Escaping the male-killer

At the moment, our yard here has many more butterflies than the "butterfly garden" at the zoo. That's mostly attributable to the zoo's complete lack of milkweed (Asclepias), which we've had going strong for around three weeks, supplemented by a huge number of coneflowers (Echinacea). After a quick burst of painted ladies, the monarchs have settled in. We've had phlox for a few days now, so we'll be watching for tiger swallowtails.

So I like this story about the rapid response to selection among Blue Moon butterflies in Samoa:

Sylvain Charlat of the University of California, Berkeley, and the University College London, along with colleagues, studied the sex ratios of Hypolimnas bolina butterflies on the Samoan islands of Upolu and Savaii, where males had dwindled to 1 percent of the populations in 2001.
The likely culprit was a male-killing parasite, Wolbachia, which lives inside the butterfly's reproductive cells, preferably female sex cells. With a female host, Wolbachia can hitch a ride to the next generation aboard the mother's eggs. Since males are "useless" for the bacteria's survival, the parasite kills male embryos.
But the male butterflies found a way to stealthily overcome the parasites. At the beginning of 2006, the scientists found the males made up about 40 percent of Upolu's butterfly population.
On Savaii, females still dominated the Blue Moon butterfly population (99 percent) at the start of 2006, but by the year's end, males made up nearly 40 percent.

Wolbachia may be the best parasites in nature, judging by the diversity of strategies that they exhibit. Killing males to direct resources toward females is the least of these. Here's a passage from a 1995 article by Stephen Hart:

Entomologists had thought that parthenogenetic wasps represented a quirk of nature -- until geneticist Richard Stouthamer of the Agricultural University in Wageningen, the Netherlands, reported in the April 1990 Proceedings of the National Academy of Sciences that he had "cured" parthenogenetic wasps with a dose of antibiotic. Tetracycline exorcises a parthenogenetic female, allowing her to lay viable female and male eggs. Her offspring can then go on to form a sexually reproducing population (Hart 1995:4).

The culprit was Wolbachia, of course, which can spur unfertilized eggs to develop. You may pause to consider whether this is good or bad for the wasps (clearly, it's bad for the male wasps!). But for the females, it leaves them 100 percent related to their offspring.

The rapid response to the high infection rate is quite expected based on the mechanism of male-killing, as long as the few remaining males are necessary, their genes will be represented in all the next generation. So it's an enormous reproductive advantage, in this case causing an allele to go from near zero to 40 percent in only 10 generations.

Still, it raises that question again -- who's better off here, the lucky males or the Wolbachia?

UPDATE (7/21/2007): A tiger swallowtail yesterday -- a dark morph female, which is the first of those we've had around here.


Hart S. 1995. When "Wolbachia" invades, insect sex lives get a new spin. BioScience 45:4-6.