An end to the blogging hiatus at last! I hope to entertain you with the fascinating story on how female wasps got rid of their men and sex in return for bacterial endosymbionts..
Despite the obvious benefits of pleasure and procreation, sex has other advantages. The genetic material of both parents gets mixed in new and unforeseen ways (like a genetic wheel of fortune) whenever a sperm and egg cell fuse. The near ubiquity of sex in the animal world should be enough proof that sex is a good way to increase the genetic variance in the population for these species.
Yet asexual species (parthenogenic species, as biologists call them) pop up every now and then during evolution. One of the most fascinating examples I recently came across is that of the parasitic wasp, Trichogramma, which is often used as biological pest control. While some of these wasp populations exclusively consist of females, this state of asexuality is not some funky route of evolution, it is induced by bacteria!
A Trichogramma wasp parasitizing on the eggs of another insect.
The mood killer in question is a particular species of Wolbachia. These bacteria inhabit the cytoplasm of their host cells. Since sperm cells don’t have any cytoplasm, male wasps are a dead end as far as Wolbachia is concerned. The only way it can be transmitted from generation to generation is via the egg cells (which do have cytoplasm) that develop into female wasps. Normally, unfertilized haploid egg cells develop into male wasps whereas the fertilized egg cells become diploid and develop into female wasps. The Wolbachia messes with mitosis in such a way that every unfertilized egg is diploid, so that only females are born.
In some populations of Trichogramma, the Wolbachia infection rate has reached 100% and males are no longer born. There’s hope for the poor wasp guys though: by treating the wasps with antibiotics, the Wolbachia die out and new male wasps can be born. These male wasps are completely normal: they can inseminate female wasps and their sperm is viable. However, the females from these previously infected populations do not use the sperm to fertilize the eggs. They have become completely dependent on their symbiont for their reproduction.
Without male wasps around to admire her, The Wasp eventually married Ant Man.
This loss of female and retention of male sexual function is fascinating. If all things were equal, both male and female genes necessary for sexual reproduction would erode away at the same rate. Since the males are sexually viable while the females are not, something else must be happening. Stouthamer and colleagues provide an interesting explanation in a recent paper published in BMC Evolutionary Biology. By modelling infection rates, wasp mating and mutation rates they found that a ‘functionaly virginity mutation’ affecting the fertilization rate in females is likely responsible for fixing asexuality in the female population.
The basic gist is that in the initial phase of infection, males will have higher mating rates because there are females aplenty and less males being born. In uninfected females, alleles that reduce fertilization rates (less fertilization, more males!) are being selected for. The uninfected females will produce fewer uninfected daughters and more males that carry the same ‘virginity mutation’. A ratchet is now in place that leads to the fixation of both the infection and the virginity mutation:
Only those females that are not yet homozygous for the mutation will mate and part of their offspring will become homozygous for the mutation. Consequently the class of females that is homozygous for the mutant allele and infected will grow relative to the class of females that is not yet homozygous and infected.
The number of uninfected females capable of normal fertilization dwindles – spelling the end for men and sex. Poor guys. Seriously though, trading in sex for a bacterial endosymbiont.. Where’s the fun in that?
Stouthamer R, Russell JE, Vavre F, & Nunney L (2010). Intragenomic conflict in populations infected by Parthenogenesis Inducing Wolbachia ends with irreversible loss of sexual reproduction. BMC evolutionary biology, 10 (1) PMID: 20667099
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I remember reading about Wolbachia in ladybirds. The same thing was happening, all the male ladybirds were dying out and only females were being born, who could still reproduce by parthanogenesis. I didn’t realize antibiotics stopped it though, that’s quite impressive.
I think it’s less “trading in sex for a bacterial endosymbiont” and more ‘evil bacteria stealing the sex’ :p
The Wolbachia sure are a nasty bunch of sex-stealers (that does have a nice ring to it!). And while parthenogenesis is uncool, killing male larvae and feminization of males is simply unmoral! Someone’s gotta stop those men-unfriendly Rickettsia ;)
[...] I come across crazy parasite stories when I’m browsing scientific archives online. But this time was different, when a story came [...]