Studies on self-incompatibility in 'Brassica napus'

Abstract

A detailed investigation was conducted on the self-incompatibility system operating in the amphidiploid species, Brassica napus. Seven synthetic B. napus lines were produced by intercrossing plants of the parental species B. campestris and B. oleracea and doubling the chromosome numbers of the F1 hybrids, so formed. Each of the parental lines used in the synthesis were homozygous for different S-alleles. B. oleracea parental lines were homozygous for S29, S14 and S2 while the B. campestris line was homozygous for Sa and Sb. Consequently, the synthetic lines of B. napus were homozygous for different alleles at each of two S-loci. Following synthesis, S-allele activity and expression was examined in the synthetics and their F1's and F2's, using genetical and biochemical methods. In addition, a study of interspecific incompatibility between B. napus and B. oleracea was conducted. It was established that the production of synthetic B. napus by ovary culture in B. campestris was more successful than production using embryo culture in B. oleracea. Amphidiploid B. napus plants produced from F1 hybrids by chromosome doubling were easily identified, exhibiting a typical B. napus morphology, producing fertile buds and reflecting B. campestris and B. oleracea isozyme banding patterns. Synthetic B. napus plants were generally self-incompatible and the self-incompatibility alleles of both B. oleracea and B. campestris were expressed and showed interlocus epistasis similar to that found in a single locus sporophytic system. Several F1 lines produced from crosses between B. napus synthetics expressed only 3 alleles in any S-allele combination, and this expression occurred only in the pistils. The B. campestris alleles were functional in F1 stigmas but not in the pollen, showing that allele activity in F1 pollen would appear to be equivalent to that of a single locus system. In the F2 generation, all 4 S-alleles were active in a given genotype and it was established, therefore, that hidden S-loci could persist in a species with a sporophytic self-incompatibility system if alleles were partially or completely recessive. Test crosses between B. campestris, B. oleracea and B. napus showed that pollen from B. oleracea usually failed to penetrate the stigmatic surface of B. napus, despite all other combinations of interspecific crosses being compatible. It was found that this interspecific incompatibility could be overcome by bud pollinations or by treatment with cycloheximide. Biochemical studies showed that stigma-specific proteins, resolved by isoelectric focusing, correlated with some S-allele expression and could be detected in synthetic B. napus. However, stigma-specific proteins, which correlated with the presence of the Sa and S14 alleles, were detected in stigmas of F2 plants even when they were not active, indicating that S-protein expression is not correlated with the full function of self-incompatibility alleles.