The molecular biology of self-incompatibility in Brassica

Self- incompatibility in Brassica is controlled by a single "S"-locus. Different self- incompatible genotypes of Brassica are correlated with the occurrence of S-locus-specific glycoproteins (SLSGs), cDNA sequences for certain of which have been determined. In this thesis two cDNAs are presented, derived from a single homozygous line of Brassica oleracea, which are both partially homologous to known SLSG genes. One of these S-like sequences is possibly a novel form of the SLSG gene (corresponding to the S allele), whilst the other appears5invarient between Brassica lines containing different S-alleles (by comparison with published sequence date). The organisation of genomic sequences homologous to both classes of S-like cDNAs was investigated by Southern blotting using genomic DNA extracts from Brassica lines containing a range of S-alleles. The expression of genes homologous to the two cDNAs has been investigated by Northern hybridization and found to be specific to stigma tissue; no transcripts of S-like sequences are detectable in anther tissue over the developmental period during which expression of male S-gene sequences might be expected. The existence of S-like sequences, homologous to those from Brassica oleracea, has been investigated by Southern hybridization using genomic DNA extracts from a range of Brassica species and other related species. Inferences are drawn from this as to the timing of the gene duplication event which led to multiple S-like sequences, in relation to the timing of the evolutionary divergence of related taxa. The divergence of S-like sequences between different lines of B. oleracea has been analysed by comparison of DNA sequence data presented in this thesis with published sequence data. The relative rates of evolution of two domains within S-like gene sequences was estimated. A further three unrelated classes of tissue - specifically expressed sequences have been selected from a Brassica stigma cDNA library. One of these encodes a glycine-rich cell wall protein; another represents an unidentified gene showing stigma specific expression; the third represents an unidentified gene showing expression in stigma, style and anther tissue, though not in leaf tissue. cDNA sequence data, genomic Southern hybridizations and Northern hybridizations are presented for all three of these classes of cDNAs