First All-Female Species Discovered
July 2, 2001

Imagine a female-only world where all newborns are clones of their mothers and any incidental males immediately undergo sex changes to restore the all-female population.

Now there's no need to imagine.

Dutch researchers announced recently they have discovered a species of spider mites that, with the unsolicited help of a certain female-preferring bacterium, exist exactly that way.

"I think this isolated case is probably quite rare as this is the first time that it has been discovered," says Andrew Weeks, a biologist at the University of Amsterdam in the Netherlands and co-author of a new study about the mites in the June 29 issue of the journal Science.

Males, Optional

In fact, there are hundreds of species that have no use for males, including other insects, lizards, snakes, and fish (although no mammals have discarded the male). The females reproduce by laying unfertilized eggs that contain copies of their own genes.

What's different about Brevipalpus phoenicis or the "false spider mite," which feasts on coffee, tea, and citrus plants in sub-tropical regions such as Brazil, Mexico, California, and Florida, is its offspring would normally include males. But a certain, yet-unidentified bacterium, has changed all that.

The spider mite, the scientists discovered, belongs to a class of organisms labeled as haplo-diploid. The term refers to the female's ability to reproduce with or without fertilization. In people, the X and Y chromosomes determine sex, but in these species, sex is determined by whether or not an egg is fertilized.

In most haplo-diploid creatures, eggs that are fertilized and that carry two copies of each gene are known as diploid and are female. And those eggs that are not fertilized and have only one copy of each gene are called haploid, and are male.

But, strangely enough, in this unusual spider mite, the population is made up of haploid females that all carry only one copy of each gene.

"This is unprecedented," says Sarah Otto, a zoologist at the University of British Columbia in Vancouver, Canada. "It's always fascinating to find an exception to a pattern. It suggests underlying assumptions could be based on a false premise."

Continued on Next Page >>



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