Infection divides two wasp species

Giving antibiotics to tiny wasps to cure them of a sex-related disease reveals the best evidence yet that infections can help make new species, say researchers in New York.

A Nasonia wasp, about 3 millimeters long, injects an egg into a fly pupa. (c) Werren

Each of two closely related species of wasps in the genus Nasonia carries two separate strains of Wolbachia bacteria, notorious saboteurs of insect reproduction, report Seth Brodenstein and his colleagues at the University of Rochester. The wasps live in separate parts of the United States, laying eggs in pupae of flies. But when wasps of the two species meet in the lab, they don’t hybridize well. However, cure them of their Wolbachia infections, and the wasps turn out hybrids as viable and fertile as same-species crosses do.

These healthy offspring add new support to speculations that reproductive-system infections helped splinter insect lineages into today’s millions of species, the team argues in the Feb. 8 Nature.

Eleven years ago, John H. Werren, a coauthor of the current paper, revitalized this old speculation. He reported a case in which an infection may have contributed to the speciation that separated two Nasonia wasps.

Both of these species carry Wolbachia. When Werren’s team cured those infections, hybridization improved considerably, but other genetic factors prevented full viability and fertility of offspring. So, it was hard to tell whether the bacteria had played a role in the split or had just infected the wasps later.

Since then, researchers have discovered Wolbachia in all orders of insects plus spiders, mites, pillbugs, and nematodes. Recent estimates put that bacterium in 75 percent of insects. Insects inherit Wolbachia through their cytoplasm-rich eggs but not through the stripped-down sperm.

To power their female-transported spread, various Wolbachia manipulate their hosts so that male offspring die, virgins bear only daughters, and certain matings prove infertile (SN: 11/16/96, p. 318).

Werren’s team examined whether such disruptions could create reproductive barriers. The researchers compared one of the species he studied earlier, the northeastern Nasonia giraulti, with the western Nasonia longicornis. In lab matings, male N. giraulti and female N. longicornis produced no viable offspring. Switching species for moms and dads permitted about 30 percent of the offspring to live. However, after three generations were treated with antibiotics, the species mated with each other as successfully as they did among themselves.

Werren speculates that geography separated ancestors of these species and each caught a different strain of Wolbachia. “This is the first clear-cut demonstration that Wolbachia can be an early barrier between species,” he says.

Ary Hoffmann of La Trobe University outside Melbourne, Australia, points out that geography separates the wasp species, too. Hoffmann adds that he doesn’t think the bacteria can split a local population. His work suggests that in a population, the more common Wolbachia infection crowds out a rare one. “It’s not as simple as, ‘You get the bug, you get new species,'” he says.

Werren agrees that the main role for the disease is probably to keep already divided populations separate.

That’s not a trivial role, notes Michael J. Wade of Indiana University in Bloomington. He points out that in one case of the 50 or so studied so far, Wolbachia contributed to insect speciation. If the bacterium factors into the creation of 2 percent of the world’s 5 million or so insect species, Wade says, “Wolbachia could play a big role.”

Susan Milius is the life sciences writer, covering organismal biology and evolution, and has a special passion for plants, fungi and invertebrates. She studied biology and English literature.