Researchers have bred a new species of all-female lizard, mimicking a process that has happened naturally in the past but has never been directly observed.
“It’s 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 considered dead-end evolutionary flukes. But in the last decade, unisexuality has been found in more than 80 groups of fish, amphibian and reptiles. It might not be such a dead end after all.
Best-known among all unisexual species are Aspidoscelis, the whiptail lizards of southwestern North America, of which 7 of 12 species are unisexual. Genetic studies suggest their unisexuality emerged from historical unions of two sexually-reproducing lizards belonging to closely-related species, the hybrid offspring of which possessed mutations needed for parthenogenesis.
In two of the unisexual whiptails, that seems to have been enough; they immediately went all-female. In the other five, it took another round of traditional sexual mating. Those species are so-called triploids, bearing two sets of chromosomes from the original mother species and one from the father.
But for all the evidence of these historical hybridizations, it’s been remarkably difficult to observe in the present. When new hybrid whiptails have been found in nature, they’ve invariably proved sterile. The same goes for laboratory efforts, including one that lasted for 29 years and involved 230 lizards from nine species. Researchers were left with a conundrum: Though adding chromosomes is clearly possible, it’s a disaster whenever seen.
“There are recognized species for which that hybridization event occurred 100,000 years ago,” said Baumann. “But there are also hybrids that have arisen in the last five years. If you go to New Mexico and look around, you can find them. They’ve also arisen in the lab, but they’re sterile.”
There was, however, one historical hint of hybrid success. In 1967, a captive A. exsanguis female, triploid and parthenogetic, successfully mated with a male A. inornata. One female offspring laid eggs. They weren’t cared for, but Baumann and colleagues suspected that they might have developed.
In the new study they revisited that experiment, again mating A. exsanguis with A. inornata. This time, it conclusively worked. Six eggs were recovered and incubated, producing four hybrid females. All went on to clone themselves. Those offspring are now into their fourth generation, fully healthy and representing “a proof of principle” for how new parthenogenetic lizards could evolve in nature.
Baumann’s team hasn’t yet decided what to name their new species, which as of March numbered 68 females with more eggs on the way. More pressing than a name is continued study. “What is the fundamental difference between these lizards and every hybrid that’s been examined in the last 40 years?” he said.
It’s 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 could reveal as-yet-unappreciated mechanisms. “By comparing and contrasting meiosis in different species, I’ve gained an appreciation for how little we know about meiosis in any organism,” he said.
If this laboratory hybridization proves analogous to naturally-occurring moments of hybridization, it could support the notion that unisexuality is not an evolutionary dead end. Baumann’s lizards have effectively just received an influx of genetic mutations, providing variety unavailable to self-cloners. He wonders if some lizard lineages might actually alternate between sexual and unisexual reproduction, depending on the pressures of each era.
“Is it really the case that, once a species is unisexual, it’s set in stone, and it will be that way until it dies out?” he said. “Or is it there a chance that material in unisexual lineages could find its way back?”
Image: 1) One of the hybrid offspring. (William B. Neaves). 2) On the left half of the image, a hybrid parthenogenetically-reproducing daughter rests between her parent species; at right are second, third and fourth generation descendants of the hybrid. (PNAS).