(Department of Botany and Plant Pathology)

Non-reciprocal cross-sterility in the popcorns.

In many intervarietal crosses between popcorns there is little or no seed set when the cross is made one way while the reciprocal cross shows a normal set of seed. The inability of South American inbreds to set seed when pollinated by Supergold inbreds has long been known as has the incompatibility of White Rice and Hulless with dent corns.

Our investigations during the summer of 1948 have shown that the phenomenon is widespread. A crossing block was set up with nine popcorn inbreds of diverse origins and one dent corn inbred. All the inbreds had been inbred for at least three years and some were inbreds of long standing. Within this crossing block all possible combinations of crosses were made reciprocally and the resultant ears were scored for sterility. Generally the results were clear-cut with either a full seed set or else a nearly barren cob with none to a few scattered seeds. Table 1 summarizes the reactions with + denoting a full set and - denoting incompatibility.

The point to be noted is that the inbreds fall into three groups: (I) those that will not pollinate one or more inbreds (Hy, S.G. 18 and 1708-4), (II) those that will not set seed when pollinated by one or more inbreds (S.A. 24, 1001-52, 4541-U, 845-1B and 4524-4), and (III) those that do not participate in incompatibility reactions as either male or female parent (A3-1 and 4519-41). Further, there is apparently no overlapping between groups. No inbred has yet been observed to function in one incompatibility reaction as a female and in another as a male parent.

These data suggest that cross-sterility is conditioned by the interaction of genic components of the first group with genic components of the second when inbreds of the first group are used as male parents. An alternative hypothesis would be the interaction of a gene or genes from Group I with the cytoplasm of Group II. However, the fact that the F₂'s of crosses of a Group I inbred times a Group II inbred segregate for sterility to Group I pollen appears to obviate this possibility.

Table 1. Sterility test - 1948

	Dent	Supergold	Baby Golden	South American	Hulless	Black Beauty	Ohio Yellow	Red	Amber Pearl	White Rice
Male	Hy	S. G. 18	1708-4	S. A. 24	1001-52	4541-U	845-1B	4524-4	A3-1	4519-41
Female
Hy	+	+	+	+	+	+	+	+	+	+
S. G. 18	+	+	+	+	+	+	+	+	+	+
1708-4	+	+	+	+	+	+	+	+	+	+
S. A. 24	-	-	+	+	+	+	+	+	+	+
1001-52	-	-	?	+	+	+	+	+	+	+
4541-U	-	-	?	+	+	+	+	+	+	+
845-1B	-	-	-	+	+	+	+	+	+	+
4524-4	-	-	+	+	+	+	+	+	+	+
A3-1	+	+	+	+	+	+	+	+	+	+
4519-41	+	+	+	+	+	+	+	+	+	+

Of Group I, S. G. 18 and Hy acted in an identical manner while 1708-4 would not induce setting in 845-1B and doubtfully in 1001-52 and 4541-U. In Group II, S. A. 24 and 4524-4 acted alike in not setting seed with S. G. 18 and Hy while 845-1B and possibly 1001-52 and 4541-U also were sterile with 1708-4. All Group II inbreds show a common inability to set seed when S. G. 18 and Hy are used as male parents with the further possibility that they may be sterile to 1708-4.

In 1947 ten Hulless inbreds were tested with six different dent corn inbreds and hybrids (H21, Ind. 210, Tr, H5, C 103, and Hy x Wf). Nine lines gave only sterile reactions while the tenth was perfectly fertile. The dent lines tested appear to carry uniformly the factor or factors giving an interaction with most Hulless lines.

In all cases investigated the inability of Group I inbreds to induce setting in Group II stocks and the failure of Group II inbreds to set with pollen of Group I seem to act as recessives in crosses with inbreds not of their own group. All crosses (without reference to which parent was used as a female) of Group I inbreds with Group II inbreds will give normal seed sets when pollinated by Group I and pollen from the same crosses appears to induce normal seed sets in Group II inbreds. In this case, however, without linked markers it cannot be shown whether or not all gametes are functional on Group II stigmas.

With regard to the genetic constitution of Group I it can be shown that the inability of another Supergold line (S. G. 16) to pollinate S. A. 24 is due to a single recessive factor. The F₁ (S. G. 16 x S. A. 24) will give full seed sets when used to pollinate S. A. 24 although as mentioned above it cannot be shown that all gametes are functional. When the F₁ is backcrossed to S. G. 16 approximately one-half of the backcross progeny will show a sterility reaction when used to pollinate S. A. 24 while the other half give normal pollinations. This is shown in table 2.

Table 2.

Pedigree			Ability to Pollinate S. A. Plants
(S. G. 16 x S.A. 24) x S. G. 16
6358	-	2, 3, 4, 5, 7, 10, 12, 15	+	(9)
	-	6, 9, 11, 13	-	(4)
6852	-	2, 3, 4, 5, 11, 12, 14, 16	+	(8)
	-	6, 7, 8, 9, 10, 13, 15, 17	-	(8)
S. G. 16 x (S. G. 16 x S. A. 24)
6359	-	1, 4, 6, 7, 9, 14, 15	+	(7)
	-	2, 3, 5, 10, 11, 12, 13	-	(7)
6381	-	1, 2, 7, 9, 10, 11, 15	+	(7)
	-	3, 4, 5, 6, 8, 12, 14, 16, 17	-	(9)
Total			+	31
			-	28

The genetic constitution of Group II inbreds has also been investigated. In the summer of 1948 a detasseling plot in which the male parents were the dent corn hybrids Ind. 813 and 210 was set up. The female rows were reciprocal crosses of an S₂ Hulless inbred with an open-pollinated South American stock together with the F₂'s and reciprocal backcrosses to the South American stock and to the S₄ generation of the Hulless inbred. The original Hulless parent and the line used in backcrossing were also included in the test. The ears were scored for sterility at harvest. The sterility of the Group II varieties is apparently a plant character since, in general, the seed sets were either normal or almost entirely lacking without the semisterility that would be expected if it were an ovule factor. The original Hulless inbred was shown to be heterozygous for the factors conditioning sterility, since it segregated for normal and sterile plants while all the plants of the S₄ inbred were completely sterile. Analysis of the backcrosses and F₂ progeny has led to the tentative conclusion that there are three complementary genes conditioning the ability of the Hulless and South American stocks to set seed with dent corn pollen. The presence of the homozygous recessive alleles at any locus will result in sterility when pollinated by dent corn.

It is postulated that non-reciprocal cross-sterilities in the popcorns are due to the interaction of the genic components of the lines of popcorn (and dent corn) designated as Groups I and II. In at least one case it is known that the interaction between a Group I inbred and a Group II inbred is conditioned in the Group I inbred by a single recessive factor. In two Group II stocks it has been postulated that three complementary genes are operative and the presence of the homozygous recessive at any locus gives plants with a sterile reaction to Group I pollen. It is further suggested that these sterility mechanisms offer a solution to the problem of producing hybrid popcorn seed free from dent corn contamination. A backcrossing program is underway at Purdue to transfer sterility factors to dent corn pollen into the best Supergold inbreds.

Oliver E. Nelson, Jr.