The male sterile stock reported herein was derived from interspecific cross, such that considerable part of S. montanum germ plasm had been placed in the cytoplasm of S. cereale. Furthermore, pollen sterility difference in reciprocal crosses was apparent. It seemed reasonable to hypothesize, therefore, that the male sterility expression resulted from the interaction of cereale cytoplasm with nuclear genes of montanum.

In order to test this assumption it was necessary at first to have male fertile clones with known genotypes in both cereale and montanum cytoplasm. The determination of genetic constitution of the clones in question was initiated by crossing MS 70/8-1 x MF 70/7-1. Two of the male fertile F₁ segregates from this cross were then intercrossed and simultaneously crossed back to the sterile parent. All the sibcross and testcross progenies segregated into male fertile and male sterile plants in ratios expected for monogenic inheritance (Table 1). It was concluded that the MS 70/8-1 clone behaved as single gene recessive and that male fertility was restored in the MF 70/7-1 clone by a single dominant allele. Three male fertile clones of S. montanum, designated KH66-30-2, KH66-30-4 and KH66-30-5 were also crossed to MS 70/8-1 in an effort to determine their genotypes with respect to the male sterility locus. Two appeared to be heterozygous, and one appeared to be homozygous recessive (Table 2).

The test of cytoplasmic-genic interaction associated with the male sterility was provided by 1) crossing reciprocally the MF 70/7-1 clone with KH66-30-2, KH66-30-4 and KH66-30-5 clones, 2) intercrossing the KH66-30-2, KH66-30-4 and KH66-30-5 clones, and 3) scoring the resulting progenies for the presence or absence of male sterility. The cytoplasmic effect hypothesis required only the progenies of crosses involving MF 70/7-1 as the female parent to be segregating for male fertile and male sterile plants, and progenies of the other crosses to be entirely male fertile. As is shown in Table 3, the actual observations were in accord with the expectations of the assumption tested.

The necessary conclusions from these results are that S. cereale has a cytoplasm capable of inducing male sterility to certain S. montanum genotypes and that the sterilizability of the genotypes is due to a single recessive gene which is without apparent effect on sterility in montanum cytoplasm. Any alternative explanation possible seems to be less reliable. It should perhaps be mentioned here that S. kuprijanovi also contains the recessive gene(s) causing male sterility when homozygous in cereale cytoplasm.

Much is yet to be learned about the genetics of fertility restoration in cereale cytoplasm. Preliminary evidence indicates, however, that the major genes for fertility restoration are dominant and the number of the genes probably is relatively few. It seems reasonable to assume that, at least in some instances, complete pollen fertility may be due to a single dominant gene, plus modifiers. It appears that modifiers can affect the degree of fertility restoration, for none of the segregating progenies studied was completely free of nearly sterile and nearly fertile segregates. The possibility that there are more than two alleles at the Rf locus is not excluded, however.

The potential hindrances to using the (cereale) - S. montanum source of cytoplasmic male sterility in a program for commercial production of hybrid rye varieties are apparent. However, one should not dismiss the possibility of practical use of this stock on the basis of theoretical consideration alone.

Summary

A case of cytoplasmic pollen sterility in wild rye is reported. The male sterility expression is due to the interaction of homozygous recessive gene(s) from S. montanum with the cytoplasm of S. cereale. Apparently this is the first published report of difference between cytoplasm of Secale species with regard to their effect on pollen sterility.

Literature cited

GEIGER, H. H. and F. W. SCHNELL 1970. Cytoplasmic male sterility in rye (Secale cereale L.). Crop Sci. 10: 590-593.

PUTT, E. D. 1954. Cytogenetic studies of sterility in rye. Canad. J. Agric. Sci. 34: 81-119.

ZDRILKO, A. F. 1969. Studies of cytoplasmic male sterility in rye (In Russian). Selekcija i Semenovodstvo, Kijev 14: 31-33.

(Received March 18, 1972)