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

Abstract

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.

Pollen and anther development malfunction in distylous flowers of Palicourea padifolia (Rubiaceae)

“…Several authors have proposed that dioecy evolves from heterostyly when pollen transfer between floral morphs is highly asymmetrical (Ornduff 1966, Beach & Bawa 1980. The classical Darwinian explanation for the transition to dioecy from distyly proposes that the pollinator-driven asymmetrical pollen transfer between floral morphs, resulting from low reciprocity among sexual organs (e.g., Feinsinger & Busby 1987, Lau & Bosque 2003, Ornelas et al 2004a, Stone & Thomson 1994, Hernández & Ornelas 2007a, García-Robledo 2008 and lower accessibility of low-level sexual organs and pollinator behavior (Liu et al 2016, Yuan et al 2017, would favor maleness in the morph with higher pollen donation efficiency (usually SS), and femaleness in the morph receiving more legitimate pollen (usually LS) (Darwin 1877, Ornduff 1966, Lloyd 1979, Beach & Bawa 1980. Asymmetries in pollen flow might be an important first step towards gender specialization.…”