Why have sex? It’s a difficult question to ask without sounding creepy. But the answer isn’t as obvious as you might think.
Evolutionary biologists have shown that sexually reproducing organisms should theoretically be at a disadvantage when competing with asexual organisms. The reason for this is that sexual females need male partners to reproduce, whereas asexuals can reproduce by themselves (“who needs guys anyway?” says Liz). Therefore, all else being equal, sexual females (which produce both males and females) will produce only half as many descendants over the long term as asexuals will. The production of males is referred to as the “two-fold cost of sexual reproduction”, and it should result in sexual forms being outcompeted by asexual forms. However, sexual reproduction is very common, especially in animals. Why sex remains so common despite this two-fold cost has been termed the “queen of questions” in evolutionary biology.
Research on this topic has focused on finding advantages of sexual reproduction that can more than offset the cost of producing males by sexual females. One clear advantage is related to the ability of sexual organisms to produce genetically distinct offspring instead of identical clones of themselves. One reason it can be good to have offspring that differ genetically from yourself is if you’re attacked by parasites that can adapt to your particular genetic makeup. This idea is referred to the Red Queen Hypothesis (the Through the Looking Glass character said “It takes all the running you can do, to keep in the same place.” The analogy to evolutionary biology is that populations need to evolve in order to remain equally successful in the face of challenge from parasites or other environmental challenges that are themselves evolving). Although this idea has received a lot of support, it’s not well understood how other features of the environment can affect the benefits of being sexual.
This month, UST junior Liz Chambers, Adam Kay, and our collaborators (Maurine Neiman from the University of Iowa and Amy Krist from the University of Wyoming) are testing a hypothesis about how the availability of phosphorus in the environment could affect the advantages of sex. Fortunately for our adventurous selves, our study system is a freshwater snail species, Potamopyrgus antipodarum, that occurs in New Zealand. It is a rare species of animals that has co-occurring sexual and asexual individuals. These snails are found in lakes across New Zealand, and different lakes have different mixtures of asexuals and sexuals. It has become a model system for studying what determines the relative success of these different reproductive strategies.
Our hypothesis focuses on a common difference between sexual and asexual taxa – their ploidy level (that is, the number of chromosome sets per cell). In our snails, sexuals have 2 sets of chromosomes (they’re diploid) while asexuals have 3 or 4 chromosome sets (triploids or tetraploids). Our hypothesis is that higher ploidy increases demands for dietary phosphorus (P), which is an important component of nucleic acids. Our prediction is that the growth and reproduction of organisms with higher ploidy (that is, asexuals) will be more sensitive to scarcity of phosphorus in their food. If that’s the case, it would indicate that phosphorus scarcity in the environment would be more detrimental to asexuals than to sexuals. In short, sexuals snails may do relatively well in low phosphorus conditions because their lower nucleic acid content and in turn their lower demand for dietary phosphorus. This finding would be important because it would indicate that the success of sexual animals could depend on the phosphorus content of the foods in their environment.
Our work this January aims to determine whether there is the potential for snails in our system to experience phosphorus limitation. We are traveling to 15 lakes across the south island to measure various indicators of phosphorus availability for our snails. The field work is pretty straightforward – we’ll have more posts about it soon (here’s a video of Liz and Lady Lake). So far, we’ve traveled up the northeast coast and across the northern part of the south Island (does that make sense?). (Here’s a video of the view from Kaikoura). I think we’ll be able to post several more times about our adventures exploring the sex life of snails over the next few weeks. Liz thinks this ends abruptly, but there’ll be more to come later…