The evolution of alternative mechanisms that promote outcrossing in Annonaceae, a self-compatible family of early-divergent angiosperms

Annonaceae flowers are generally hermaphroditic and show high levels of outcrossing, but unlike many other early-divergent angiosperms lack a self-incompatibility mechanism. We reassess the diversity of mechanisms that have evolved to avoid self-pollination in the family. Protogyny occurs in all hermaphroditic flowers in the family, preventing autogamy but not geitonogamy. Herkogamy is rare in Annonaceae and is likely to be less effective as beetles move randomly around the flowers in search of food and/or mates. Geitonogamy is largely avoided in Annonaceae by combining protogyny with floral synchrony, manifested as either pistillate/staminate-phase synchrony (in which pistillate-phase and staminate-phase flowers do not co-occur on an individual) or heterodichogamy (in which two phenologically distinct and reproductively isolated morphs coexist in populations). Unisexual flowers have evolved independently in several lineages, mostly as andromonoecy (possibly androdioecy). Functionally monoecious populations have evolved from andromonoecious ancestors through the loss of staminate function in structurally hermaphroditic flowers. This has been achieved in different ways, including incomplete pollen/stamen development and delayed anther dehiscence. Angiosperms display an enormous diversity of mechanisms to promote xenogamy, many of which are easily overlooked without fieldwork. Floral phenology is particularly important, especially cryptic differences in timing of organ maturation or abscission.(c) 2013 The Linnean Society of London, Botanical Journal of the Linnean Society, 2014, 174, 93-109.