Genetic Basis of Heterosis

8/10/2019 Genetic Basis of Heterosis

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GENENTIC B SIS

OF

HETEROSIS

General Statements and Explanations of some

Basic Genetic Principles.

Th e crossing of unre lated individuals often

results in offspring with increased vigor. This is

called heterosis or hybrid vigor, and the genetic

background for it is not known with certainty.

There are two general modes in which het-

erosis is expressed. The firs t is an increase in

size or num ber of parts. It is the resul t of grea ter

cell activity or greater number of cells. The

second way is by an increase in biological effici-

ency such as reproductive rate or survix-a1ability.

In addition to this, there may be a reduction of

grow th and/or survival rate . that is, hybird

weakness, reversed , or negative heterosis. This

is not common, however, it does occur (18). Stern.

1948, has found one exampl e of negative hetero-

isis in Drosophilla, Hemizgotes and homozygotes

for a serie s of position alleles R+ and +3 possess

norm al venation. However, the heterozygote

R+ /+ 3 exhibits various degrees of deficient ven-

ation. Thus the genotypic combination of the

two, or the

heterozygosity at a single locus de-

ceases effectiveness (26).

Heterosis has been utilized for many plant

and animal improvements.

Farm crops: Maize, sugar beets, sorghums, forage

crops, and grasses.

Horticultural crops: Tomatoes, squashes, cucum-

bers, eggplants, onions, and annual orna-

ments.

Silkworms

Livestock: Swin e, poult ry, beef and milk ca ttle.

Vegetatively propagated plants (13).

The greatest development of heterosis has

been in

Zea

Mays (33). One hundred per cent of the

corn now grown in the corn belt is hybrid corn(l3).

Commerically, hybrid corn seed is produced by

thz double-cross as follows:

Inbred Inbred Inbred Inbred

A

B C D

(AxB)

(AxB) x (CxD)

(double-cross) (25)

A

pape r presented in a n Animal Breeding class

at Louisiana Polytechnic Institute.

The expression of each charac ter, as a rule ,

is independent of other characters. Thus. the

plan t will not be heterotic as a whole. Brieger.

1950. rep ort s that the heterotic characters affect-

ed in maize, are: height, position of e ar . size of

leaves. chlorophyll formation, root system, resist-

ance to disease, pests. and unfavorable conditions,

size an d numbe r of kernels, width and len gth of

ear, size and bran ching of tassel, and th e amount

KRUSE

of pollen shed. Earliness, lateness, row nu

plant and kernel color are not affected by

erosis (3). However, Leng reports tha t

number is affected in widely different degre

heterosis, and tha t number of kernels p er

(ear length) is the only primary yield comp

in

which large positive effects of heterosi

manifested consistently (20).

Chambers, et al, (4), reported that in

hybrid vigor is evident in number of pig

litter, and litter weights at birth but it incr

as the litter becomes less dependent upo

direct mothering ability of the dam.

Vigo

expressed to a greater extent in the incr

viability of pigs and productivity of two

cross gilts than in the increased gro wth ra

individual pigs. The ext ra number of pig

litter in most cases

was sufficient to accou

a larger percentage of th e increase in tota l

weight. Therefore, since heterosis is express

both number of pigs survived and gro wth

per pig, total weight of litter seems to be th

best over all measure of performance for com

ison of lines or crosses 4).

When a heterozygote A1 A1 resembles in

otype of the homozygotes AI AI or

A

At.

allele A1 is said to be dominant or rece

Dominance is when AI At. resembles

Dominance is absent when A A: is ex

midway between A1 AI and A: A ? . Domin

is incomplete when A1 Ar is intermediate bet

AI A, and At. A? . Overdominance is when

is more extreme-for example, larger-t

ei ther AI A1 or A. A? (25).

Mutations constantly produce delete

mutants. Natural selection quickly eliminate

dominant gene alleles but cannot eliminate re

ive gene alleles because they are covered

overpowered by good dominants. Thu s in

the deleterious alleles build up in a popula

Grea t loads of these delete rious recessive a

are present in normally crossbred popula

(25). This has been proven tr ue by Dobzh

in Drosophila pseudoobscura where a m

ity of the individua ls carry heterozygous re

ives. When these were inbred. the offspring

more highly homozygous but it resulted

in

of vigor. This vigor was restored when t h

bred lines were intercrossed (8).

Inbreeding increases the proportion o

mozygotes in the population.

For exampl

F.L f a monohybrid cross AA x aa is 25AA

25aa. 50 percent are homozygous; 50 percen

heterozygous. Suppose all plants ar e s

The en tir e progeny of homozygous individua

main homozygous, but only half of the hete r

ous, individuals remain heterozygous, the

half become homozygous. This is illus trat

Table I.

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HUNTSVILLE ha

HUNTSVILLE Historical Huntsville

preparing a welcome-of-tomorrow for the annu

meeting of two agricu ltura l associations set f

April

5 6 7.

While not in official sessions. the Nation

Association of Colleges and Teachers of Agricu

ture and Delta Tau Alpha Honor Fraternity w

have an opportunitv for tours in an area that

fast becoming the leading tourist attraction in t

great Southwest.

Meeiings will be held

n

Sam Houston Sta

Tecchers College's glamorous new 1.8 millio

Lowman Student Center.

Conve~itiondelegates will enjoy a tour o

the historic campus.

The Austin College Buil

ing. built in

1851.

is the oldest building west

the Mississippi still in use for educational pu

poses. Thc newest ol the college's ma ny faci

ties is the Farrington Science building. Stu den

study here under ideal conditions

in

well-equi

ped laboratories. Jus t one of the many adva

tages for study that the science students have

Lhe planatnrium.

The Graphic Arts Building houses the on

complete graphic arts department in the worl

Along with studies in journalism, photograph

and photo-engraving. the complete Southwe

School of P rin ting is housed in this building .

The modern new agriculture laboratory w

be of interes t to convention delegates. The 89

acre Country Campus offers agricultural studen

splendid opportunities to demonstrate and stud

in agronomy. horticulture. dairy. livestock. an

poultry farming. Country Campus is also th

home of Sam Houston' s excellent 9-hole go

course in the rolling hills of Walker County.

Historic Huntsville is the home of th e St a

Prison. The well-kept buildings of both th

main unit and the farm units. are a clear refle

tion of Texas' advanced prison system. In tun

with their striving for rehabilitation. the priso

ers are entertained each October by well-know

performer s at the annu al Prison Rodeo. Th

rodeo staged for the prisoners is visited by hund

reds of rodeo lovers from the Lone Star sta

as well as other states.

Across from the prison, on the shortest high

\vay in the state. is the grave of G enera l Sam

Houston, first president of the Republic ol' Texa

Nestled in the tall pines of east Texas

Sam Houston Park . The serene park grounds fea

tur e a museum in Sam Houston's honor. Adja

cent to the museum is the small home of th

farnous Texan, a vivid exa mple of how the gr ea

man lived. small log cabin houses th e law

books he used in his practice. Behind t he whi t

frame house is the newly built War and Peac

House. A tri but e to Texas' own soldiers is show

here in World War I and I1 relics.

en N


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of ommerce

Overlooking the park is the i~ostalgic team-

boclt House the two-story home fashioned after

a

lMississippi steamboat in which the Texas Gen-

eral spent his last days.

Only nine miles from the Sam Houston Park

is Huntsv ille State Park. Thousands of Texans

flock annually to the 21.000-acre recreational

area. Pleasant holidays are spent fishing camp-

ing boating and skiing.

Driving back into scenic Huntsville. NACTA

and DTA delegates trave l through par t of the

155 000 acre National Forest. Driving through

this beautiful wooded area. they end up back on

campus under th e tall spires of Old Main.

News Bureau. Sam Houston State

Huntsville. Texas

M RCH

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GENETIC BASIS . . .

(Continued From Page 9)

the interaction can be described in ter ms of the

ability to synthesize specific metabolites. Emerson

decribed this Neurospora heterocaryon (physio-

logical apprcximation to a heterozygote in a

diploid organism) with a one-gene difference be-

tween the component nuclei showing superior

growth to either homocaryotic strain.

Biochemi-

cally, it appear that one homocaryotic strain

synthesizes too little vitamin para-amino benzoic

acid,

wheress the other produces an excess that

promotes a reaction deleterious to growth. The

heterocaryon achieves a proper balance that is

enough for growth and not to the extent that it

is deleterious (1).

Bonnier, 2). concluded that the single

assumption of overdominance will make many

accessory assumptions unnecessary. As long as

there is not biochemical proof to the contrary,

the assumption of overdominance seems to be the

simplest explanation for heterosis

2).

Other Facis and Ideas on Heierosis.

The simplest kind of t ru e heterosis--euheter-

osis-is the

r sult

of a superior dominant allele

sheltering a deleterious recessive mutant

9).

Thus heterosis proper or euheterosis is a product

of muta tion and selection pressures. Luxur iance

of heterozygotes is an evolutionary accident (8).

Luxuriance is observed in some hybrids between

normally self-ferterlized species, races. or strains.

This luxur iance is not due to sheltering of dele ter-

ous genes nor due to overdominance. Luxur iance

is from the evolutionary standpoint, an accidental

condition brought about by complementary action

of genes found in the parental form crossed. These

cases do not seem to be bet ter adaptable and also

ar e more frequently found in domesticated species.

Luxuriance is, really. pseudoheterosis 9).

It is interesting to note in the roport of

Wellhausen that Chalqueno corn. a hybrid of

Conicc and Tuxpeno: two distinct races of corn

will

not outdo either parent in their respective

location as measured by yield. yet in a new

envi ronment i t has an excess of vigor.

Certainly

the genotype is no different. The difference in

this case must be due to different interactions be-

tween over-all gene action and environment (31).

Jn Drosophila paeudoobscura both of th e two

kinds of heterosis ar e reasonably well known and

understood. Firs t, there is the presence of deleter-

ious recessive mutant gene being sheltered by

their normal dominant alleles in a population.

Accumulation of these deleterious genes is a by-

product of the muta tion process.

Secondly, there

are

complexes of linked polygenes which give spe-

cific heterotic interaction effects in heterozy-

gotes (overdominance). This kind of heterosis is en-

gendered by natura l selection and a form of

ada pta tio n of species to its environment

(8).

Hayman said that heterosis is a

composite

phenomena: possible causes are epistasis. over-

dominance, and accumulation of favorable domin-

ants in the heterozygotes (14).

Many factors may influence hybrid vigor.

Genes that have no dominance may be the ones

that have a major effect on heterosis.

Then

chromosomal deletions may occur which is

complete elimination of a normal locus.

T

are also dominant unfavorable genes pre

In the homozygote, they are not completely le

but seldom produce seed or pollen.

In t he h

ozygote there is a marked reduction in

growth, and reproduction as compared wi th e

parent (18).

In choromosomal resrrangements, suc

inversions and translocations. genes without

ation are placed in different spatial relations

other genes. In altered position they have d

ent effects 18). Dobzhansky and Rhodes in

suggested a method of locating genes by para

tric inversions. In plant s heterozygous for

inversions. the crossovers that occur within

inverted segments are only rarely recovere

viable gametes. Thus. the inve rted segme

inherited as a urlit and all genes contained w

the segment are completely linked with very

exceptions. In plants without inversions. w

crossing over breaks up favorable combina

of minor genes. the existance of them is diff

to demonstrate. Thus. if the same characteri

show up in all followinq generations. then i

be assumed that these characteristics were loc

in the inverted section. Sprague, 1941. used

method and found that plants with heterozy

inverted segments were superior to homozy

inverted segment in yield and kernel weigh

the difference as significant. Chao, study in

bred lines of corn by this method, found tha t g

increasing ear height in most of these inbred

were present in chromosome 6 of the ln3 a s

and in the long arm of chromosome 3. These g

did not show overdominance (5).

Loci

are

known that have different effec

the different parts of an organism. These g

may be so close together that they never

crossing over or they may be compound g

with multiple effects. That a chromosome

linear arran gement of loci. each of which is a

of a single gene with a one-effect-function.

over-simplification. There are also single g

with multiple effects. This has been proven

especially tru e in growth production illustrate

chlorophyll formation (18). However the dev

ment of cholorophyll is affected by man y g

all, in this case. involved in the developme

.this heitable characte r. Each recesive a

when homozygous allows formation of par ti

no pigmentation resulting in an albino.

generally believed that the majority. if no

genes for albinism, a f f ~ c t different step

chlorophyll production. Thus if one step fai

go to completion. albinism results (16).

It has also been suggested that heterosis

be the result of t he interaction between genes

cytoplasm. Within species, differences in rec

cal crosses are rare; however, in two diff

flowering types of tobacco, crosses show a m

nal effect (18).

A

I?

Shull preferred

explanation that heterosia was due to a stim

resulting from a changed nucleus on a relat

unaltered cytoplasm.

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Whaley reported that heterosis is associated

with the ability of the hybrid to synthesize or

to utilize one or several specific substances involv-

ed in fundam ental growth processes of organ-

isms. Much evidence indicates that primary

heterosis effect is concerned with growt h sub-

stances whose predominant activity is registered

in the earl y part of the development cycle. Many

hybrid plants gain their advantage a few hours

after germination. The primary gro wth activities

at thi s stage involves the unfolding of t he

enzymatic pattern. Here the hybrid has its ad-

vantage (32). Gartner,et

al

(1953), found that the

degree of heterosis in the experimental plants of

A n i i r r h i n u m majus L. (snapdragon) was greatly

influenced by the amount of solar radiation.

Heterotic ability of the F1 to retain the indol-

acetic acid has been demonstrated. The greater

abili ty of the hybrids to retain and utilize growth

substance under high light conditions permits

greate r expansion of plant tissue and t hus gives

the additional growth increment that can cumu-

latively result in heterosis

(1

1).

Cases are reported where a deleterious re-

cessive gene caused the heterozygote to be no

bette r than the homozygous normal. These genes

are lethal or nearly lethal. They are all unfavor-

able

(17).

Sternei al, 26), tested the viability

of Drosphila melanogasfer for 75 sex linked recess-

ive lethals. It was found tha t on the average the

viabi lity of the heterozygote for lethals was .965.

a decrease in viability of approximately

4

percent

(27).

By way of a summary, it can be said tha t

heterosis can involve only recombinations of al-

ready existing alleles, unless by rare chance

mutation occurs. We are thus concerned with an

interpreta tion limited to diffe rent types of re-

combinations. and to different kinds of gene action

resul ting from these recombinations (32). The

basic principle of all explanations is tha t different

gene combinations from the same restricted pool

of genes will give many different phenotypes (30).

In conclusion, heterosis is apparently not due to

any single genetic cause.

1

Beadle, G. W.. 1953, Heterosis, Journa l of Here -

dity, 44:88

2. Bonnier, G., 1961 Esperi ments on Hybr id Super-

iori ty in Drosophila melanogaster: Egg Layi ng

Capacity and Larval Survival, Genetics. 46:9-24

3.

Brieger, F. G., 1950, Genetic Basic of Hete rosis in

blaize, Genetics, 35: 420-45.

4.

Cham bers , D. and Whatley, J. A., 1951, Heterosis in

Crosses of Inbred Lines of Duroc Swine,

Jou rna l of Animal Science, 10:505-15.

5.

Chao, C. Y., 1959, Heterotic Effects of a Chromoso-

mal Segment in Maize, Genetics. 44:657-77.

6. Crow, J. F., 1948, Alternative Hypothesis of Hy-

brid Vigor. Genetics. 33:477-87.

7. Crow, Jam es F., Dominance and Overdominance,

edited by John W Gowen, 1952 Heterosis. Iowa

Sta te College Press, Arnes. Iowa.

8. Dobzhansky, T.. 1950, Genetics of National Pop-

ulations: Orgin of Heterosis Through Natural

Selection in Population of Drosophila pseudoob-

scura, Genetics, 35:288-302.

9. Dobzhansky,

T.

H., Natu re and Orgi n of Heteros

edited by John W. Gowen, 1952, Heterosis, Io

Statc College Press Ames Iowa.

10

England. D. C. and

L M.

Winters , 1953, Effects

Genetic Diversity and Performance of Inb

Lines per se on Hybrid Vigor in Swin

Journal of Animal Science, 12: 836-47.

11. Gartner,

J. B.,

and others , 1953, Effect of Indo

acetic Acid and Amount of Solar Radiation

Heterosis in the Snapdragon, Science. 117:5

5.

12. Go\r.en, Jo hn W., Hybr id Vigor in Drosophil

edited by John

W.

Gowen, 1952, Hetero

Iowa State College Press, Ames. Iowa.

13. Hayes. H. K., Development of th e Heterosis C

cept, edited by Jo hn W. Gowen, 1952, Hetero

Iowa Sta te College Press, Arne.. Iowa .

14. Hayman,

R.

L., 1957 Interaction, Hetcrosis, A

Diallel Crosses. Genetics. 42:336-55

15. Henderson,

M.

T., 1949. Consideration of

Genetic Explanations of Heterosis, Agrono

Journa l, 41 :123-6.

1G. Irwin.

M.

R. Specificity of Gene Effects, edi

by Joh n W. Gowen, 1952. Heterosis, Iowa S t

College Press, Ames, Iowa.

17. Jones, D. F.. 1945, Heterosis Resu lting fr

Degenera tive Changes. Genetics., 30527-42

18. Jones, Donald

F.,

Plasmagenes and Chromoge

in Heterosis, edited by Jo hn W. Gowen, 19

Heterosis, Iowa State College Press, Ames, Iow

19. Jones,

D. F.,

1957, Gene Action i n Heteros

Genetics, 42:92-103.

20. Leng, E.

R.

1954, Effects of Heterosis on th e Ma

Components of Grain Yield in Corn, Agrono

Journal, 46:502-506.

21. Rickey, F. D.. 1945, Bruce's Explanation of Hyb

Vigor, Journa l of Hered ity , 36:243-4.

22. Shu ll, G.

H.,

1948, Wha t is Heterosis? Genet

33:439-46.

23. Shul l, George Harrison, Beginning of th e Hetero

Concept, edited by John W. Gowe n, 1

Heterosis. Iowa Sta te College Press, Ames, Io

24.

Sierlr, C.

F.

and L. &I. Winters . 1951, Study

Heterosis in Swine, Journ al of Ani mal Scien

10:104-11.

25. Sinno tt, Edmund W.. Dunn, L. C.. and Dobzhans

Theodosius, 1958. Principles of Genetics, M

Graw

-

Hill Book Company, Inc., New Yo

Chapter 19, Pages 254-268.

26. Stern, C. 1948, Negative Heterosis and Decrea

Effectiveness of Alleles in Heterozygote

Genetics, 33:215-19.

27. Ste rn, C., an d other s. 1952, Viability of He te

zygotes for Lethals. Genetics, 37:413-49.

78 Strauss,

F.

S., and J. W. Gowen, 1943, Hetero

Its Mechanism in Terms of Chromosmes Un

in Egg Production of Drosophila melanogasr

Abstract. Genetics, 28:93.

9. Tantaway, A 0 1957, Heterosis and Gene

Variance in Hybrids Between

Inbred Lines

Drosophila melanogaster. in Relation to the Level

Homozygosity, Genetics, 42535-43.

30. Wagner, Robert P., and Mitchell, Herschell. 1

Genetics and Metabolism. Joh n Wiley So

Inc., New york.

31.

Wellhausen,

E.

J., Heterosis in a New Populatio

edited by John W. Gowen, 1952, Heterosis, Io

State College Press, Ames, Iowa.

32. Whaley, W. Gordon,

Physiology of Gen e Act

in Hybrids, e&ted by John W. Gowen, 1

Heterosis. Iowa S tat e College Press , Ames, Io

33. Zirkle, Conway, Early Ideas on Inbreeding

Crossbreeding, edited by John W. Gowen, 1

Heierozis, Iowa State College Press. Ames, Io

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Genetic Basis of Heterosis