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IX. Genetic Control of Hybrid Vigor in Sorghum

IX. Genetic Control of Hybrid Vigor in Sorghum

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GENETIC CONTROL OF FLOWERING AND GROWTH I N SORGHUM



147



has been due to the fact that no mechanism to control hormone levels

had been recognized. If such a mechanism has now been recognized, there

is reason to revise some of the theories that are the basis of practices in

plant breeding. The theory that varieties of self-pollinated crops such as

sorghum are burdened with numerous, small metabolic deficiencies is now

untenable.

Hageman et al. (1967) has postulated that hybrids are superior to parents in having better balanced metabolic systems. They suggested that the

fundamental metabolic systems involved in growth and yield needed to

be recognized and, particularly, the optimum levels of activity of each enzyme. The idea that the genetic control over growth is hormonal is probably not in conflict with their concept and they have recognized that “a

single enzyme, hormone, vitamin, or growth factor could be solely responsible for the enhanced growth of a hybrid.” Nevertheless, they have stated

that “the complexities of metabolism preclude a single factor from being

the universal underlying cause of hybrid vigor.”



A. EFFECT

OF HETEROZYGOSITY

AT MATURITY

LOCI

Data in Table I11 (Quinby and Karper, 1946) show that plants heterozygous at locus 1 when locus 2 is homozygous recessive were later to

flower than either homozygous genotype. But when locus 2 was homozygous dominant, the genotype heterozygous at locus 1 was earlier to

flower than the later homozygous genotype. The heterozygous genotype

Malmalma2ma, was much later to flower than either homozygous genotype

and produced a much greater yield of heads. However, the difference in

duration of growth between genotypes was great and the influence of duration of growth and of hybrid vigor cannot be separated. However, the

heterozygous genotype Ma,maIMa,Ma, was only 3 days earlier to flower

than the homozygous genotype MalMalMa,Ma, but produced a yield of

heads 60% greater due largely to more heads per plant. Heterozygous

genotypes for maturity were different from homozygous genotypes in both

maturity and yield of heads and heterozygosity was important largely because of interaction among genes at different loci rather than between alleles within a heterozygous locus.

Differences, due to gene interaction, between pairs of hybrids that differ

only in being homozygous or heterozygous at one locus are shown in Table

IV. These data were presented previously (Quinby and Karper, 1948),

but, at that time, the maturity genotypes of most of the parents were not

known. Now that the maturity genotypes are known, it is possible to draw

conclusions regarding the genetic cause of hybrid vigor that could not be

drawn in 1948.



148



J. R. QUINBY



TABLE 111

Effect of Heterozygosity at the ma1 Locus on Days to Flower and Yield of Heads ‘ , b



Class



Genotype



Days to

flower



Head

weight

per plant

(g)



Planted Sune 20, 1944

Homozygous (SM60)

Difference

Heterozygous

Difference

Homozygous (60M)



ma,malma?ma~maamaa

Ma4Mac



51

32**

13**

70



94

55**

149

19**

130



50

43**

93

3*

96



91

149**

240

90 *

150



83



Ma1 Malmazmazrnaama3Ma4 Mac

Planted June 9, 1942



Homozygous (SM90)

Difference

Heterozygous

Difference

Homozygous (9OM)



malmalMa~Ma~ma3maaMa,Ma4

M a l m a lMazMazma3maaM a rM a 4

Ma1MalMa?Mazma3maaM a 4 M a 4



Data from Quinby and Kaper (1946).

Plant populations were grown at Chillicothe, Texas.



It is assumed that the paired varieties listed in Table IV differ only in

the genes controlling maturity. Many plant breeders or geneticists, including Schuler (1954), are skeptical that two varieties could differ only at

one or two gene loci. Such skepticism is difficult to allay because the skepticism originates in the misconception that so-called quantitative characters

are, necessarily, complex in inheritance.

Part of the data from the 1948 paper are presented again in Table IV

with the yield figures converted to grams. Only the data from pairs of hybrids that differ in one allele are presented. Quinby (1967) has presented

evidence that multiple allelic series exist at the maturity loci; but, because

the information has no bearing on the point under discussion, the multiple

allelic designations are not shown.

Heterozygous Masmas in the HEGARI x TEXAS MILO hybrid as compared

to homozygous recessive ma3ma3in the EARLY HEGARI X TEXAS MILO hybrid resulted in little difference in days to flower but in a large difference

in grain yield. The same was true in the HEGARI x SOONER MILO and

EARLY HEGARI X SOONER MILO hybrids. Heterozygous Masmas as compared to homozygous dominant Ma,Ma, in TEXAS BLACKHULL

KAFIR x EARLY HEGARI

and TEXAS BLACKHULL KAFIR x HEGARI

pair of hybrids resulted in earlier flowering and greater grain yield. TEXAS



TABLE IV

Maturity Genotype, Days to Flower, and Grain Yield of Parents and Hybridso.b



Variety or hybrid



Days to

flower



Genotype



Grain yield

per plant

(g)



%

'

Higher



SOONER

MILO

T E X A S MILO



EARLYHEGARI

HEGARI

EARLYKALO

KALO



HEGARI

x



TEXAS MILO



EARLYHEGARI X

HEG.AR1



x



TEXAS M I L O



n



S O O N E R MILO



EARLYHEGAHI



x SOONER



TEXAS

BL.ACKHT-LL

TEXAS

BLACKHULL



KAFIR



TEXAS

BLACKHULL

TEXASB L A C K H U L L



KAFIR



KAFIR



KAFIR



MILO



x E A R L Y IIEGARI

x HEGARI

x KALO

x E A R L Y K.4LO



E

3lalma1-lla2Ma?Ma3ma~M

a4ma4

.Ifa ~ r n a ~ M a 2a2Ma3Ma3&1

M

a4ma4



m a ~ m a ~ M a ~ m a da&

f a ~a&

M



a4



malma1M a2Ma2Af aaMa3Ma4Ma4



Data from Quinby and Karper (1948).



* Plant populations were grown at Chillicothe, Texas, in 1941.

Significantly greater than yield of other member of pair at 0.01 level.



98

104



l?4c

94



3?



57

56



130c

96



37



1



z



3

CL



P

W



150



J. R. QUINBY



BLACKHULL KAFIR x KALO and TEXAS BLACKHULL KAFIR x EARLY KALO

hybrids are similar in maturity to the commercial hybrids in general use.

Heterozygous Mu2mu,, in the former hybrid as compared to homozygous

Mu2Mu2 in the latter caused a 37% increase in grain yield even though

the days to flower of the two hybrids differed by only one day.



B.



EFFECTOF HETEROZYGOSITY

AT O N E HEIGHT

LOCUS



Graham and Lessman (1968) presented data at the 1968 meeting of

the Crop Science Society that showed hybrid vigor due to heterozygosity

at the dw, height locus in sorghum. This hybrid vigor was not the point

of their presentation and the abstract contains no reference to the yield

of the plants heterozygous for height.

The 2-dwarf parent used in their crosses was TEXAS MILO and the

3-dwarf parent was CALIFORNIA 38 MILO. Both parents are

Mulma2mu3Ma4for maturity but CALIFORNIA 38 MILO is recessive dw,

whereas TEXAS MILO is dominant Dw,,for height. Because both are MILO

varieties, they are in similar genetic backgrounds.

The pertinent data are shown in Table V. Plants heterozygous Dwz dw?

produced more grain than plants of either homozygous parent.

TABLE V

Effect of Heterozygosity at the diup Height Locus

I N MII.Oa*h



Entry

%dwarf X %dwarf, FI

2-dwarf X 3-dwarf, FI

%-dwarfparent (dwIDw2!hJ3d21)4)

3-dwarf parent (dw~dwzDiuaDwa)



Gtain

yield per

plant

(9)

241c

238"

228

197



Data from Graham and Lessman (1968).

Plant populations were grown at Lafayette, Indiana, in 1963 and 1965.

Significantly above either parent at 0.01 level.

a



C. DISCUSSION

OF EFFECTS

OF HETEROZGOSITY

The information presented leads to the conclusion that heterozygosity

at one height or one maturity locus, due to interaction between loci or

epistasis, results in greater grain yield; and this is true regardless of the

dominant or recessive condition at the homozygous locus.



GENETIC CONTROL OF FLOWERING AND GROWTH I N SORGHUM



151



Assuming that maturity genes control hormone levels, it appears that

heterozygous genotypes, due to gene interaction, produce levels of auxin

and gibberellin that are different from the levels produced by homozygous

genotypes. Some heterozygous combinations apparently produce levels of

hormones more favorable to growth than any homozygous combination,

but some heterozygous combinations would produce hormone levels more

favorable than others. Quinby (1963) presented data from two high- and

one low-yielding hybrid. Patanothai and Atkins ( 1971 ) presented growth

curves from two hybrids showing medium and high heterosis, but a growth

curve for the low heterosis hybrid was not shown because that hybrid was

not significantly superior, in the characters measured, to midparental

values.



X.



Sorgnum Genotypes as Experimental Subjects



Sorghum hybrids came into use in 1957 after a female parent was produced using cytoplasmic male-sterility (Stephens and Holland, 1954).

Prior to that time, farmers grew true-breeding varieties. Plant breeders,

for about 40 years, worked at producing improved varieties. Because

farmers saved mutations and plant breeders were interested in genetics,

the sorghum species now includes a number of varieties or genotypes that

are useful experimental subjects. If the hypotheses presented here are taken

seriously, they will need to be confirmed or refuted. Strains will be identified in this section that are in similar genetic backgrounds, but differ at

only one or two loci that affect growth. Physiologists working on the flowering process have, for the most part, neglected to use varieties, and have

thus been working without a check.

A.



THEMILOMATURITYGENOTYPES



A tropical sorghum variety reached the United States in 1879 and was

called “MILLO MAIZE” (Karper and Quinby, 1947). In spite of its tall

height and late maturity, farmers grew the variety and by 1910 had selected

earlier maturities and short statures from the original variety. All the varieties that originated in this way are MILOS and differ from one another

only in maturity, height, or pericarp color. It was determined about 30

years ago that four mutations at maturity loci, two mutations at height

loci, one mutation for pericarp color, and one mutation for Periconia rootrot resistance had been preserved (Quinby, 1967).

In the process of studying the genetics of duration of growth, eight maturity genotypes were produced by selection from a cross between two va-



152



J . R. QUINBY



rieties of the genotypes Malmarrna3Ma, and malMa2Ma3Ma4.A linkage

between height and maturity was broken and the resulting eight genotypes

are all in the same genetic background and are recessive at three height

loci. When the eight genotypes are grown in long nights, they flower at

about the same time (Miller et al., 1968a); but in the short nights of temperate zones in the summer, they flower at different times as shown in

Table I. Subsequently, a second mutation at the third maturity locus was

found in a 3-dwarf MILO variety (Quinby and Karper, 1961) and named

RYER MILO. The new variety was identified for maturity as being

Malma2ma,RMa, and was given the designation of 4 4 M . 38M was then

obtained as a segregation product out of a cross between 44M and SM60.

The list of MILO maturity genotypes now includes the ten strains shown

in Table I.

B.



MATURITY

GENOTYPES

RECESSIVE

AT

LOCUS FOURFOR MATURITY



The only recessive known at locus 4 was identified in HEGARI. Because

all the MILO maturity genotypes are dominant at locus 4, 4-recessive maturity genotypes in pure MILO could not be obtained. But several useful

maturity genotypes using recesives from BLACKHULL KAFIR, MILO, and

HEGARI

have been produced. These genotypes are KMH-1

(Ma,ma,ma,ma, ), KMH-2 ( ma,mu,ma3ma,), MH-3 ( mulma2nza:,Rma4).

C. PAIRSOF VARIETIES

THAT DIFFER

AT ONE MATURITY

Locus

Fourteen pairs of genotypes that differ at only one maturity locus exist

.among the MILO maturity genotypes shown in Table I. In addition, KALO

and EARLY KALO differ at locus 2; and HEGARI and EARLY HEGARI at lOCUS

3. The origins of KALO and EARLY KALO and of HEGARI and EARLY HEGARI

have been presented previously (Quinby, 1967). Maturity loci 1 and 2

have been assumed here to control the synthesis of auxin and loci 3 and

4, the synthesis of gibberellin. These pairs of genotypes might be used to

verify or refute these assumptions.



D. TEMPERATE

AND TROPICAL

PAIRSOF



VARIETIES



In an effort to make tropical germplasm readily available to plant breeders of sorghum in temperate zones, more than 100 tropical varieties have

now been converted to temperate zone adaptation. Most tropical varieties

are dominant at all four maturity loci and the conversion to temperate



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IX. Genetic Control of Hybrid Vigor in Sorghum

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