This is ttae not only in Egyptian normal x Sea Island crinkled, but also ia G. baq'bage~e oar. bras£iense (Trinidad Red Kidn.ey) x Sea Island crinkled, and in crosses 'between a Peruvian type of bar~cde~se (Tanguis) and Sea Islaad crinkled. Hutchinson & @hose (1937) believe this sligh~ faik~re to signify that the last stages of the evolution of dominance of.normaI in 6a.rb~dense may. have taken place in the last century, since the separation of Egyptian from the Perennial× Sea Island hybrid population from which it was selected. They deduce fm'ther that if crinkled is transferred to the back- ground of a barbcde.~e tree cotton which has not been under cultivation, dominance should break down. Since these preli.minary observations, farther work has established that in ~he case of Egyptian normal x Sea Island crinkled, or Sea Island normal x Egyptian crinkled, the slight cUsturbance of dominance is found only in the F~. C?mplete dominance is restored even in the first backcross. There is thus no reason to anticipate failure of dominance in primitive tree cottons, especially when it has been shown g):at the potent barbade~se normal is nearly dominant even on the super-crinkled background.

The slight impairment of detain.ante in some }nter~ba~'badense crosses is capable of a relatively simple explanation. Assuming dominance to be

SYDIgEg CROSS H~L~4~ND ~.ND OL~_~ ~{. ATTI~CJ~ 45

very. ancient in barbade~se on aeGdunt of the e~steucs in the allied A~'~io~'.ia~m diploid species of the powerful normal allele, ~he dominance mechanism of ba~'ba, de~se would tend, according to the writer's theory of gent d~if~ (tIarlana &Att, eek, 193g), to become constructed in slightly ffifferent ways in different ecological areas. On crossing, the now diver- gent .dominance mechanisms would tend to disintegrate each other. E~m/p~iag and Sea Island, and possibly other barbade~zse types, possess slightly different modifier complexes, produced in response to isolation

. , . .

and:divergent ecological conditions. Thus even when a dominance mechanism has been perfected, it is not in a static but in a dynamic cdndition and the genes contributing to its make-up mnsg be contimtally cganglng according to selection pressure tending to seleet alleles of the modifiers for other more important functions. The dominance ~elation is analogous to a useful s~rueture. I~ must be of physiological raise, for otherwise i~s construction in so many different ways can hardly be e.rplgined. But once attained it must preserve its integrity, and it cannot do this unless it. possesses enough genetic variance to reeons~ruc~ its i~odifier ;ys~em when mutation in any gent constituting such a system results in an allele strengthening some other more important physio- logical process, resulting in. the tdtimats dissemination of this new gens through, the population.

There is, finally, one aspect of t]ae Fisher effect which it is necessary %o discuss, and that is the conditions under which it is tike]y to become operative. The first condithon is ~hat ~ considerable a.mount of out-" breeding must take place to provide for di:ffusion ~hrough the whole Sl)ecies of modJt3.ers affecting only the Aa and aa phases. ThisTcondition is. proyided by such genera, as D~'o,so2hga, and Zea, by all self-s~eKle plants and generally by cott, on, espeeiMly in cultivated forms. Since it has been shown that crinkled modifiers simultaneously a,:ffect both the : Aa and aa phases in ]g~'s~t,i~m, the diffu.sion process would be accelerated by natu.ra.l crossing between aa and the other two phases. I~ is not~ -improbable that~ this bus taken place in cotton. The very precise degree • 0f adjusbmen~ of the normal alleld~bo its modifier complex, whate.ver the m.ethod of dominance construction, is diNcult it, explain except on the assumption that the .last stage of Jmprovemen~ of Aa to completely normal was efl~cte~[ "by cross-~ransference of modifiers from recessivss improved by direct n.ataral selection. It is easy to understa,nd the im- provement of Aa to nearly normal by a.cemnulsiion of modifiers, b~tl) When the stage is reached of an excessively faint degree of crinkling, i6 is difBcult to imagine ~hat this faint manifestation could be removed by

46 TAe 6~s~e~-ics of Co~o~

direst selection of heterozygol~es~, i t seems necessary in ~his ptarticular ca, so to invoke the recessive as responsible for the last stage.

There is also evidence of an important subsidiary mechauism for diffusion of dominance modi.[iers, that is, the possible selective adva~.ta,ge of the Aa phase o~er the AA phase. I..n a tdrevious paper (1985) th~ writer attempted ~o show that the Aa phase ia type 9/d'rsa~t,~.~~ was not only not at a select:ive disadvantage compared, wbh the AA phase, but even slightly superior. Latez" and[ more extensive experiments, of which only 5he summarized results are no w avMiable, demonstrated that it was only under favourab]e condition,s ~hat the Aa phase was stighdy superior. It was generally inferior under bad conditions, especialiy wRh inte-~.tse eorape~Rioo. ~Now under the intensMy fluctuating conditions found in nature the Aa phase ~vouid sometimes be under stdngen~ selection which wouldtend to provide a margin of safety. At o~her times so:me Aa plants migh.t have a multiplication rate exceeding Chat, of the normal, which ~von[d a.ssist in the diffusion of modifiers ~hroughout the species. Further experiments are obviously required on the selective value of tha AA and Aa phases for a large ntm].ber of different ge~.es.

S LuI~{AI% Y

I. The dominance relations of the crinkled mutant of Gos.s~b.~.m b.r~.rbs~s~ss L. have been studied in an extensive series of backcrossing experiments, involving six species of New World cottons.

2. It is believed that there have been t~,vo methods by which dominance at the crinkled locus in the six species of New World Goss~pi~ns has been a~tained. The first method is th.ae proposed by Fisher, whereby dominance has been reached by modification of ~he heterozygous phase (hi<s~m,~n, ya~'#~reses~,s, and Tai~e~zse). The modi~ers improving the

heterozygous phase have simultaneously improved the recessive. The second method is that proposed by Haldane, in which dominance is a~eained by the employment of a norm.M aRele of great dominance potency (b~rbc~de.~ss, m~ze~ztesz~.m, and ~Da,~zd~ii), the recessive phase being relatively unmodified.

3. Some cqndRions ~nder which the Fisher effect is operative ~re discussed.

4. Evidence is brought forward incEcating that the normal allele of bc~'bade~se, C ~s, may" become mutable on the genetic background of c.n.e type d p~,r~p'~raaoe~s.

S Y D > < c Y @t¢OSS HABLAND AND 0LGA ~[. ATTECK 4 7

1%EFE1%BNOES

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])warf' in go~,sypi~m~/%ra'~aam L." J . gc'~,ct. 3~, 437. ]~:[~LE~, t~, J, (1932). Fttr~her studies on the nature ,~nd causes of gene mutations.

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gone in~raoMon. J. ge, n, eL 38, 229. W~m~T, S. (1929). Fisher's them T of domina.nee. Amo. iv°at. 63, 276.

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