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Microbe lets mite dads perform virgin birth

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11:25am, June 27, 2001
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For the first time, biologists have found an animal, a gender-bent mite, living and procreating quite well despite the supposed handicap of carrying genes from just one parent.

Among other animals, the mom-and-pop plan rules, explains Andrew R. Weeks of Monash University in Clayton, Australia. People, parrots, perch, polyps, and all the animals in between usually inherit one copy of a set of chromosomes from each of their parents, a system called diploidy.

Arthropods do push the limits. Among many ants, bees, wasps, mites, and thrips, for example, unfertilized eggs develop into haploid males, which have just one set of chromosomes. However, their sisters retain the double set of chromosomes, and both parts of it come into play during reproduction.

The suspicious case of the plant-eating mite Brevipalpus phoenicis has teased biologists for decades. This mite exists mainly in a female form that procreates without bothering to have sex. Each individual carries two chromosomes. Is the mite diploid, carrying two copies of a single chromosome, or haploid, carrying one copy of a two-chromosome set?

Now, Weeks and his colleagues report that the mite's two chains of genes differ from each other. The experiments, Weeks contends, demonstrate that the mites are haploid. So far, nobody has seen or created a diploid B. phoenicis.

"It's really hard to argue with their conclusions," says Sarah Otto of the University of British Columbia in Vancouver. "This is the first example of an animal that lives and reproduces mainly in a haploid form."

Biologists have fretted for decades over the tradeoffs of single- and double-strength chromosomes. Bacteria, many fungi, and certain stages of plants survive well as haploids. However, conventional wisdom holds that the perils of developing a big body, with lots of cell division, favor a redundant set of chromosomes. "Many people consider diploidy to be the pinnacle of evolution," Otto says.

The researchers also report in the June 29 Science that these virgin-mother mites are actually males in the grip of a severe bacterial infection.

The mite, roughly 250 micrometers long, first captured scientific interest by attacking a wide range of beloved crops, including coffee, citrus, and palms.

To sort out B. phoenicis' genetics, Weeks and his collaborators turned to techniques that make various genetic materials fluoresce under a microscope. The researchers' special stains and other techniques detected a complex called a nuclear organizing region on one chromosome but not the other.

The researchers also screened about 10 mites from each of 45 laboratory-raised lineages. Even though the seven chromosome spots tested consist of a type of DNA that varies readily, the researchers found these genetic regions to be identical within each line.

During the studies with fluorescence, Weeks noticed abundant bacteria infesting the specimens. He managed to coax lab mites to eat leaves treated with tetracycline, which cured some of the mites of their bacterial infection. In the next generation, Weeks found that the bacteria-free mites grew up morphologically male, similar to closely related mite species. He's watching to see if they ever mate with the bacteria-carrying mites.

Weeks and his colleagues discovered that the influential bacterium doesn't belong to the much-discussed Wolbachia genus of gender-benders (SN: 11/16/96, p. 318) but comes from a different phylum. The newly identified species represents only the second bacterium found to disrupt gender and reproduction.

Molly S. Hunter of the University of Arizona in Tucson says that the bacterium Weeks isolated is "essentially the same organism" as the one she and her colleagues have found causing virgin births in wasps.

She predicts that the discovery of this second reproduction-disrupting bacterium will shake up the field. "If it's like Wolbachia, that will be pretty interesting," she says. "If it's not like Wolbachia, that will be pretty interesting, too."

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