Jnoj pm 'ursaJ unds pm loyoale alnlosqe jo am1xp 1.1 e '1oyo3p alnlosqe jo sa%uey:, inoj) ~oyoap jo savas pap& e u! pateq6yap aiam saldtur?~ "1uauIlva.q y 21 io j 31,)qa3e [hmin ~ S ' Z JO pealsu! a m ~ ~ ~ a d w a , woo2 qv y S-0 JOJ loqo3p %OS + aplaae ~huwn pleJnjvs u! pa3eld aJam saldws ayk -g S" 1-1 JOJ ' 2 ' ~ ~d 'asah1owhz jo 8ur 9 p m Z!$ip~ j q . t x I 'Q

7 1 3 ~ N 9'0 8uru!e1uo3 ~a j j nq aioydsoyd JU 1.0 JO VOI jo uo!1n1os a u! L~~ x 5 ' ~ JO uogex~ua3uo3 e la papuadsns aiaM sIla3 ayA '(p) urn10 p m ~ a p p r ~ JO spowaw aw 01 %qp~o33e paxy aaaM splualu! 1uaJaJpp a palsamy s11a3

d d 0 3 ~ 0 ~ 3 ! ~ UOAj3ala

-3 ,~s JO ainleiadura1 a ~ ! p y s a ~ pue 3,9z JO a~n1~~adtua1 a~!sspmd aq1 le yloq lno pap.1~3 aJaM suo!le~muods p m 3,92 JO a~n1e~drera1 a ~ ~ s s v a d aw 1e ]no p a p 3 slam suogeqn3ul ' V ~ A pup a d A il! q3~012 a~!lwa%a~ 11v 10d 'T.U/,QT x s JO 6qsuap II~:, e 1e p+qds u! papuadsnsa~ uay1 pue M ~ S p w IaleM palI!ls!p aIr;nals rq!~ q3ea aauo paysem aJam sl1a3 ayL aaseyd 301 01 MOJ% 01 palnoip aiaM pue v/ ,QI x 5 jo 6puap 11a3 e IE v d ~ 01~1 papuadsnsa~ aJaM sIla3 ' W M O J ~ JO y 02-81 ialjv ' adA p!nb!l a q ur qw/ 01 x I JO h~!suap 11a3 e be pa1e1n3oul aiam C=J~A pqos uo sAp S-S ioj UMQB sIia3 pyod !~

.mypaw JO aqg ad auls61 pue 'aurus!Waw 'aupep ?u~u!2re 'auluap~ JO $ & a ~ £ y1lm pquawalddns araM e l p w atp go1v .alqaae urnysse~d %Z 8uyp3u03 ( ~ 6 s ) elparen uo!1el~uods pue talela3e wunm!ss~1od sgl pus? 9uo1dd %Z '13eqxa )seah-olaeg % u ~ u ~ ~ u o ~ wnFaeu uoy -e1ruodsa;rd f.m%tz-013~~ u o ! ~ e ~ ~ ~ ~ p g o s JOJ PUB 'auo1dd -013~8 %Z 'pa~1xa ~seah-013~8 'asalxap %Z 8u!u!a1uo3 ( a c $ ~ ) urn!pam a ~ y q a % a ~ :pasn =am elpaw % U ! M ~ ~ O J aqA

. ( u o ~ ~ ~ ~ ~ u n m o ~ ~vuosad '011sodsg -9 -8) p!oldnam sq hem u!e.x1s a y ~ ley1 so paAIoAu! s! aua% auo uey1 aiow raqla l e v sage~pu! uopei3a~das nqn2a.q 1nq 1!eil aAlssa3ar e se palyaqeap

SelpuI ' E W S ~ ~ - leydw~ 'aqua3 h!s~a~!un PuyIaN ~e~mvph iq 'sa@ua!3~ a j q jo uols!~la :ssarppe luammad,

OsaFpoq a10d alpulds 'qds funmrpaw uo!1ep1ods 'yqds f w n ~ p w uoyqruodsaid ' V ~ A fwn!paw -a3qa%a~ ' adA txaldmo3 gemauo1deuhs61od '3sd thpoq asuap -mapnu 'qpu fxaldwo3 leruauo,dvuAs '3s :sNouvnAauraaV

s! uoyelnw 01 ods aqA .aseydoid qo!am 1e s.1n330 Isam pue '($3 arnte~adwaq aAg3gsaJ ayl 1e aaeld sayel s!saquhs VNgI 3!lo!ayd '3,PE 1p Pal!q!V! 1nq 3,92 1" Iemou s! uo!1elwod~ s30~s pue gz 1e ~ e m o u s! ~ I M O J ~ .(g) ~uarsnaqnba~ 1a.u e siq 11 pue 1-z sk]/ 1-2 skj 1-9 ZJD/ 1-p 8.i~ 1-z gpo/ 1-% apv 01 ods/01 ods o ~ / o ~ lo/v ST adLtoua% sq - IPS up4s p!o~d!p pnpuqs E WOJJ saiodso3se JO s!saua8elnun 9aIopwqn 6q paqiap np.13~ 1ua!agap-u0!1e~Puods a~!)!suas-ain1e~adwa1 e s! 01 0ds/01 ods JOJ sno%hzouroy uleqs ,sea6 ayL

spq1at.u pm s p ~ ~ a p q , q ' guoy

-a3~unmmo:, pnoslad 'zqoydtqg 's) aogZ )B pa~epuoels uayw ~ o u 1mq 3,$~ IF? aseydo~d 31po~aku le palsam uaym sasd JO raor13npoad 3ygo~d B seq ~p ads u ~ ~ 4 . s ISBPL IVY) U M O ~ S S! uoda~ s ~ y ~ u~ 33s jo U O F S S ~ J & ~

aq1 al~1n8a~ ley1 s ~ o l 3 ~ j 3paua8 lo 6jv3~may3 'le3~sICqd "Iauoggnu 8u11epsl 01 LBM ay) uado s u o p p A asau ' (L '9 's 'p 'c 7) s3sd puv s3s jo uo~ssa~dxa argg 8sn

sa3uaajjrp alqepa~dd~ an? slay) ley) ale3Fpur s)s~ah $0

suwes swopn ur slsolam uo suor)sMasqg '(1) U O F ) E ~ ' $ H

-%as amssorero~y~ pue 6uopenFqsuoaa~ qaua8 68%npled amosomoq:, sno8ojounoq na UOFI:,UWJ 03 pa%papotq - 3 ~ SF 3s aqo salokyma %ur:,ngssda~ Rjpnxas n1

enur urn of a1 led l ~ w p e ~ ~ , ] .<apu sal suep 3s ap uopep1~1n33a mop e h 11 '3s sap arn?aar-a[Ia uo!13npoid EI

sed alsaD,u s p x ~ . ~ '3s sap uo!l3npo.xd el assa3 au anb JueAe an13a~ja~s 01 ods aqhpnos a1 suep 1 w . 1 anb awnsyd u g esanbp -yio1deaahshlod saxaldwo3 sap 1uaaruayuoD qpu sap snld no %05 la sp.1aw8e.g s p w 'qu2up1;do~d 1uaaruaLAap s 3 ~ sal '1p.m un ~!npo~d as vo saplla3 sal suva .(qpu) asuap a~le?13nu sd.103 un 1uaAnos la (3s ) sanbrm?uo1deds saxaldu103 sap 's?nblgdnp xneriosnj sa.xla3god sdlro:, ap na! un 1ua!1uo:, nehou anbey3 go aseydo~d el ap a!Alns 1sa anbyoy.u s aseqd el 'alomon a~n~e~?dnna~ anna?,l no aun,I v 3,92 y uou s~ re r '3,~s q anb!loyw aseydsid cl ap I ~ U E un n d a?s1~?13e.ap?:, isa ainAa1 ap 01 08s aq3nos e?

682-882 '09 "14d3Y30!8 'f .(au!s!na~a3 sa3,Czero~qm~s) 01 ods 4 lseah tq saxa[dmo3 ~erenauold~n6s pue Isam 3!1o!a~q (2861) '3 'npnny q *a .d ' s uao ]~

-qpu ayl ur lepa1vw 3s JO uo!1e~nwnmv UR %errsne3 s n q 'jlasl! no!13npod 3s w a ~ j e IOU saop Inq norg~npo~d 3s JO mmopjnys 01 iopd sanmo 1sam 01 ods 1eyl pamnsse sr 11 *saxalduro3 lerenauolda3uLs6lod u!eluo3 sqpu a q JO a ~ o w JQ %OS p u ~ pa1uaur8e.g 1nq 1rraywoad amo3aq s 3 s aq1 slga3 palsam UH -(qpu) hpoq asnap-xal3nu e Lgluanbaij p m ' ( ~ 1 3 ~ ) saxap5.uo:, 1xxuauo1d~u6s 'sarpoq a1od alpu!ds pa1e~ldnp JO )as e su!e)~o3 s~na13raan yDea aiaym a s ~ y d o ~ d hq pamolloj s! asvqd s 3gopw ~ e m o u cunlvradmaa Jaylla 1~ '3,92 1" IOU inq 3,~& 1~ i sam aseqdoid a!ioyaw hq p a z ~ a ~ ~ e ~ e y 3 s! 01 ods qasts ]sea6 ayLg

Can

. J. B

ioch

em. D

ownl

oade

d fr

om w

ww

.nrc

rese

arch

pres

s.co

m b

y U

nive

rsity

of

Shef

fiel

d -

Sub

Lib

rari

an o

n 11

/11/

14Fo

r pe

rson

al u

se o

nly.

MOENS AND KUNDU

changes of pure Spurr resin by centrifugation) and finally samples were embedded in Spurr resin. Serial sections were collected on Formvar-covered single-hole grids and stained with 2% uranyl acetate for 2 min and lead citrate for 1 min. Observations were made on a Philips EM 200 at 60 kV.

The SC, pSC, and the ndb are observed only, but not necessarily, at meiotic prophase. In nonarrested cells these are transient structures so that their absence in a given nucleus at the time of fixation does not provide information on the mcurrence of the structure prior to fixation or, had it not been fixed, afterwards.

Results 26°C. 0 h sporukation

The ultrastmcture of the yeast nuclei was recorded at the time the logarithmically growing cells were trans- ferred from acetate YBA to SPM. All nuclei were found to be free from meiotic characteristics such as SCs (Fig. 3), pSCs (Fig. 4), ndbs (Fig. 3), and enlarged spbs (Fig. 3). In a sample of 13 completely analysed cells, 4 were in S phase or G2, judging from the presence of duplicated spbs and 9 were in Go or GI with a single spb. The cells with two spbs complete mitosis before enter- ing into meiosis.

26"C, 6 h sporuiation Spo 18 diploids grown at the permissive temperature

of 26°C show the normal pattern of progress through meiosis and spomlation (Fig. 1). After 6 h on sporula- tion medium a random sample of 12 serially sectioned nuclei was found to consist of 5 nuclei with a single spb. Four nuclei had a duplicated spb but no axial cores or SCs, and three cells were in the pachytene stage of meiosis having nuclei with two spbs and SCs (Fig. 3). This distribution is representative of a larger sample of nuclei observed in single sections. Two of the pachytene nuclei but none of the others had a ndb (Fig. 2).

26"@, 8-10 h sgorulation In a sample of 16 complete nuclei, six cells had not yet

duplicated the spb, four of these had developed a ndb inside the nucleolus, and 1 nucleus had two spbs but no SCs. Nine of the 16 cells were in meiotic prophase judging by the presence of two spbs, and axial cores or SCs. The ndb was present in three of these nine cells (Fig. 2). Presumably in six of the nine cells the ndb had already disappeared.

2Q°C, 12-14 h sporulatisn A large sample of 38 complete nuclei was examined

to verify the beginning of spore formation at this time and to compare this culture with the 34°C cultures which fail to initiate the first meiotic division. Of the five cells with unduplicated spbs, two showed signs of having entered meiosis judging by the presence of ndbs but h e e did not seem to make much progress. One cell had

D I u l s I m s and Spores

Hours in Sporulalion Medium

FIG. 1. Graphs of percentages of spo 10/spo I0 nuclei at consecutive stages of meiosis at the permissive (26°C) and restrictive temperature (34°C). Samples were taken from the SPM at the times shown and fixed for electron microscopy. A totd sf 200 complete nuclei were examined for the presence of a single spb (premeiosis), for the presence of a duplicated spb with or without SCs (meiotic prophase), and for dividing nuclei or ascospores (divisions and spores). Clearly the spo 10 strain at 34OC arrests in meiotic prophase while the same strain sporulated at 26°C goes on to form spores at 12 and 24 h.

duplicated spbs but no SCs. Most of the nuclei, 20 out of 38, were in meiotic prophase, all with two spbs and SCs. Of these 11 had a ndb and 9 did not, while none had a pSC, a ffiquent feature of 34°C arrested cells at this time (Fig. 2). The 12 cells that had started the meiotic divisions all came from a 14-h sample. Of the 12 cells, 18 were in meiosis II and 2 had young spores.

26"C, 20-24 h sporulation At this time 26 of the 31 cells examined had spores,

and one ascus was in the first meiotic division. Only 4 of the 31 cells had not initiated the meiotic divisions. Of

Can

. J. B

ioch

em. D

ownl

oade

d fr

om w

ww

.nrc

rese

arch

pres

s.co

m b

y U

nive

rsity

of

Shef

fiel

d -

Sub

Lib

rari

an o

n 11

/11/

14Fo

r pe

rson

al u

se o

nly.

286 CAN. B. BIOCHEM. VOE. 60, 1982

BOO 2G°C ndb these, two cells had one spb, one cell had two spbs, and

one cell had two spbs and SCs. All four had ndbs.

H w r s In Sporulet.lon nedlw

FIG. 2. Histograms of percentages of ndbs and pSCs in 200 spo PO cells sporulated at 26 or 34°C. At 26°C the number of nuclei with ndbs decreases dter 8-18 h as cells progress to meiotic divisions and spore formation. The 34°C culture, however, arrests in meiotic prophase so that the number of ndbs increases. After 10 h s o w of the ndbs contain pSCs and the frequency of ndbs with pSCs increases gradually.

34"C, 6 h sporulation In a sample of 18 cells the proportions of cells in

consecutive stages of meiosis are nearly identical to the proportion observed at 26°C. The only difference is that the ndb is more prevalent in the 34°C culture (Fig. 2). It was present in three of the nine cells with one spb, in four of the six cells with two spbs, and in two of the three cells with two spbs and SCs; that is 58% of the cells whereas at 26°C only 17% of the cells had developed a ndb at 6 k (Fig. 2).

34"C, 8-10 h spsmlatisn A large proportion of cells, 22 out of 30, had already

progressed into meiotic prophase (Fig. I ) , somewhat more rapidly than at 26°C. Also the single spb compart- ment contained only 20% of the cells whereas at 26OC 48% was still at that stage. Again, at 34°C the incidence of ndbs is much higher (70%) than at 26°C (44%). Also, one cell of 34°C was found to have a pSC whereas none were observed at 26OC (Fig. 2).

FIG. 3. A s p 10 nucleus from a eel1 in sporulation medium at 34°C for 24 h. Visible is the duplicated spb; a SC, continuous in ake two sections; the ndb; the nucleolus, no; gnd the nuclear envelope, ne. The SC consists of two lateral elements and a central element between them. The structure of the nucleus indicates hat this is a slowly developing cell so that it has not entered the arrest as yet. Scale bar = 1 pm, magnification x 30000.

Can

. J. B

ioch

em. D

ownl

oade

d fr

om w

ww

.nrc

rese

arch

pres

s.co

m b

y U

nive

rsity

of

Shef

fiel

d -

Sub

Lib

rari

an o

n 11

/11/

14Fo

r pe

rson

al u

se o

nly.

MOENS AND KUNDU 287

FIG. 4. A sps 10 nucleus arrested in meiotic prophase after 24 h in sgorulation medium at 34°C. The ndb contains pSCs. The pSCs are not two-dimensional ribbon-like structures as are the SCs; instead the pSCs are three-dimensional crystals of SC central element material. It is assumed that the ndb manufacture SC material and that when normal regulation is interfered with, excess SC material accumulates and auto- assembles within the ndb.

34"C, 12-14 h sporulation At this time the effect of spo 10 is well expressed.

Clearly the cells fail to progress from meiotic prophase into the meiotic divisions. At 26°C some 30% of the cells are at meiosis XI and later, while 0% of the 34°C cells had advanced that far, A total of 27 out of 33 cells were arrested in pachytene with two spbs and SCs (Fig. 1). Of the 27 cells, 11 had pSCs (Fig. 5 and 4), also a feature not found in 26°C cells. Of the remaining 16 cells, 12 had a ndb and 4 had no ndb or pSC (Fig. 2).

34"C, 20-24 h sporulation The spo 10 phenotype is most obvious at this time.

Instead of 80% asci with spores, as at 26"C, there is 0% of such cells at 34°C after 24 h (Fig. 1). Most nuclei (19 of the 22, or 82%) were arrested in pachytene with duplicated spbs and SCs. Of these 12 had a pSC and 7 had ndb (Fig. 2). The percentage of nuclei with a pSC is variable; in m earlier test run apparently 108% of the nuclei had a pSC. Cells arrested for prolonged periods show degenerative features such that in my one section the synaptonemal complexes appear as uncommonly

dense structures, but when traced from one section to the next they lack their customary continuity. Also, the tripartite structure of two lateral elements with a central element is not well defined. 'The same defective SCs have been reported for cdc 4 homozygotes sporulated at 34°C and arrested in meiotic prophase (5 ) . 26"C, 8 h sporulation and return to YPD, 3 h

Return to vegetative growth medium apparently removes meiotic characteristics rapidly. Although the 8-h sample must have had 60% of the cells in meiotic prophase no SCs, duplicated spbs nor ndbs were found after 3 h on YPD at 26 or 34°C. In a sample of 32 nuclei, nine cells had formed buds and four of the buds had a nucleus. 34°C' 8 h sporulation and return to YPD, 3 h

'The results were the same as above, no meiotic structures were found. Of 33 cells grown at 34OC, 14 had well-developed buds, others had small buds. The nucleolus seemed unusually strongly developed. Of 19 cells returned to YPD at 26"C, 7 had developed large buds at 3 h and other cells had a small bud. 34OC, 8 h sporulation and shift to 2@C, 8 h sporulation

The presence of asci with ascospores in this sample indicates that spo 10 action at 34OC is still reversible after 8 h. 26"C, 8 h sporulation and shift to 34"C, 8 h sporulation

The presence of cells undergoing the second meiotic division indicates that at least the more advanced cells can no longer be arrested. Other nuclei had SCs, but no pSCs were observed.

SPO ~ ~ r y o t y p e Four nuclei were examined in their entirety. In two

the SCs were untraceable, in one the SCs could be followed but not the individual lateral element, and in one nucleus a reasonable tracing was made (Fig. 5). In the two latter nuclei only 15 complete SCs were found, 2 less than observed in other strains. The lengths in micro- metres were 3.07, 2.25, 2.18, 2.10, 1.87, 1.85, 1.73, 1.65, 1.28, 1.20, 1.16, 1.11,0.64,0.62, and0.57 with an average of 1.58 Fm. Total length of SCs was 23.67 bm. In addition there was a single core attached to the nucleolus.

Discussion The regulation of meiosis is more accessible for study

in the yeast (Saccharomyces cerevisiae) than it is in higher organisms. The complex cellular interactions that trigger meiosis in higher organisms are absent in this single-celled organism and the encumbrances of gamete development are lacking. Without these constraints the meiotic process itself has been found to be quite flexible (10, 11, 12, 13$ 14).

In most yeast strains the nuclear aspects of meiosis include the duplication of enlarged spbs, the formation of a ndb inside the nucleolus, the presence of SCs, md

Can

. J. B

ioch

em. D

ownl

oade

d fr

om w

ww

.nrc

rese

arch

pres

s.co

m b

y U

nive

rsity

of

Shef

fiel

d -

Sub

Lib

rari

an o

n 11

/11/

14Fo

r pe

rson

al u

se o

nly.

CAN. Je BIOGHEM. VOL. 60, 1982

FIG. 5. A stereopair of SCs in a spo I0 nucleus. The arrangement. total length, and individual lengths are normal. Only 15 complete SCs could be found as opposed to the normal 17. The asterisk marks a nucleolus attachment site.

the formation of a meiosis I spindle with spindle elonga- tion followed by spb duplication, meiosis 11, and spore formation (2, 3, 4, 5, 6). In some strains, but not in others, the ndb may contain stacks of SC material, the pSCs. The interest in spo 10 lies in the prolific production of pSCs when arrested in meiotic prophase at 34°C (Fig. 4). The accumulated observations on meiotic prophase indicate that cells progressing normally through meiosis form the ndb during meiotic S phase (15), then the SCs, next the ndb disappears in late prophase, and finally the SCs disappear at the time the meiosis I spindle forms (3,4, 5,6). If meiotic prophase is arrested as in cdc 4 at 34°C (5, 6 ) and in spo 10 at 3rb°C, the ndb fails to disintegrate so that it is found in most of the prophase nuclei and an increasing number of ndbs contain pSCs as the arrest is prolonged. pSCs have also been reported in the spomlation proficient triploid CBS 5525 (3) and strain 11 B (4), both of which, incidentally, produce a high percentage of inviable spores.

From these observations it has been assumed that the ndb is the site of SC central element manufacture ( 3 , Q ) . Under normal circumstances the ndb products move directly to the synapsing chromosomes and no pSCs form. In the case of CBS 5525 and 1 1 B it is assumed that a genetic imbalance in the regulation of central element production or utilization causes excess central element to accumulate as pSCs in the ndb. In the case of cdc 4 and spo 10 at the restrictive temperature it is assumed that the signal leading to the transition from ckbl.omosome synapsis (zygotene-pachytene) to diplo- tene md metaphase H is deficient and as a result synthesis of central element material continues to excess in the ndb.

The primary cause of temperature-sensitive meiotic prophase arrest in spo I 0 is not known. cdc 4 with a similar arrest has only very limited meiotic DNA synthesis (15) whereas spo 10 has a wild-type level of DNA synthesis (9). Both cdc 4 and spo 10 can be rescued by return to growth medium at 26OC and cdc 4 gives evidence of meiotic levels of recombination under those circumstances (15). Both strains are capable of completing sporulation when shifted to 34OC after 8 or more hours in SPM at 26OC. The analogy of structural characteristics and effects of temperature shifts between the two strains suggests that although spo 18 has a wild-type level of meiotic DNA synthesis it may have an incomplete or defective meiotic specific DNA metabo- lism of zygotene or pachytene DNA repair (16). Much the same meiotic prophase arrest has been observed in a rad B/rad 6 repair deficient strain (S. Kundu, personal observations).

An observation of no direct bearing on the nature of meiotic arrest in spo 10 is the possibility that spo 18 is aneuploid or has some chromosome rearrangements.

In five nuclei of strains unrelated to spo 10 (DC x 374, DC x 4 16, G. R. Fink), 17 SCs can be recognized without too much difficulty in each nucleus (17). The total length of SCs varies from 24 to 27 pLm per nucleus. There is usually one SC longer than 3 km, three SCs are between 2 and 3 pLm, nine SCs between 1 and 2 pm, and four SCs are less than 1 p. Byers and Goetsch (5) have reported tracing of thee cdc 4 cells where they found 15, 15, and 17 SCs with total lengths of 27.7,22.4, and 33.1 km. There is some difficulty in tracing yeast SCs. In two spa 18 nuclei only 15 SCs and a short single core were found. The SCs and their distribution in the nucleus are shown in Fig. 5; all ends are attached to the

Can

. J. B

ioch

em. D

ownl

oade

d fr

om w

ww

.nrc

rese

arch

pres

s.co

m b

y U

nive

rsity

of

Shef

fiel

d -

Sub

Lib

rari

an o

n 11

/11/

14Fo

r pe

rson

al u

se o

nly.

MOENS AND KUNDU 289

nuclear envelope except the end marked with an asterisk which ends in the nucleolus (5). The image was obtained by recording, on computer file, the X , Y, and Z coordinates of the lateral elements where they occurred in subsequent sections and by drawing, with a computer- driven plotter, smooth lines through the points of each lateral element, The stereo image was obtained by rotating all points around a Y-ascus in the middle of the nucleus and drawing a plot of the rotated points (17). The total length of SCs in two reconstructed nuclei is within the n ~ m a l range and the individual lengths of the SCs are also within the range normally found. Aneu- ploidy or rearrangements we possible but errors in tracing are not excluded.

Acknowledgements The spa HQ strain was isolated by M. S. and R. E.

Espsito and the EM phenotype was originally observed by S. Klapholtz, of the Department of Biology, Univer- sity of Chicago. We appreciate their generosity in supplying the strain and their assistance with the project. Technical assistance was provided by Mary Lou Ashton of the Department of Biology, York University, and financial assistance was provided by the Natural Sciences and Engineering Research Council of Canada.

1 . Rasmussen, S. W. Bs Holm, P. B. (1980) Hereditas 93, 187-216

2. Moens, P. B. & Rapport, E. (1971) J. Cell Biok. 50, 344-36 1

3. Msens, P . B . Br. Rapport, E. (197 1 ) J . Cell Sci. 9 , 665-677

4. Zickler, D. & Olson, L. W. (1975) Chrornosorna 50, 1-23

5. Byers, B . & Gsetsch, L. (1975) Proc. Natl. Acad. Sci. U.S.A. 72,5056-5060

6 . Horesh, O., Simchen, G. & Friedman, A. (1979) Chromosoma 75, 10 1 - 1 15

7 . Moens, P . B . , Barclay, B. J. & Little, .I. G. (1981) Chromosoma 82, 333-340

8: Esposito, M. S. & Esposito, R. E. (1969) Genetics 61, 79-89

9 . Esposito, M. S . & Esposito, R. E. (1974) Genetics 78, 215-225

10. Sherman,F. &Roman, H . (1963) Genetics48,255-261 1 1 . Moens, P. B. (1974) J. CellSci. 16, 519-527 12. Moens, P . B., Mowat, M., Esposito, M. S. & Esposito,

R. E. (1977) Philos. Trans. R . Soc. London, Ser. B: 277, 351-358

13. Klapholtz, %. & Esposito, R. E. (1988) Ge~getics 96, 567-580

14. Klapholtz, S . & Esposito, R. E. (1980) Genetics 96, 589-61 1

15. Simchen, G . &Hirschberg, J. (1977) Genetics86,57-72 16. Stern, H. & Hotta, Y. (1980) Mol. Cell. Biochern. 29,

145- 158 17. Moens, P . B. & Moens, T. (1981) J. Ultrastruct. Wes.

75, 131-141

Can

. J. B

ioch

em. D

ownl

oade

d fr

om w

ww

.nrc

rese

arch

pres

s.co

m b

y U

nive

rsity

of

Shef

fiel

d -

Sub

Lib

rari

an o

n 11

/11/

14Fo

r pe

rson

al u

se o

nly.