Floral phenology and breeding system of Desmos chinensis and Dasymaschalon trichophorum (Annonaceae): protogyny and intra-individual floral synchronicity to promote out-crossing in early-divergent angiosperms

Abstract

Early-divergent angiosperms differ from more derived lineages in their strategies to promote out-crossing and reduce selfing, often lacking genetically controlled biochemical mechanisms of self-incompatibility. Flowers of Desmos chinensis and Dasymaschalon trichophorum (members of the early-divergent family Annonaceae) are hermaphroditic with a floral chamber. Desmos chinensis has six petals in which the floral chamber is formed by three inner petals. Dasymaschalon trichophorum only has three petals which are morphologically homologous to the outer petals which form the floral chamber. Both species show intrafloral dichogamy (protogyny) with a reproductively inactive phase (interim phase) between the pistillate and staminate phases which limits autogamy. The floral chambers in Desmos chinensis and Dasymaschalon trichophorum have small basal apertures between the petals during the sexually matured phase (closed in staminate phase in D. trichophorum), limiting entry of larger, non-pollinating flower visitors. Small beetles, the major pollinator group of the Annonaceae and other early-divergent angiosperms, were observed to stay inside the floral chambers in both species throughout the pistillate and interim phases and subsequently leave the chambers during the staminate phase. They are inferred to be the effective pollinators. Intra-individual floral synchrony was observed in both species, with synchronized receptivity of flowers within individuals, and maturation of a new cohort of flowers occurring after 48 hrs, hence limiting opportunities for geitonogamy. Occasional asynchrony was observed, however, and results from controlled pollination experiments and analysis of inter-simple sequence repeats (ISSR) markers indicates that Desmos chinensis has a mixed-mating system. It is possibly an evolutionary advantage to retain a level of geitonogamy, particularly in small populations and when there is a limited availability or patchiness of pollinators.