2. Studies on su_x.

In the following table the effects of dosage of genes and interaction between su₁ and su_x can be appreciated. At the top of each cell in the table, the genotypic constitution of the endosperm with respect to su₁ and su_x 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 su_xsu_xsu_x, 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 su_x, 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 su_x. 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 su_x doses, for its normal allele.

On homozygous su_x 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 su_x. 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 su_x 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 su_x, the F₁ grains are starchy and the F₂ must be theoretically as follows:

9: starchy

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

1: (+ + su)(su_xsu_xsu_x) (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 su_x. Owing to the type of interaction between su₁ and su_x, 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 su_x and at the same time is the F₂ for su₁. The mother F₁ plant, 45.6013-3, having the genotype (+/su,su_x/su_x) 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)(su_xsu_xsu_x) 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)(su_xsu_xsu_x), 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 (+++ su_xsu_xsu_x) actually appears as starchy; and part of what should be the smooth sugary class (++su)(su_xsu_xsu_x) is changed into wrinkled.

The smooth sugary class with (++su)(su_xsu_xsu_x) endosperm constitution is especially useful in breeding for high sugar content, when it is desired to transfer su_x to ordinary sugary stocks. The F₂ smooth sugary grains are chosen which certainly will be homozygous su_x 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)(su_xsu_xsu_x) 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 su_x in wrinkled grains.

S. Horovitz

A. H. Marchioni

H. G. Fisher