Pollen-ovule ratios are strongly correlated with floral reciprocity, in addition to sexual system, in Mussaenda (Rubiaceae)

Heterostyly is a floral polymorphism that increase inter-morph pollen transfer and promote disassortative mating. Breakdown of heterostyly has happened many times, either leading to dioecy or monomorphism. Mussaenda is a genus with diverse sexual systems including distyly, dioecy, floral monomorphism, and homostyly, making it an ideal system to study the evolution of floral traits and their relationship with sexual system shifts. Here, floral traits and pollen-ovule (P/O) ratios were estimated and used to test hypotheses about the relationships among P/O ratios, floral trait evolution, and sexual system shifts. Our results revealed that there is no significant difference of reciprocity in upper level sexual organs between species with different sexual systems. The reciprocity indices of the lower level sexual organs, however, are smaller in species with functional dioecy than in those with distyly. P/O ratios in dimorphic Mussaenda species were relatively lower than in monomorphic outcrossing species, but did not differ significantly between species with distyly and functional dioecy. Populational P/O ratios were negatively correlated with reciprocity index in the dimorphic species. We suggest that the loss of function in lower level organs in species with functional dioecy has resulted from less strict reciprocity. The relationship between P/O ratios and reciprocity indices strongly support that efficient disassortative pollination of distylus flowers may have promoted evolution towards low P/O ratios.