Self-incompatibility and mating system evolution in Linanthus section Leptosiphon (Polemoniaceae)

Self-incompatibility was found to be evolutionarily dynamic in Linanthus section Leptosiphon (Polemoniaceae), a group of spring-flowering annual species with a center of distribution in California. The genetic control of self-incompatibility was studied in Linanthus porviflorus. Compatibility relationships among full-siblings were assessed by examining for pollen tube growth in hand cross-pollinated flowers. The results were consistent with sporophytic control of self-incompatibility. Linanthus section Leptosiphon was found to comprise both self-incompatible and self-compatible species. The phylogeny of the section was reconstructed using sequence data for the internal transcribed spacer region of rDNA. Geographically distinct populations of L. bicolor, a selfing species, appeared in three well-supported clades, suggesting that morphological convergence associated with selfing has resulted in the circumscription of a polyphyletic species. Parallel evolution toward reduced flower and inflorescence size also is evident in other self-compatible lineages. Reconstruction of breeding system transformations using parsimony methods indicated that self-incompatibility has been lost at least three times in the section.; The causes and consequences of mating system evolution in Linanthus were investigated in population level studies of three sympatric species with contrasting mating systems: L. parviflorus, a self-incompatible species, L. bicolor, a highly selfing species, and L. jepsonii, a partially selfing species. The magnitude of inbreeding depression was determined for populations of L. jepsonii and L. bicolor. Cumulative inbreeding depression was low for both species, less than 0.228 in all populations. Consistent with theoretical expectations, the magnitude of inbreeding depression was significantly negatively correlated with the fixation indices estimated from isozyme data, a measure of prior inbreeding. Inbreeding depression in L. jepsonii, though greater than in L. bicolor, is not sufficient to prevent evolution of increased selfing rates in this species, according to theoretical models of mating system evolution. Pollen limitation of reproduction, assessed by pollen supplementation experiments, was greater in populations of L. parviflorus than in the self-compatible species, suggesting that selection for reproductive assurance was a major factor in the evolution of selfing in these taxa. Wind pollination was demonstrated experimentally in L. parviflorus and appears to provide some reproductive assurance to this self-incompatible annual.