Pollen-ovule ratios are strongly correlated with floral reciprocity in addition to sexual system in Mussaenda (Rubiaceae)
2 May 2017
Yuan, Shuai; Chen, Shi; Deng, Xiaofang; Duan, Tingting; Luo, Zhonglai; Zhang, DianxiangHeterostyly is a floral polymorphism that increase inter-morph pollen transfer and promote disassortative mating. Breakdown of heterostyly happened many times, which can either lead to dioecy or to 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 its relationship with sexual system shifts. Floral traits and pollen-ovule (P/O) ratios were estimated and used to test the 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 in species with different sexual systems; the reciprocity indices in lower level organs, however, are smaller in functional dioecy than distyly. P/O ratios in dimorphic Mussaenda species were relatively lower than monomorphic outcrossing species, and didn’t differ significantly between species with distyly and functional dioecy. Populational P/O ratios were negatively correlated with reciprocity index in all dimorphic species. We suggest that the loss of function in lower level organs in species with functional dioecy resulted from less strict reciprocity. The relationship between P/O ratios and reciprocity indices strongly support that efficient disassortative pollination in distyly could have resulted in the low P/O ratios.