The evolution of key functional floral traits in the early divergent angiosperm family Annonaceae

Abstract

Potential key functional floral traits are assessed in the species‐rich early divergent angiosperm family Annonaceae. Pollinators (generally beetles) are attracted by various cues (particularly visual, olfactory and thermogenic), with pollinators rewarded by nectar (generally as stigmatic exudate), heat and protection within the partially enclosed floral chamber. Petals sometimes function as pollinator brood sites, although this may be deceptive. Annonaceae species are self‐compatible, with outcrossing promoted by a combination of protogyny, herkogamy, floral synchrony and dicliny. Pollination efficiency is enhanced by pollen aggregation, changes in anthesis duration, and pollinator trapping involving a close alignment between petal movements and the circadian rhythms of pollinators. Most Annonaceae flowers are apocarpous, with syncarpy restricted to very few lineages; fertilization is therefore optimized by intercarpellary growth of pollen tubes, either via stigmatic exudate (suprastylar extragynoecial compitum, EGC) or possibly the floral receptacle (infrastylar EGC). Although Annonaceae lack a distinct style, the stigmas in several lineages are elongated to form ‘pseudostyles’ that are hypothesized to function as sites for pollen competition. Flowers can be regarded as immature fruits in which the ovules are yet to be fertilized, with floral traits that may have little selective advantage during anthesis theoretically promoting fruit and seed dispersal. The plesiomorphic apocarpous trait may have been perpetuated in Annonaceae flowers since it promotes the independent dispersal of fruit monocarps (derived from separate carpels), thereby maximizing the spatial/temporal distance between seedlings. This might compensate for the lack of genetic diversity among seeds within fruits arising from the limited diversity of pollen donors. This article is protected by copyright. All rights reserved.