First All-Female Species Discovered

ABCNews.com

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."

The Sex-Changing Bug

Until now, scientists have assumed that no species would evolve to be entirely haploid. The thinking has been that being diploid and having that spare set of genes—as humans do—provides a critical buffer against mutations that can occur during development. Particularly in big animals (like people) that undergo lots of cell division, if one gene turns sour, there's a spare to back it up.

So how could a species—made up entirely of organisms with single gene copies—possibly be better off?

In fact, the mites may not be better off. Instead, it's likely another creature is reaping the benefits.

Weeks and his colleagues were shocked to find the eggs of the false spider mite were laced with bacteria. Wondering what role the bacteria might play, the scientists treated the mites' eggs with antibiotics to see if any changes might occur.

Big changes did happen. Eggs that remained infected developed into females, as expected, but the cured offspring became males.

Although Weeks doesn't yet understand how the bacteria changes the sex of a developing mite, he suspects he knows why.

Sperm are too tiny in size to transmit an infectious bacteria from parent to offspring. But a female's eggs are roomy enough to host a bacterium just fine. By possibly tinkering with hormones and keeping the mite population all female, the bacterium ensures the survival of its future generations.

One other bacterium, called Wolbachia, is known to play a similar trick. These bugs can transform young male pill bugs (the critters commonly found under a wet log) into females. Weeks isn't sure whether Wolbachia and this newly discovered bacterium work in the same way, but he suspects they might.

"It would make sense they both use a similar mechanism, but I am just guessing at this stage," he says.

Men Need Not Fear

Both Otto and Weeks are interested in long-term observations of the false spider mite to find out if there are any possible benefits to being all haploid and all female. If the females do flourish, even under environmental stresses, then the sex-changing bacterium may not be the only trigger behind the mites' unique evolution.

And, as for any men who may be concerned about a similar bug infecting human populations, rest assured, the scientists say this is very unlikely.

"We have something called genomic imprinting which would prevent anything like this from ever happening to humans," says Weeks. "But then again, who knows?"

(c) 2001 ABCNews.com