Undesirable sex partners; bacteria manipulate reproduction of insects and other species - Wolbacteria cause cytoplasmic incompatibility in arthropods

Science News, Nov 16, 1996 by John Travis

An explanation rests in the fact that wasps, like bees and ants, have an unusual mechanism for determining sex. In wasps, eggs fertilized by sperm contain a maternal and a paternal set of chromosomes and develop into females. Unfertilized eggs, with only a maternal chromosome set, develop into males.

When an infected male jewel wasp mates with an uninfected female, the paternal chromosomes from the sperm seem to fragment and fail to join the maternal set, says Werren. Consequently, only males result from such a mating. This indirectly aids the spread of Wolbachia by reducing the number of uninfected daughters produced by uninfected females, explains Werren.

Wolbachia sometimes takes a more feminist approach. In many parasitic wasps, which lay their eggs in developing insects that they have killed, Wolbachia infections eliminate the need for males. An infected female reproduces via an asexual process known as parthenogenesis. The unfertilized eggs simply duplicate their one set of chromosomes and develop into females.

These parthenogenetic wasps had long been a biological curiosity until a few years ago, when Richard Stouthamer, working with Werren, showed that the phenomenon stemmed from Wolbachia infection. With antibiotics, "you can cure a line of its parthenogenesis and make it sexual," says Stouthamer, now at the Wageningen Agricultural University in the Netherlands.

The clear preference for females isn't limited to Wolbachia strains that infect wasps. At the Bar Harbor meeting, Thierry Rigaud of the University of Poitiers in France, reported finding the bacteria in the wood louse Armadillidium vulgare.

In these lice, Wolbachia frequently overrides genetic inheritance. The bacteria, says Rigaud, "feminize" an embryonic wood louse that is genetically male by disrupting the production or effects of masculinizing hormones during its development. The increased number of daughters allows Wolbachia to spread quickly.

While scientists continue to tally the animals that Wolbachia infects, as well as the outcomes of those infections, they are also trying to unravel the mechanisms by which Wolbachia distorts its host's reproduction. Are cytoplasmic incompatibility, parthenogenesis, and feminization distinct strategies pursued by Wolbachia, or are they merely reflections of different ways in which host species react to the bacteria?

As the best-known phenomenon, cytoplasmic incompatibility has received the most attention. An initial hypothesis to explain it was that proteins made by Wolbachia in the testes bind to the chromosomes inside sperm and disrupt their later union with maternal chromosomes.

That simple idea has a major flaw, however. Researchers haven't found any Wolbachia molecule incorporated into sperm. Nor are whole bacteria present in the sperm. As sperm mature, they lose most of their cytoplasm and squeeze out any Wolbachia.

The latest theory about cytoplasmic incompatibility comes from Timothy L. Karr of the University of Chicago. He argues that Wolbachia's influence during the maturation of sperm somehow disturbs the carefully choreographed maneuvering that later brings together the sperm's chromosomes with the egg's. "It looks like the paternal and maternal chromosomes are out of sync," says Karr.