Science and sensibility

Sex is one of the most intriguing puzzles in evolutionary biology. Its very presence seems counterintuitive. If organisms are acting to get as many of their genes into the next generation as they can then why would they go to the extreme lengths that they do to mix their genes with someone else before they make babies?

In fact, as John Maynard Smith pointed out, producing males incurs another cost for organisms that go in for them. Imagine a population of sexually reproducing organisms in which each female produces two offspring. In this case the population size will remain static because on average each female will produce one son and one daughter; the daughter will go on bear the same and so on ad infinitum. Now, imagine a mutant female crops up that can reproduce by cloning. She will be able to generate two females that will themselves each bear two further females. This will lead to a rapid expansion of the cloning organisms to the extent that they will take over. To put it another way sexually reproducing organisms put half their reproductive energy into males who do not bear more young. This is sometimes called the two fold cost of sex.

All this has lead people to wonder if sex is really worth all the investment that goes into it. One particularly exciting theory as to why it just might be was proposed by the Oxford biologist W.D. Hamilton. Like many of Hamilton's theories it involves parasites. Parasites and their hosts are in constant arms races, hosts are developing better defensive mechanisms just as quickly as parasites develop better tricks for getting around them. During such an arms race the best strategy for a parasite to employ is have a trick for getting around the most common defensive mechanism in a population (more targets equals a better chance of propagation) Consequently the best strategy for a potential host organism to take is to have a rare defensive mechanism. Hamilton hypothesised that the mixing of DNA during sexual reproduction may be mechanism for generating offspring with rare genetic make-ups which are therefore less likely to suffer parasitic infection

So whats all this got to do with gastropods? The first evidence for Hamiltons theory came from an unlikely source- the New Zealand Mud Snail, Potamopyrgus antipodarum

P. antipodarum is a little odd in that most individuals reproduce by asexual parthenogenic means but there are some populations like those in my back yard, the South Island's lakes, that have mixed populations with sexual males and females as well as asexual individuals. In New Zealand P. antipodarum is parasitized by a trematode worm.[1]

Those two facts combine to make a great chance to test Hamilton's theory. If he was right and sexual reproduction is a mechanism for generating rare genetic make ups then we would expect to see more sexually reproducing snails where the risk of infection is high and less where the risk is low. In 1987 Curt Lively reported that lakes where infection with trematodes was common had a lot of sexually reproducing snails while those from lakes with little or no parasitism where mainly asexual. This suggests that in the absence of parasitism asexual females replace sexuals while the threat parasitism in other lakes maintains sexual reproduction. That certainly supports Hamiltons notion that sex is worth all the investment that goes into it because it provides genetic make ups that stayone step ahead of parasites in evolutionary arms races. Similar results have been found in American fish species that suffer parasitism.

Since that initial report lots more interesting research has been done on these snails, you can see a description of some of it (and some lovely photos of the South Island) out at Lively's departmental website.

[1] Sadly when the snail is free from this parasite its population can grow unchecked. The NZ mud snail is actually a dangerous invasive species in the USA. I can't help but think this is some sort of strike back at the northern hemisphere from NZ's biota which faces invasive plants, insects, fish, mammals and birds!