2. Studies on sux.

 

In the following table the effects of dosage of genes and interaction between su1 and sux can be appreciated. At the top of each cell in the table, the genotypic constitution of the endosperm with respect to su1 and sux is stated. Second row indicates grain phenotypes. Percentage numbers in the third row are total contents corresponding to the sum of reducing and non-reducing sugars, expressed as inverted sugar, and calculated on a dry matter basis. These figures represent the average of two or more analyses and could be better substituted by the nearest round unit without loss of accuracy.

 

It is interesting to note in this table that in supersugary grains having the endosperm constitutions, sususu suxsuxsux, the sugar content is 25% as compared to 5% found in ordinary sugary. This very high sugar content in super-sugary can be perceived even during milling operations previous to the chemical analysis, because the ground product sticks to the mill, the flour obtained is not powdery but forms granulous aggregates, its color being darker than that of the flour from ordinary sugary.

 

Increase in sugar content is due to the interaction between su1 and sux, and not only to doses of alleles. This is shown by the complete dominance of each normal allele in absence of interaction of su1 with sux. Moreover, in the tetraploid from ordinary sugary, whose endosperm cells contain six doses of su1, the sugar content is 5%, just as much as in its own diploid whose endosperm cells contain four doses of su1.

 

On homozygous su1 background, as in those endosperm constitutions represented in the top row of the following table, sugar content raises in a marked progression with additional substitutions of sux doses, for its normal allele.

 

On homozygous sux background, as in those endosperm constitutions shown in the last column of the table, even a more striking progression in raising sugar content results from increasing su1 doses.


 

Doses
of su1

(su1) (sux)

(su1) (sux)

(su1) (sux)

(su1) (sux)

Observations

 

 

 

 

 

 

 

(sususu)(+++)

(sususu)(++sux)

(sususu)(+suxsux)

(sususu)(suxsuxsux)
(super-sugary)

Endosperm

3

wrinkled

wrinkled

wrinkled

wrinkled

Phenotype

 

sugar = 5.1%

8.2%

15.9%

24.9%

Sugar content

 

 

 

 

 

 

 

(+susu)(+++)

(+susu)(++sux)

(+susu)(+suxsux)

(+susu)(suxsuxsux)

 

2

starchy

starchy

starchy

wrinkled

 

 

sugar = 2.2%

2.1%

3.1%

8.7%

 

 

 

 

 

 

 

 

(+ +su)(+++)

(+ +su)(++sux)

(+ +su)(+suxsux)

(+ +su)(suxsuxsux)

 

1

starchy

starchy

starchy

smooth sugary

 

 

sugar = 2.3%

2.0%

2.0%

4.8%

 

 

 

 

 

 

 

 

(+ + +)(+++)

(+ + +)(++sux)

(+ + +)(+suxsux)

(+ + +)(suxsuxsux)

 

0

starchy

starchy

starchy

opaque

 

 

sugar = 1.8%

1.9%

1.8%

3.2%

 

 

 

 

 

 

 

 

0

1

2

3

Doses of sux


It may be observed also that grains heterozygous for su1 are already neatly wrinkled (two doses su1) or smooth sugary (one dose su1). This fact is interpreted as an inversion of dominance; the ordinarily completely recessive wrinkled su1 becomes dominant over its starchy allele in grains which are homozygous for sux. The increased sugar content and wrinkledness, just described, will be referred to in this report as the "dosage interaction effect". This year we shall have tetraploid seeds homozygous for sux and super-sugary in enough quantities for chemical analysis.

 

Unfortunately, such a high amount of sugar has not been corroborated in other super-sugary stocks. Some of them have gone down to 15% or even lower. But this is due, partly at least, to the fact that super-sugary grains deteriorate very easily before attaining complete maturity. This deterioration is caused by bacterial and fungous attacks that partially liquify its endosperm, destroy sugars and even seriously affect the embryo's vitality. In order to obtain viable seeds, super-sugary ears must be harvested not later than 30 days after fertilization and immediately dried in the shortest time. Sometimes grains, outwardly normal, have already lost their germinating ability and have tasteless endosperm.

 

In crosses of su1 with sux, the F1 grains are starchy and the F2 must be theoretically as follows:

 

9: starchy

 

1: sux (opague, with a slight roughness, appreciated better under the incidence of light over the grain's surface)

 

1: (+ + su)(suxsuxsux) (smooth sugary that Dr. Shafer described in the 1946 Corn Letter as: "Dented and translucent, but not wrinkled".)

 

5: (wrinkled including four ordinary sugary and one supersugary) We join these two classes because it is not easy to distinguish them from one another.

 

In some of our stocks Ga of chromosome 4 is present, and, of course, it must alter the normal ratios in segregating cultures. Nevertheless, this alone does not seem to account for the extra­ordinary ratios reported by Dr. Shafer in the 1946 Corn Letter, neither for the ones we ourselves obtained in some crosses, as those shown in the next table. Items 1 and 2 in the table are the recipro­cal backcrosses to su1 involving the same two individual plants; at the same time, they are the F2 with respect to sux. Owing to the type of interaction between su1 and sux, we have to bear in mind that there is the possibility of mistaking the genotype (+/su) with (su/su); each one might be attributed to a given wrinkled grain before the progeny test is made. Item 3 is a backcross to sux and at the same time is the F2 for su1. The mother F1 plant, 45.6013-3, having the genotype (+/su,sux/sux) was produced by a wrinkled grain.


Item

Crossed plants

 

 

Progeny

 

Female

Male

 

 

Starchy

Opaque (sux)

Smooth sugary

Wrinkled

Totals

 

 

 

 

 

 

 

 

 

 

1

45.6013-2

45.6012-6

Obs. numbers

 

52

12

0

71

135

 

wrinkled x

starchy

Obs. ratio

 

3.08

0.71

0

4.21

8

 

su +/sux

+/su+/sux

Theoretical ratio

n.i.

3

1

0

4

8

 

 

 

 

i.

3

0

0

5

8

 

 

 

 

 

 

 

 

 

 

2

45.6012-6

45.6013-2

Obs. numbers

 

51

8

7

60

126

 

starchy x

wrinkled

Obs. ratio

 

3.24

0.51

0.44

3.81

8

 

+/su+/Sux

su +/sux

Theoretical ratio

n.i.

3

1

0

4

8

 

 

 

 

i.

3

0

1

4

8

 

 

 

 

 

 

 

 

 

 

3

45.6013-3

45.6012-2

Obs. numbers

 

152

12

26

143

333

 

wrinkled x

starchy

Obs. ratio

 

3.65

0.29

0.62

3.44

8

 

+/su sux

+/su +/sux

Theoretical ratio

n.i.

3

3

0

2

8

 

 

 

 

i.

3

1

1

3

8

 

 

 

 

 

 

 

 

 

 

Theoretical ratio,

n.i. =

without "dosage interaction effect"

 

 

 

 

 

 

i. =

with "dosage interaction effect".

 

 

 

 

 


The "opaque" grains observed in item 1 have presumably the (+susu)(suxsuxsux) constitution and, according to the "dosage inter­action" hypothesis, they should be wrinkled. With no dosage inter­action effect these grains should be opaque, as actually was observed in this case.

 

The smooth sugary grains in item 2, according to dosage interaction hypothesis, should have the constitution, (++su)(suxsuxsux), and they should appear in one eighth of the population. In o observed ratio, smooth sugary appears in half that frequency, the other half being presumably changed into the opaque class. The smooth sugary class can only appear as a result of dosage interaction effect.

 

Comparing the ratio observed in item 3 with the ratio cal­culated according to dosage interaction hypothesis, it seems that part of what should be the "opaque" class (+++ suxsuxsux) actually appears as starchy; and part of what should be the smooth sugary class (++su)(suxsuxsux) is changed into wrinkled.

 

The smooth sugary class with (++su)(suxsuxsux) endosperm constitution is especially useful in breeding for high sugar content, when it is desired to transfer sux to ordinary   sugary stocks. The F2 smooth sugary grains are chosen which certainly will be homozy­gous sux with no danger of losing su1 (heterozygous). The progeny of selfed plants from smooth sugary grains produces only opaque, smooth sugary and wrinkled grains. Fifty per cent of the latter are super-sugary and the other 50 per cent will segregate su1. The super-sugary with (sususu)(suxsuxsux) endosperm are those among the wrinkled grains, whose progeny produces wrinkled ones only.

 

The previously mentioned complicated segregations in certain stocks do not invalidate the practical usefulness of smooth sugary grains in selection, as outlined above. The use of smooth sugary in F2 simplifies the procedure of selection, avoiding to resort to cross tests in order to detect and preserve sux in wrinkled grains.

 

S. Horovitz

A. H. Marchioni

H. G. Fisher