Asexual Revolution

In the chaparral and old growth Douglas fir on the west coast of the US, live the Timema, a little stick insect which has several distinct lines who have all sworn off sex for the past million years. There have been no signed pledges here. They are all female and prefer to clone themselves rather than have any males cluttering up the place. This curious behavior has been brought into the daylight by Schwander et al of Simon Frazer U in a recent report (1,2).

The received wisdom is that cloning is not a good strategy for the long-term survival of a species and that sex is the more rewarding way to go. However, two of these female-only lines have been going happily on with virgin birth after virgin birth for a million or more generations and at least three others for half a million.

Their mitochondrial DNA is a good indicator of their parentage and it is interesting that chunks of their nuclear DNA (alleles) showed greater divergence in individual lines than that found with related species who rely on a regular sexual preference for continuation of the species. So all this wild making out that we see in most species leads to a more standardized DNA with less drift than occurs with an asexual approach to life.

The puzzling question is why did they end up this way. They are a complicated animal species and it is a fair bet that somewhere on their development path their ancestors were sexually active. Did they “decide” at some point that producing males was just too darned expensive.

Digging around, I read that a parasitic bacteria can cause parthenogenesis with some insect species and honey bees do a bit of both, as do some crayfish. Sharks in aquariums have been observed to resort to it and turkeys can, apparently, be bred to it. In 2004 and 2007, stem cell lines from human embryos which were produced by parthenogenesis. If too much of this catches on, our chances of getting lucky on a night out will drop to that of winning the lottery!

T. Schwander, L. Henry & B.J. Crisp, Current Biol., 21, 1129, (2011).

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