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To make female pill bugs, just add bacterial genes

Bits of Wolbachia DNA infiltrate an arthropod’s genetic makeup and change sex determination

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4:51pm, September 30, 2016
pillbug

SEX LIVES Wolbachia bacteria have done some remarkable things to sex in terrestrial crustaceans commonly called pill bugs — even when the actual bacteria are long gone.

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ORLANDO, Fla. — When sex chromosomes among common pill bugs go bad from disuse, borrowed bacterial DNA comes to the rescue. Certain pill bugs grow up female because of sex chromosomes cobbled together with genes that jumped from the bacteria.

Genetic analysis traces this female-maker DNA to Wolbachia bacteria, Richard Cordaux, based at the University of Poitiers with France’s scientific research center CNRS, announced September 29 at the International Congress of Entomology.

Various kinds of Wolbachia infect many arthropods, spreading from mother to offspring and often biasing their hosts’ sex ratios toward females (and thus creating even more female offspring). In the common pill bug (Armadillidium vulgare), Wolbachia can favor female development in two ways. Just by bacterial infection without any gene transfer, bacteria passed down to eggs can make genetic males develop into functional females. Generations of Wolbachia infections determining sex let these pill bugs’ now-obsolete female-making genes degenerate. Which makes it very strange that certain populations of pill bugs with no current Wolbachia infection still produce abundant females. That’s where Cordaux and Poitier colleague Clément Gilbert have demonstrated a second way that Wolbachia makes lady pill bugs — by donating DNA directly to the pill bug genes.

The researchers, who share an interest in sex determination, have built a case that Wolbachia inserted feminizing genes into pill bug chromosomes. The bacterial genes thus created a new sex chromosome.

“Incredible,” said Steve Perlman after hearing the talk, not in disbelief but in wonder at the biology. Perlman, of the University of Victoria in Canada, studies symbiosis and parasitism and says this new example of far-flung gene transfer is part of “a big thing in the field now.” Such transfers provide exotic genetic variation that fuels evolutionary processes. Audience members Ellen Martinson and Vincent Martinson, both of the University of Rochester in New York, were themselves coauthors of a 2016 paper describing microsporidian fungus DNA that has become a venom gene in some wasps.

Citations

R. Cordaux. Evolution of new sex chromosomes by lateral genome transfer of bacterial symbiont in pillbug. International Congress of Entomology, Orlando, Fla., September 29, 2016.

E.G. Martinson et al. Laterally transferred gene recruited as a venom in parasitoid wasps. Molecular Biology and Evolution. Vol. 33, April 2016, p. 1042. doi: 10.1093/molbev/msv348

Further Reading

R. Cordaux, D. Bouchon and P. Grève. The impact of endosymbionts on the evolution of host sex-determination mechanisms. Trends in Genetics. Vol. 27, August 2011, p. 332..doi:10.1016/j.tig.2011.05.002

S. Milius. Lone survivor of ancient flowers is gluttonous gene consumer. Science News Online, December 21, 2013.

S. Milius. Malaria mosquito dosed with disease-fighting bacteria. Science News Online, May 9, 2013.

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