All biology students learn that animals are diploid, carrying one set of chromosomes from their mother and one set from their father. In sperm and eggs, these double sets are halved to produce single, "haploid" ones, which unite during fertilization. But in a paper in the 29 June issue of Science, researchers from the Netherlands and the Czech Republic describe a mite that is entirely haploid. The oddity is caused by a scheming bacterium that tampers with its host's reproductive organs and turns male mites into females.
Mites are already an exception to the all-diploid rule. Like bees, wasps, and ants, these spiderlike arthropods have an unusual system for determining which egg turns into a son and which into a daughter: Unfertilized eggs develop into haploid males, while fertilized ones become diploid females, says geneticist Andrew Weeks from the University of Amsterdam. However, some species, like the citrus pest Brevipalpus phoenicis, consist entirely of females, which produce daughters via virgin birth.
When Weeks and his colleagues started to study the genetics of B. phoenicis, they discovered something strange: Whereas most related mite species have four chromosomes (two from each parent), B. phoenicis individuals have only two. Using genetic probes, Weeks showed that each chromosome carries different genes, which means they are a haploid set. While peering through his microscope, Weeks also spotted the possible culprit: Lots of bacteria of a previously unknown species were living inside the insects' cells. When Weeks killed the bacteria by feeding the mites on bean leaves floating in an antibiotic solution, the mites started producing males as well. But although they mated with the females, these males did not father any offspring.
Weeks suspects that the as-yet-unnamed bacterium manipulates the developing hormone glands in young, haploid males, turning the hapless youngsters into the unusual haploid females. This is in the bacterium's best interest. Because it lives intracellularly, it cannot easily infect new victims; instead, it needs to hitch a ride in eggs to reach the next generation. That makes it a sensible strategy to force any male host to become a transsexual and start laying eggs.
"This is very exciting work," says Ary Hoffmann, who works on similarly manipulative bacteria in fruit flies at La Trobe University in Melbourne, Australia. Researchers already knew that other bacteria species, such as Wolbachia, could meddle with their hosts' sex lives (Science, 11 May 1993). But Hoffmann says the new species may have different ways of manipulating its host, and he urges scientists to look for it in other invertebrates to see whether it has similar effects.