What could be better than two types of sexes? For one organism, the answer isn’t three, but seven! And to top it off, these seven sexes aren’t evenly distributed in a population, although researchers have now developed a mathematical model that can accurately estimate the probabilities in this crap-shoot game of sexual determination.
Meet Tetrahymena thermophila, which in addition to its seven different sexes—conveniently named I, II, III, IV, V, VI, and VII—has such a complex sex life that it requires an extra nucleus. This fuzzy, single-celled critter has a larger macronucleus that takes care of most cellular functions and a smaller micronucleus dedicated to genetic conjugation.
The other odd thing about this one-celled wonder is that the population of the seven sexes are skewed, leading Unversity of Houston researcher Rebecca Zufall and her colleagues to ask: What gives?
To answer that question, they created mathematical models of T. thermophila populations, and discovered that different versions of the same gene, or alleles, gave advantages to different sexes. Unlike humans, in which an individual’s sex is fully determined by its genes, the genotypes of these creatures provide only probabilities of developing certain sexes—probabilities that are influenced partly by genetics and partly by surrounding temperatures. The sex-influencing gene is called mat, and different alleles make certain sex types more likely than others: A T. thermophila born with the mat2 allele, for example, has no chance of becoming type I, a 15% chance of being type II, a 9% chance of developing into type III, and so forth. For convenience, this mat2 gene variety and the other 13 known alleles are also grouped into an A-group or a B-group: The A-group alleles lead to the I, II, III, V, and VI sexes, and the B-group alleles to the II, III, IV, V, VI, and VII sexes. In terms of evolutionary advantages, the researchers’ models back up exactly what we’d expect: The alleles that produce a variety of sexes outcompete the alleles that produce only one; variation is a nice thing to have, after all.
Epilogue: You may still be wondering why a species would have seven sexes; researchers are wondering the same thing. Some past research suggests that it’s to maximize the choice of sexual partners (pdf), since partners must be of a different sex type. The jury’s still out as to why they’re specifically at the seven-sex level now. Tiago Paixão, the study’s lead author, says that “one possible answer is that this is the number of sexes that populations of T. thermophila typically support, and further increases in number of sexes would not lead to any noticeable increase in fitness.”
That said, keep in mind that scientists are playing loose with the term “sex” when they define these protozoa; as Cleveland State University professor F. Paul Doerder and his colleagues write (pdf), “To recognize suitable partners, ciliates [such as T. thermophila] are differentiated into mating types, a kind of self-not-self discrimination system, as opposed to true sexes.”
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Image: Wikimedia Commons / Ayacop