ESA: Making sense of sex
Well, we’ll never make sense of human sex, but we might make some sense of snail sex.
Adam Kay of the University St. Thomas in presented work on Potamopyrgus antipodarum, the New Zealand Mud Snail, which comes in both sexual and asexual flavors. When considering the snails it is unclear why sex should have evolved and why it should persist. The same is true of biology more generally. There is a cost to males, who are now not guaranteed to reproduce, and asexual populations just reproduce much, much faster.
According to Kay, there are more than 20 hypothesis for why sex exists, mostly genetic, focusing on phenotypic results of recombination. But their group has got another theory that I think is quite dashing in its simplicity. Most asexual snails have not just two chromosomes, but three (a little extra buffer against deleterious mutations usually considered to be good news). Chromosomes and associated RNA are made of nucleic acid, and nucleic acid contains tons of phosphorus.
“In invertebrates, DNA and RNA can make up a large fraction of organismal dry weight,” says Kay.
I bet you can guess where this is going. The hypothesis is that in places with limited phosphorus, the snails can’t afford to have all that nucleic acid, and so sexual snails are selected for.
So far, their evidence consists of a clear pattern of asexual snails being stuffed with phosphorus and sexual snails being phosphorus lean. And there you have it. Snails have sex because they are broke.
UPDATE: Kay's collaborator, Maurine Neiman, a biologist at the University of Iowa, tells us about the cool snail image above: "The snails are not naturally colorful, but were painted to identify four different lineages in an experiment studying how diet affects their elemental composition. They are also very tiny; the grid is 1 cm x 1 cm."
Nieman also asked that I mention the undergraduates from St. Thomas who did a lot of the work and will be authors on the paper, Katherine Theisen and Madelyn Mayry.
See also Evolution: Scandal! Sex-starved and still surviving
