Bacterial Control of Gene Expression And Horizontal Gene Transfer In Pillbugs

Pillbug. Roly-poly. Woodlouse. Doodle bug. This endearing creature, which goes by many names, is common throughout North America and Europe. It seeks moisture, scurries from light, and rolls into a ball when threatened. And it often finds itself embroiled in an evolutionary war with a bacterium. At stake is its very sexual identity.

In pillbugs, sex is determined by two chromosomes: Z and W. Individuals who inherit two Zs develop as males, while ZW individuals become female. But in some populations, these rules are overwritten by a microbe called Wolbachia.

Wolbachia infects the cells of pillbugs and only passes down the female line; only mothers can transmit the bacterium to their young. Male embryos are dead ends to Wolbachia, so when it runs into them, it feminizes them by interfering with the development of hormone-producing glands. The result is that all young pillbugs infected with Wolbachia grow up into females, even those that are genetically male. In such populations, the W chromosome tends to disappear altogether. Eventually, all the pillbugs are ZZ, and it’s the presence or absence of Wolbachia that dictates whether they become female or male.

It’s astonishing enough that a microbe should so totally take the reins of sex determination from its host. But this story, which a group of French scientists have pieced together over the last 40 years, now has an even more baffling twist. 

In the 1980s, the French researchers showed that some pillbugs do not have Wolbachia, but act as if they did. They’re all ZZ, but some still develop as females. The researchers proposed that the bacterium has transferred a piece of its DNA into the pillbug’s genome, and that this “feminizing element”—or f-element—was now dictating the animal’s sexes, even in the microbe’s absence. 

With the technology of the 1980s, the researchers had no way of testing their idea. But 30 years on, Richard Cordaux, from the University of Poitiers, has come very close to proving that they were right.

From The Atlantic.

What if this happened to humans instead of pillbugs?

Horizontal gene transfer and long term symbiotic parasitic infections with subtle behavioral effects are not unknown in humans. Maybe it already happens. We are pretty confident that gender variants like homosexuality and transgender identity are biological and usually arise early in life. But, our understanding of why this happens is murky at best. We know that it isn't simply a matter of having or not having a gene, but a variety of theories are competing to explain why this does happen.'

The only fictional portrayal of a fairly similar phenomena is Greg Bear's 1999 novel Darwin's Radio. But, it would surely be a stunning and confusing phenomena to live through and might take decades to understand, just as it did in the case of the pillbugs.

Yet, there is really no good reason why this couldn't happen to humans just as it did to the pillbugs, some day in the future. Humans need some kinds of bacteria to live, so they can't simply destroy all bacteria in their environments. Too strong an antibiotic in your system can cause as much harm as a mildly harmful bacteria. So, the possibility of this kind of horizontal gene transfer or parasitic control of gene expression is always open.