Pollen and sperm heteromorphism: convergence across kingdoms?

Sperm competition theory predicts that males should produce many, similar sperm. However, in some species of animals and plants, males exhibit a heteromorphism that results in the production of at least two different types of sperm or pollen grains. In animals, sperm heteromorphism typically corresponds to the production of one fertile morph and one (or more) sterile morph(s), whereas in plants two or more pollen morphs (one of which can be either sterile or fertile) are produced in all flowers but sometimes in different anthers. Heteromorphism has arisen independently several times across phyla and at different phylogenetic levels. Here, we compare and contrast sperm and pollen heteromorphism and discuss the evolutionary hypotheses suggested to explain heteromorphism in these taxa. These hypotheses include facilitation, nutritive contribution, blocking, cheap filler, sperm flushing or killing for animals; outcrossing and precise cross-pollen transfer or bet-hedging strategy for plants; cryptic female choice for both. We conclude that heteromorphism in the two phyla is most likely linked to a general evolutionary response to sexual selection, either to increase one male's sperm or pollen success in competition with other males, or mediate male/female interactions. Therefore, although sperm and pollen are not homologous, we suggest that heteromorphism represents an example of convergence across kingdoms.