Every animal was thought to carry at least one backup copy of each of its chromosomes. No longer - researchers have discovered that an entirely female species of mite has only one copy of its chromosomes1.

The mite, Brevipalpus phoenicis, owes its singular state to a bacterium that turns males into females. This enables the mite to do without unproductive males, a situation that presumably allowed the infected asexual form to outdo its sexual forebears.

This strategy may also have contributed to the mite's current status as a thriving pest of crops including coffee and citrus fruits. "This species has been extremely successful, with a worldwide distribution and over 100 different plant hosts," says Andrew Weeks of the University of Amsterdam.

The bacteria perform their male-morphing trick because males are no use to them - only eggs transport the bug through the generations.

Weeks and his colleagues used DNA staining and fingerprinting to prove that the two chromosomes of B. phoenicis are not a matching pair. Treating eggs with antibiotics caused the mites to develop into males.

Evolutionary biologist Sally Otto of the University of British Columbia, Vancouver, says that this study will "challenge the presumption that all animals are diploid". Some have suggested that diploidy (having two copies of each chromosome) is advantageous because one set can go wrong without disastrous effects.

Instead, diploidy may be a 'frozen accident'. Perhaps, once it arose over one billion years ago, the machinery of cell division and development became unable to cope with chromosomes unless they were in pairs. "[Animals] may not be diploid because it is favourable, but because they can no longer lose it," says Otto.

By masking harmful mutations, diploidy allows them to accumulate and pass through the generations, points out Philippe Jarne, who works on the population genetics and mollusc evolution at the University of Montpellier, France. We'd be lost without our backup genes, he says: "If you were a diploid, and were suddenly put into the haploid [single-chromosomes] state, you'd probably die two or three times over."

Its haploid state may help B. phoenicis to avoid one of the pitfalls of asexuality: the build-up of harmful mutations without the genetic shuffling of sexual reproduction. "Haploid organisms are very efficient at purging deleterious mutations," says Jarne.

The mite's populations would once have included males with one set of chromosomes and females with two. This condition has evolved many times in mites and insects, such as ants.