“Do sperm really compete and do eggs ever have a choice? Adult distribution and gamete mixing influence sexual selection, sexual conflict, and the evolution of gamete recognition proteins in the sea”

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Don R. Levitan

Sperm compete and eggs choose in externally fertilizing species

Male sea urchins spawning off the Pacific coast of Canada. Individuals are tagged, mapped and genotyped for parentage analysis and sequencing of their sperm proteins.
(Credit: Kevin Olsen)

There is a long history of studies that examine male competition and female choice, but in many cases females mate with multiple males and it is not clear whether the sperm from one male directly competes with the sperm from another male in the race to fertilize an individual egg. When sperm arrive simultaneously to an egg, it provides an opportunity for eggs to choose sperm from competing males. Don Levitan examines this question using sea urchins by conducting underwater experiments off the Pacific coast of Canada. He finds that when sea urchin density is high, sperm from multiple males can mix to the degree that sperm competition and egg choice are likely and that the winner of this competition is predicted by the sperm proteins that interact with the egg surface. When sea urchin density is lower, eggs are surrounded by the sperm from single males and the male that wins is determined by which sperm arrives first and not by protein interactions. These results shed light on patterns of sperm competition and egg choice in the sea and more broadly how ecological conditions can influence the evolution of compatibility between sperm and eggs. Read the Article