Lab-Created All-Female Lizard Species

The researchers bred a new species of all-female lizard, mimicking a process that occurred naturally in the past but has never been directly observed.

“It’s about recreating the events that lead to new species,” said cell biologist Peter Baumann of the Stowers Institute for Medical Research, whose new species is described May 3 in the Proceedings of the National Academy of Sciences. “It relates to the question of how these unisexual species arise in the first place.”

Female-only species that reproduce by cloning themselves — a process called parthenogenesis, in which embryos develop without fertilization — were once thought of as dead-end evolutionary flukes. But over the past decade, unisexuality has been found in over 80 groups of fish, amphibians and reptiles. It may not be such a dead end after all.

The best known of all unisexual species are Aspidoscelis, the whiptail lizards of southwestern North America, of which 7 of the 12 species are unisexual. Genetic studies suggest that their unisexuality arose from historical unions of two sexually reproducing lizards belonging to closely related species, whose hybrid offspring possessed mutations necessary for parthenogenesis.

In two of the unisexual whiptails, that seems to have been enough; they immediately became entirely feminine. In the other five, another cycle of traditional sexual mating was required. These species are said to be triploid, carrying two sets of chromosomes from the original parent species and one from the father.

But despite all the evidence of these historical hybridizations, it has been remarkably difficult to observe in the present. When new hybrid whiptails have been found in the wild, they have invariably turned out to be sterile. The same goes for laboratory efforts, including the one that lasted 29 years and involved 230 lizards of nine species. The researchers faced a conundrum: although adding chromosomes is clearly possible, it’s a disaster every time you see it.

>Baumann’s team has yet to decide what to name their new species, which in March had 68 females with more eggs on the way.

“There are recognized species for which this hybridization event occurred 100,000 years ago,” Baumann said. “But there are also hybrids that have appeared in the last five years. If you go to New Mexico and look around, you can find them. They also appeared in the lab, but they are sterile. “

There was, however, a historic hint of hybrid success. In 1967, a captive A. bloodless female, triploid and parthenogenetic, successfully mated with a male A.inornata. A female offspring laid eggs. They were untreated, but Baumann and his colleagues suspected they might have developed.

In the new study, they revisited that experience, mating again A. bloodless with A.inornata. This time it worked conclusively. Six eggs were retrieved and incubated, producing four hybrid females. All continued to clone themselves. These offspring are now in their fourth generation, perfectly healthy and representing “proof of principle” for how new parthenogenetic lizards might evolve in the wild.

Baumann’s team has yet to decide what to name their new species, which in March had 68 females with more eggs on the way. More pressing than a name is the pursuit of the study. “What is the fundamental difference between these lizards and all the hybrids that have been examined over the past 40 years? he said.

This is a question with multiple implications. Baumann’s expertise is in cell division; comparing sexual cell division, known as meosis, in the new species with other infertile lizards may reveal mechanisms that are still unclear. “By comparing and contrasting meiosis in different species, I realized how little we know about meiosis in any organism,” he said.

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