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Post date: Nov 16, 2017 2:37:50 AM
Auanema rhodensis is a very weird nematode. The first unusual thing about A. rhodensis is that it has three sexes: males, females, and hermaphrodites (which can self-fertilize and mate with males, but cannot mate with females). You might expect that this would not be a stable evolutionary equilibrium, and that the species would rapidly evolve to be exclusively female/male or hermaphrodite/male. But previous work from the Pires-daSilva group showed that XX individuals can develop either as hermaphrodites or females depending on whether they were starved, suggesting that each has advantages depending on the environment, which could allow both XX sexes to persist evolutionarily.
But the more we learn about A. rhodensis, the weirder it gets. Sex chromosome segregation in A. rhodensis turns out to behave very unusually. Building on previous work, the Pires-daSilva group shows in "Gender- and gamete-specific patterns of X chromosome segregation in a three-gendered nematode" that males only produce sperm with an X chromosome, whereas hermaphrodite sperm mostly have two X chromosomes, and hermaphrodite oocytes have no sex chromosomes at all. (Note that A. rhodensis males, like many other nematodes, have a single X, rather than an X and a Y.) This means that male/female crosses only produce females and hermaphrodites; male/hermaphrodite crosses only produce males; and hermaphrodite self-mating produces hermaphrodites and females (with an additional low rate of males). It's very interesting to think of how this could have evolved. It has tantalizing similarities to sex-distortion meiotic drive systems. In particular, the males only produce sperm with a single X because the meiosis II products without an X do not mature into sperm. Perhaps this started with a male-specific X chromosome drive element, and the other changes evolved in response.
The other question that immediately springs to mind is what kind of sex ratios this system will produce in the population. Is it even workable for the species to have certain crosses that only produce progeny of one sex? I ran some simple simulations (below), and it turns out to be pretty stable, with similar sex ratios after 10 generations regardless of the starting ratios. The key is that if there are both females and hermaphrodites in the population, then crosses to males will generate both XX and X progeny (respectively), and overall the population is okay. You could have a problem, however, if XX progeny were much more likely to be hermaphrodites than females, because then you'd get "too many" males, or if the XX progeny were much more likely to be females, because then you'd get "too many" females. This could be another reason that both females and hermaphrodites have persisted evolutionarily in A. rhodensis - their sex chromosome segregation system requires it!
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