Mitey haploids aren't hapless after all
Friday, 29 June 2001
Not only do the adults undergo "virgin birth" , but the only time baby boys are produced is when the mother is treated with antibiotics.
The false spider mite Brevipalpus phoenicis is the only animal known to exist entirely in the "haploid" state - with only half the full complement of chromosomes, according to a report in today's Science.
Most animals are diploid, that is they carry two copies of each chromosome - except when sperm and eggs are produced. Although there are a few cases where only the males of a species are haploid.
This led to the view that the diploid state was superior, probably because the extra gene copy could "hide" bad mutations which would otherwise be expressed if the animal had only one set of genetic instructions.
But the mite is a daring exception to the rule and suggests the doubling of genes may be more an accident of evolution than a biological necessity.
Measuring just a quarter of a millimetre in length, the creature is "extremely successful" from an evolutionary point of view, says Monash University's Andrew Weeks who led the research. It lives in rainforests as well as coffee, citrus and tea plantations around the world, including Queensland.
It seems the unusual reproductive strategy arose because of infection with a bacteria, Dr Weeks told ABC Science Online. The females produce only female offspring through eggs which are all infected with an as yet unnamed bacterium.
It's thought the mite's ancestors included both males, which were haploid, and females, which were diploid. The males would have arisen from unfertilised haploid eggs.
But at some time in the evolutionary past, infection of those unfertilised eggs with bacteria had caused them to develop as females - probably by blocking the production of male hormones.
Weeks' team showed the all-female mites of today could be made to produce males if they were treated with antibiotics.
The mites are known as false spider mites to distinguish them from their close relatives, the spider mites, which spin webs.