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X. Sorghum Genotypes as Experimental Subjects

X. Sorghum Genotypes as Experimental Subjects

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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.







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


have been produced. These genotypes are KMH-1

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





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.





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



adaptation can be accomplished by substituting a recessive maturity allele

for a dominant one. The process is to cross a short, temperate variety to

the desirable tropical variety, to select short and early plants from the

segregating population, and to backcross to the original tropical variety.

The backcrossing is continued for four or five backcrosses or until the temperate, short-statured strain looks like the original tropical variety, except

for being short, when grown in the winter in Puerto Rico or Jamaica.

The Texas Agricultural Experiment Station and the U.S. Department

of Agriculture have distributed 62 converted lines and have several hundred more in some stage of conversion. The Pioneer Hi-Bred Company

also has numerous converted varieties. When selections are mzide from the

segregating population of the last backcrosses, it is possible to select lines

that are either dominant or recessive at the first maturity locus. The two

lines will look alike in the tropics in the winter but will differ in time of

flowering in temperate zones by 20 or 30 days. If any physiologist is interested in this kind of material, it would be possible, with a little advance

notice, to obtain a number of temperate and tropical pairs of varieties.

There are now converted varieties from low elevations in Nigeria and

from extremely high elevations in Ethiopia, where lowland varieties will

grow but not shed pollen. There are also varieties that are grown in the

summer in India and others that are grown in the winter. These varieties

should be ideal subject for certain studies of temperature effects.



Three height genotypes exist in MILO, all of which are of the maturity

genotype malMa2MaJMa4,The first is recessive at dw,; the second, at dwl

and dw,; and the third at dwl, dwL, and dw,. In addition, a number of

pairs of isogenic strains exist. The first member of each pair is a 3-dwarf

of the genotype dw,Dw,dw,dw,. The second member of each pair arose

as a tall mutation at locus 3 and has the genotype dw1Dw2Dw3dw4.

Early HEGARI and HEGARI are both DwldwLDw3dw4for height but the

former is unstable for height while the later is stable and produces no tall

mutations. Early HEGARI is Ma,Ma,ma3ma, for maturity while HEGARI is

MalMa2Majrna,.There is a possibility that recessive maj is, in some way,

associated with the unstable condition at height locus dw, in EARLY

HEGARI. Even though this might be due to an influence within a linkage

group, the linkage can be broken because KARPER (1953) distributed

HI-HEGARI, a forage hybrid of HEGARI maturity, that was selected from a

cross between HEGARI and a tall-mutant strain from EARLY HEGARI. Because of the instability, presumably at dwr, 2-dwarf and 1-dwarf height

genotypes exist in EARLY HEGARI.




Summary and Discussion of Genetic Control of Growth in Sorghum

If the control of growth is as simple as suggested in this chapter, there

is reason to wonder why the genetic control has been unrecognized for

so long. One may wonder, also, why the identity of the flowering stimulus

has remained so elusive.

The inhibitory effect of high as well as low levels of auxin might explain

the failure to hasten floral initiation with many species of plants. using applications of auxin to short-day plants growing in long days. These failures

have caused physiologists to conclude that endogenous auxins do not play

a central role in the process that leads to floral initiation. The fact that

applications of gibberellin to many long-day plants growing in short days

hastens floral initiation could indicate that an excess of auxin exists in such

plants. In such a case, additional auxin would not be expected to hasten

floral initiation. Short-day plants such as Xanthium, will initiate floral buds

following exposure to one long night and must need only a little auxin to

allow floral initiation. Such plants under short-night treatment should not

contain too much auxin; nevertheless, they do not respond to applications

of auxin. Perhaps the auxin level that promotes floral initiation in such

plants is so low that applications of auxin result in auxin levels high enough

to inhibit rather than promote floral initiation. The inhibition of both high

and low levels of auxin could explain why the floral stimulus was not

recognized long ago.

For 50 years plant breeders have been taught that inbred lines are burdened with numerous cryptic, recessive, deleterious genes. Assuming that

such deleterious genes prevent normal growth, how could the control of

growth be simple genetically? The notion about the complexity of the genetic control of plant growth spawned the development of the disciplines

of population and quantitative genetics. The interest in population genetics

diverted attention away from the obvious fact that a few genes such as

the maturity genes of sorghum have profound effects on duration of growth

and plant size.

Sorghum has many advantages as an experimental species. In the first

place, it is self-pollinated and inbred lines are vigorous. Only a few varieties were introduced to the United States about a century ago, and the

two most important ones were tropical varieties. The varieties were too

late and too tall to satisfy farmers who promptly selected shorter and

earlier maturing types that suited them better. As a result, a number of

dwarf and early varieties in similar genetic backgrounds originated.

In the 1930’s it seemed desirable to make a shorter SOONER MILO and

the tall EARLY MILO was crossed to DWARF YELLOW MILO. When F, rows

of this cross were grown, it became apparent that a linkage between tall



height and early maturity existed and that the inheritance of duration of

growth was relatively simple. Ultimately, the inheritance of maturity in

MILO was determined and three genes were recognized (Quinby and

Karper, 1945) .

The different maturity genotypes appeared to be the same size if grown

in 14-hours nights under which treatment they flowered at about the same

time. It was assumed, therefore, that the maturity genes, in some way,

controlled synthesis of a floral hormone that had no influence except on

time of floral initiation. At long last, an effort was made to see whether

the maturity genes of MILO might influence growth rates when the maturity

genotypes were grown in short nights and the genotypes had different times

of floral initiation and durations of growth. Contrary to expectation, maturity genes were found to influence growth rates (Quinby, 1972a). This

disclosure led me to the conclusion that the floral stimulus was probably

a combination of common hormones, and prompted the thinking that led

to the hypotheses presented in the previous pages and summarized in the

following paragraphs.

The genetic control of flowering in sorghum appears to be genetically

simple because only four gene loci have been recognized. The continuous

variation in flowering is thought to result from allelic series at the four

loci and because of complementary action between gene loci.

The floral stimulus appears to consist of auxin and gibberellin, and an

interaction between the two hormones produces the stimulus that changes

a vegetative bud into a fruiting bud. Auxin is produced largely during darkness, and gibberellin during daylight. Temperate varieties, because of less

inhibition by phytochrome, produce more auxin than tropical varieties during daylight. Early-flowering temperate varieties produce more auxin and

more gibberellin during daylight than late ones.

The PT3"form of phytochrome appears to inhibit synthesis of auxin

during daylight, and the PGGO

form to inhibit the synthesis of gibberellin

during darkness. Both forms of phytochrome are present in plants during

the day. Alleles at loci 1 and 2 are assumed to cause differences in sensitivity to inhibition by P,,",and alleles at loci 3 and 4 to cause differences

in sensitivity to inhibition by P,,,.

Dominant alleles at the maturity loci cause sensitivity and recessive alleles less sensitivity to ,inhibition by phytochrome. As a result, recessives

at loci 1 and 2 allow the synthesis of some auxin during daylight to supplement that produced in darkness. Recessives at loci 3 and 4 allow the synthesis of more gibberellin during daylight. The maturity genes appear,

through differences in sensitivity to inhibition, to regulate levels of auxin

and gibberellin that, in turn, control time of floral initiation and growth.

High, as well as low, levels of auxin inhibit floral initiation. Low levels

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X. Sorghum Genotypes as Experimental Subjects

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