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THE false spider mite is the first animal in the history of science to make
do with only one copy of its chromosomes. The discovery is overturning theories
of evolution.

The cells of most multicellular animals are 鈥渄iploid鈥濃�攖hey carry two
copies of each chromosome. This makes evolutionary sense: if a mutation strikes
a gene on one chromosome, a flawless version on the other can compensate. But no
one has ever found an animal where both sexes are 鈥渉aploid鈥濃�攃arrying a
single set of unpaired chromosomes.

Until now, that is. A team led by Andrew Weeks of the University of Amsterdam
in the Netherlands has shown that the false spider mite Brevipalpus phoenicis, a
pest of crops such as citrus fruits, tea and palms, fits the bill. 鈥淚t seems
that in biology there are exceptions to every rule,鈥� says Weeks. 鈥淣ever before
has a female from the animal kingdom been found to be exclusively haploid,鈥� he
says. Almost all the mites are female and produce only female offspring from
unfertilised eggs.

The cells of false spider mites contain two chromosomes. But before Weeks鈥檚
study, no one knew whether these chromosomes were similar, suggesting a diploid
state, or unrelated, indicating a haploid state. Using standard sequencing
techniques, Weeks鈥檚 team found the mites鈥� chromosomes to be very different. The
scientists couldn鈥檛 find two identical copies of any particular genes in the
mites. They concluded that the species is exclusively haploid.

鈥淭his is quite a surprise鈥攊t goes against the dogma I was taught,鈥� says
Sarah Otto, an evolutionary biologist at the University of British Columbia in
Vancouver. Because they鈥檇 never found an all-haploid species before, biologists
assumed that the strategy couldn鈥檛 work.

But Weeks thinks being exclusively haploid might give the animals an
evolutionary advantage. Their lack of chromosome copies might, in the long term,
help the species by ensuring that dangerous mutations kill the individuals
carrying them rather than spreading down the generations. This genetic state may
be rare simply because other diploid animals haven鈥檛 had the opportunity to make
a similar transition.

Weeks believes the mites once consisted of diploid females and haploid males.
He has shown that infection by an as yet unclassified bacterium feminised males
in the wild, possibly by blocking the secretion of a crucial male hormone. The
offspring of these infected mites would eventually develop into haploid females,
leaving few males for the diploid females to mate with.