Evolution of distyly breakdown in Palicoureeae Robbr. & Manen and Psychotrieae Cham. & Schltdl. (Rubiaceae)

Abstract

Distyly is a floral polymorphism with reciprocal placement of male and female structures, heteromorphic self-incompatibility, and other ancillary traits. However, breeding system breakdowns and loss of polymorphism are common. Here we traced the diversification of breeding strategies in the type genera of tribes Palicoureeae and Psychotrieae and discussed the evolution of distyly in a phylogenetic framework. We used literature and field information for breeding systems transitions in 46 species of Palicourea and Psychotria. Beyond distyly, we found four additional breeding systems, including monomorphism with herkogamy, homostyly (without herkogamy), monoecy and dioecy. Breeding transitions arose independently and were mostly derived from distyly. Only two species presented monomorphism as an intermediate state into gender specialization. It was not possible to evaluate the origin and evolutionary pathways for distyly in Psychotria and Palicourea as a whole, since distyly seems to be ancestral to their diversification. Breeding transitions in Psychotria and Palicourea appeared to be phylogenetically and biogeographically independent and occurred mostly in islands or isolated forest fragments, with distinct divergence times. Breeding transitions were not related to changes in ploidy. We propose that evolution of breeding transitions in Psychotria and Palicourea represent phylogenetically independent strategies to reproductive assurance in isolated or disturbed habitats.