Drs Tim Littlewood and Andrea Waeschenbach (Zoology) have collaborated with colleagues from Switzerland and Japan on a paper, recently published in the Proceedings of the National Academy of Sciences, that attracted widespread media attention from the science and popular press. Headlines such as “‘Worm porn’ sheds light on evolution of sperm” (MSN Science) and “X-rated worm movies reveal sex secrets” (Nature News) reflect the more restrained coverage.
It is a continuing challenge in science to explain why sex evolved in different species in such a variety of forms - internal or external fertilisation; separate sexes or hermaphrodites; mate selection; number of offspring; number of mates; timing of reproduction; and many other questions. Sperm in particular are intriguing - these are highly specialised cells with the function of exchanging genetic material, evolved to survive and function in quite different situations in different species. The huge variety of different sorts of sperm cells reflects the variety of different reproductive strategies in various groups of organism.
This study looked at a number of related species of a small transparent flatworm - Macrostomum. The team used a robust molecular phylogeny (developed by TL and AW) using DNA to define evolutionary history of the worms. They then looked at mating strategies, the morphology of the bodies and the types of sperm in different species.
They found that one group of worms had very complex sperm with spines and a pattern of hermaphrodite exchange of sperm cells. However, a different strategy had evolved in one member of this group and in four worms in another group in which the sperm is injected by one worm into the body of another. In these injecting species, the form of the sperm has evolved to become simpler, losing certain characteristics such as spines: the form of sperm seems to be related to mating techniques. It seems possible that the hypodermic injection gives certain advantages in some species - this might be to avoid competition from the sperm of rivals, or to avoid female rejection of sperm, but more work will be needed to answer this question.
The team was led by Dr Lukas Scharer (University of Basel, Switzerland) and included Dr Dita Vizoso (Basel) and Dr Wataru Yoshida (Hirosaki University, Japan).
Schärer, L., Littlewood, D.T.J., Waeschenbach, A., Yoshida, W. & Vizoso, D.B. (2011). Mating behaviour and the evolution of sperm design. Proceedings of the National Academy of Sciences USA 108:1490-1495.