How sperm evolve

Sperm, the male reproductive cell produced by an animal's testes and released from its penis, varies tremendously among species. Although they all share the same basic function, sperm differing in size, shape and number probably evolved due to post-copulatory competition. This arises when sperm from different males compete in the female reproductive tract for the honour of fertilising her egg.

The EU-funded SPERM EVOLUTION (Towards an evolutionary ecology of spermatogenesis) project used a simple flatworm to study how sperm evolve, how they are produced and what role the testes play in sperm variation.

Macrostomum flatworms are tiny organisms that contain both male and female sexual organs. When paired up, these worms seem to take turns being male or female during mating. However, when placed in groups where competition is high or, conversely, when isolated from any mates, flatworms change their reproductive strategy.

In a social environment, M. lignano alters the speed with which it produces sperm. Presumably, this flexible production rate allows the worm to modulate its amount of sperm according to the number of competitors in its environment.

When mates are not available, things get even stranger. As a fall-back strategy, individuals of the species M. hystrix use their penises to inject themselves in the head with their own sperm.

Researchers also found that differing sperm competition levels led to changes in genetic regulation throughout the Macrostomum genome. This will help researchers to understand how variation in sperm production occurs over both small and large evolutionary timescales.

A better understanding of the male reproductive cell and its production by the testes is useful to both ecologists and evolutionary biologists. In particular, this project's outcomes may help bridge evolutionary and basic biomedical research into, among other things, male fertility.