As a grad student, Horacio Frydman was studying sterile fruitflies when something unusual caught his eye. “One of the mutant lines had a strange pattern of staining in the part of the ovary close to the egg cells,” he recalls. “Almost everybody thought it was an artefact.” But after taking a closer look with an electron microscope, he concluded that the staining was caused by infectious bacteria — more precisely, Wolbachia.

Wolbachia is extremely adept at infecting all types of insects. It also invades small parasitic worms, such as the filarial nematodes that cause river blindness and elephantiasis in places such as Frydman's home country, Brazil.

The bacterium typically inhabits cells in the testes and ovaries of its host, and is passed on from generation to generation through the mother. But until recently, no one knew which cellular mechanisms Wolbachia exploits to infect its host and spread so successfully down the generations

“I saw Wolbachia as an opportunity to combine my interest in fruitfly development with studying host–parasite interactions in tropical diseases,” says Frydman.

So after completing his PhD work in the lab of stem-cell researcher Allan Spradling, at the Carnegie Institution of Washington, Frydman wrote a proposal for examining the transmission of Wolbachia in fruitflies (Drosophila). He then looked for a lab that would take him. “That put me in the unusual position of having a project without having an actual job,” he says.

Eventually, he found a home in the laboratory of Nobel laureate Eric Wieschaus at Princeton University in New Jersey. “Although Eric had never previously worked with Wolbachia, he generously allowed me to bring my own project to his very successful lab,” enthuses Frydman.

Credit: ANGELA BASILE

His first step was to improve the reagents and methods used to visualize Wolbachia in living and fixed tissues. Armed with the right tools, Frydman found that the bacteria clustered in two places in the germarium, a part of the fly's ovary where the development of eggs begins.

Having trained in Spradling's lab, Frydman quickly realized that the clusters were in a region known to include two or three of the somatic stem cells that generate structures to protect the embryo during development. But the Wolbachia was not inside the stem cells themselves. Instead, as reported on page 509 of this issue, the bacteria were found in the surrounding ‘niche’, a microenvironment that provides stem cells with the factors needed for their specialized properties.

“This positioning probably facilitates Wolbachia transmission and may have a role in the success of the parasite,” says Frydman. Indeed, it is possible that by homing in on the niche the bacteria are able, like stem cells, to renew and amplify themselves, and then infect many eggs.

Frydman now plans to explore what Wolbachia can teach us about the stem-cell niche itself. “We are only just learning about the niche, but the bacteria have been recognizing and living in it for millions of years.”