How all-female species avoid the shrinkage of their gene pool is among the animal kingdom’s great mysteries. Now biologists think they’ve discovered the trick.

According to a study published Sunday in Nature, egg-producing cells in a Aspidoscelis tellesata, a ladies-only species of whiptail lizard, contain double the standard genetic complement. They pick the healthiest set of chromosomes, preventing the loss of vital variation.

“There’s an absence of sperm, and genetic information is never provided by another source. Anything that’s lost is lost for good,” said Peter Baumann, a University of Kansas cell biologist. “If there’s a way to prevent the loss, then how is that accomplished? That’s what our paper explains.”

In all animal cells, genes are contained in DNA packages called chromosomes. Cells have two copies of each chromosome, and thus of each gene: In sexually reproducing species, one copy comes from mom, and the other from dad.

During reproduction, germ cells duplicate their chromosome set, then go through two rounds of cell division. The result is a sperm or egg cell with one copy of each chromosome. Some genetic variation is lost in the process, but it doesn’t matter, since sperm and egg soon fuse. Losses are offset by the mixing of their union.

In creatures that develop from unfertilized eggs — a process called parthenogenesis, found in A. tellesata, plus about 70 other fish, reptile and bird species — both chromosome copies come from mom. Lost genetic variation is unrecoverable, and ought to accumulate over many generations, eventually producing an animal unable to survive.

But as Baumann’s group shows, A. tesselata germ cells start with four sets of chromosome copies, not two. When the cells finish dividing, the resulting eggs contain a standard set, one that’s assembled, they found, from two loss-free copies.

“This is an elegant mechanism,” said Baumann. And though they don’t yet know how A. tesselata‘s germ cells evolved this trick, “you can imagine that it happens by a relatively simple change.” That could explain why parthenogenesis reproduction has emerged in so many species.

Whether parthenogenesis is an evolutionary aberration or viable strategy remains debated. Even if these species maintain their gene pool, they still lack the genetic mixing that gives sexual reproducers a steady flow of new adaptations.

According to Baumann, A. tesselata is lucky: It appears to be descended from a union of two related species, giving it a hybrid vigor. As for populations lacking built-in durability, he said asexual reproduction may be a useful short-term strategy. It could maintain lineages through periods of isolation, with species reverting to sexual reproduction when suitable partners were available.

However, Baumann cautioned against assuming that all parthenogenic species used the mechanisms seen in A. tesselata.

“I think it’s going to be widespread, but nature often surprises us<” he said. “We think we know how something works, then find out that nature came up with many ways of doing it.”

Images: 1) Whiptail lizards/Peter Baumann. 2) A comparison of chromosomes in the gametes of a related, sexually-producing lizard species (above) and the whiptail (below).

See Also:

Citation: “Sister chromosome pairing maintains heterozygosity in parthenogenetic lizards.” By Aracely A. Lutes, William B. Neaves, Diana P. Baumann, Winfried Wiegraebe & Peter Baumann. Nature, Advance Online Publication, February 21, 2010.

Brandon Keim’s Twitter stream and reportorial outtakes; Wired Science on Twitter. Brandon is currently working on a book about ecological tipping points.