Function and Evolution of Aggregated Pollen in Angiosperms

The evolution of different forms of pollen aggregation (tetrads, polyads, pollen threads, pollinia) from individual monads is a recurring transition in angiosperm history, having occurred independently at least 39 times. Aggregation should evolve only under special circumstances, because diminishing returns associated with pollen removal and receipt instead favor monads that act largely independently. All forms of aggregation result in sibling pollen grains acting together, but they seem to evolve to ease different limitations on siring success: tetrads may evolve most commonly when pollinators visit infrequently, pollen threads may be most beneficial when ovules become available synchronously, and pollinia greatly increase the probability that a pollen grain removed by a pollinator reaches a conspecific stigma. Once pollen aggregation evolves, its implications for gametophytic competition and the relatedness of seeds within fruits probably influence further reproductive evolution, especially the frequency with which pollen from a single donor sires all seeds in a fruit. This latter effect, rather than improvements in pollination efficiency, probably accounts for the common association of pollen aggregation with low pollen:ovule ratios. The ability of orchid pollinia to reduce diminishing returns during pollination may explain both the floral diversity and the widespread occurrence of deceit pollination in this clade.