When I wrote Nature’s Nether Regions, I struggled a lot with sperm and ova. Should I include the evolution of sex cells (or “gametes”) in the book or not? In the end, I did so, but only minimally. After all, the evolution of the multitude of shapes and traits, forms and functions of sperm cells and egg cells is a mer à boire just as complex as that of genital organs themselves. All the sexual selection forces that mold the shape of penises and vaginas do their tricks all over again when confronted with the cells that are channelled by these genitals.
A new article in this week’s PLoS Biology gives us a peek into the amazing world of gamete evolution. Janice Ting of the University of Toronto and co-authors studied that workhorse of developmental biologists, the tiny nematode worm Caenorhabditis. They discovered that, when they tried crossing one Caenorhabditis species with another, not ony did they not get any offspring, but also the female would be the worse for wear afterwards: she often died shortly after the deed. As it turned out, this was due to the sperm cells she had received from the other species’ male. Ting and colleagues saw how these had broken out of the uterus and were wreaking havoc with the female’s internal organs.
The reason probably is that that worm species with such rogue sperm was a sexually reproducing one, whose sperm has been continuously evolving to outcompete other males’ sperm (with the female reproductive system keeping up, evolutionarily, by bolstering the organ walls and the like). The species to which the hapless female belonged, however, was a species that mostly reproduced as a hermaphrodite (that is, it is male and female at the same time), by self-fertilizing–in nematodes evolution causes repeated shifts between sexual and hermaphroditic species. Since in such species there is not so much competition among the sperm of different males, these species have not been adapted to withstand the onslaught of aggressive sperm cells.
If true, then this is an interesting new reason why different species often have reproductive barriers: they cannot cross-breed because their female reproductive system cannot cope with the aggressive sperm cells of the other species. In one of my previous books, Frogs, Flies & Dandelions, I describe how female tsetse flies sometimes die after mating with a male from a different part of Africa because their vaginas are simply ripped apart by those males’ genitals. This new worm study again shows how barriers between species may evolve simply because sexual strategies do not keep up with one another.