MULTIPLE ORIGINS OF SELF-COMPATIBILITY IN LINANTHUS SECTION LEPTOSIPHON (POLEMONIACEAE): PHYLOGENETIC EVIDENCE FROM INTERNAL-TRANSCRIBED-SPACER SEQUENCE DATA.

Phylogenetic reconstruction based on sequence variation in the internal transcribed spacer (ITS) region of rDNA was used to investigate the evolutionary dynamics of homomorphic self-incompatibility in Linanthus section Leptosiphon (Polemoniaceae), a group of annual plant species. Hand-pollination experiments revealed that five species were self-incompatible and four were self-compatible. Optimization of breeding systems onto the tree resulting from maximum-likelihood analysis, with no assumptions made about the ancestral condition, indicated that self-incompatibility has been lost four times in this section. An alternative tree rearrangement conforming to the hypothesis of three losses of self-incompatibility did not have a significantly lower likelihood than the maximum-likelihood tree as determined by a paired-sites test, but all rearrangements resulting in fewer than three losses were statistically rejected. Linanthus bicolor, a selfing species, was found to be polyphyletic, with populations from different geographic regions occurring in three well-supported clades. Morphological similarity in these distinct lineages is likely to have resulted from convergent evolution of traits associated with self-fertilization. Selection for reproductive assurance is hypothesized to have played an important role in the recurrent transformations from self-incompatibility to selfing in this group of annual species.