No Sex Needed: All-Female Lizard Species

"Since the 1960s scientists have known that some species of whiptail lizards need a male even less than a fish needs a bicycle. This all-lady lizard species (of the Aspidoscelis genus) from Mexico and the U.S. Southwest manage to produce well-bred offspring without the aid of male fertilization.

But how do they—and the other 70 species of vertebrates that propagate this way—do it without the genetic monotony and disease vulnerability that often results from asexual reproduction? "It has remained unclear" and "has been the topic of much speculation," report a team of researchers who aimed to answer just that question. Their results were published online February 21 in the journal Nature. (Scientific American is part of Nature Publishing Group.)

These lizards and other "parthenogenetic species are genetically isolated," explains Peter Baumann, an associate investigator at the Stowers Institute for Medical Research in Kansas City, Mo., and co-author of the study. Species as diverse as Komodo dragons and hammerhead sharks do it asexually if necessary, but some species, like these little lizards, don't have a choice. "They can't exchange genetic material, and this loss of genetic exchange is a major disadvantage to them in a changing environment," he says. Unless an animal can recombine the DNA they already have, they will produce an offspring with an identical set of chromosomes, in which any genetic weakness, such as disease susceptibility or physical mutation, would have no chance to be overridden by outside genetic material from a mate.

The new research by Baumann and his team reveal that these lizards maintain genetic richness by starting the reproductive process with twice the number of chromosomes as their sexually reproducing cousins. These celibate species resulted from the hybridization of different sexual species, a process that instills the parthenogenetic lizards with a great amount of genetic diversity at the outset. And the researchers found that these species could maintain the diversity by never pairing their homologous chromosomes (as sexual species do by taking one set of chromosomes from each parent) but rather by combining their sister chromosomes instead. "Recombination between pairs of sister chromosomes maintains heterozygosity" throughout the chromosome, noted the authors of the study, which was led by Aracely Lutes, a postdoctoral researcher in Baumann's lab."

http://www.scientificamerican.com/article.cfm?id=asexual-lizards