Self-incompatibility in Senecio squalidus L (Asteraceae)

The Asteraceae, one of the largest families of flowering plants, shows an e xtreme diversity of breeding systems with many species having a pronounced flexibility in breeding strategy-a factor thought to be crucial to the succ ess of this family in colonizing most major biogeographical regions. Despit e self-incompatibility (SI) being the most common mode of outcrossing in th e Asteraceae, then have been few detailed studies of SI within this family. This paper discusses the SI system operating in Senecio squalidus (Oxford ragwort), an invasive alien species that has colonized most parts of the UK within the last 150 years. In S. squalidus, SI is sporophytic and controll ed by a single multiallelic S locus. Preliminary findings suggest that UK p opulations contain few S alleles. Pseudo self-compatibility (PSC) has been found to operate in S. squalidus and leads to total or partial loss of S-sp ecific discrimination as females, though pollen S function is unaffected. E xceptional compatible and semi-compatible crosses in normally incompatible groups suggest that a cryptic gametophytic element (the G gene) can influen ce the outcome of certain crosses. It is suggested that PSC, mediated by mo difier loci unlinked to the S locus, together with the apparent activity of a G gene in determining SI, may have provided flexibility in the SI system of S. squalidus that was crucial to its success as a colonist. Basic aspec ts of the cell biology of the pollen-stigma interaction during compatible a nd incompatible pollinations in S. squalidus are discussed in the light of previous studies of species in the Asteraceae and Brassicaceae. (C) 2000 An nals of Botany Company.