Sporophytic self-incompatibility in Senecio squalidus L. (Asteraceae)--the search for S

Senecio squalidus (Oxford Ragwort) is being used as a model species to study the genetics and molecular genetics of self-incompatibility (SI) in the Asteraceae. S. squalidus has a strong system of sporophytic SI (SSI) and populations within the UK contain very few S alleles probably due to a population bottleneck experienced on its introduction to the UK. The genetic control of SSI in S. squalidus is complex and may involve a second locus epistatic to S. Progress towards identifying the female determinant of SSI in S. squalidus is reviewed here. Research is focused on plants carrying two defined S alleles, S(1) and S(2). S(2) is dominant to S(1) in pollen and stigma. RT-PCR was used to amplify three SRK-like cDNAs from stigmas of S(1)S(2) heterozygotes, but the expression patterns of these cDNAs suggest that they are unlikely to be directly involved in SI or pollen-stigma interactions in contrast to SSI in the Brassicaceae. Stigma-specific proteins associated with the S(1) allele and the S(2) allele have been identified using isoelectric focusing and these proteins have been designated SSP1 (Stigma S-associated Protein 1) and SSP2. SSP1 and SSP2 cDNAs have been cloned by 3' and 5' RACE and shown to be allelic forms of the same gene, SSP. The expression of SSP and its linkage to the S locus are currently being investigated. Initial results show SSP to be expressed exclusively in stigmas and developmentally regulated, with maximal expression occurring at and just before anthesis when SI is fully functional, SSP expression being undetectable in immature buds. Together these data suggest that SSP is a strong candidate for a Senecio S-gene.