Going it alone

The all-female lizard species Aspidoscelis tesselata successfully carries on without any males. Peter Baumann discovered how the ladies do it.

The deserts of the Southwest are home to some species of whiptail lizards that produce fertile offspring without fertilization by males. The female-only species arose from crosses between two related species that reproduce sexually. But how they—and the other 70 species of vertebrates known to propagate without the continuous mixing of genes from males and females—avoid the dangers associated with asexual reproduction has remained unclear. Yet, asexual species of whiptail lizards managed to avoid genetic monotony and disease vulnerability and hold on to a healthy mix of the founding species’ genes over hundreds of years.

When Howard Hughes Medical Institute early career scientist Peter Baumann, an associate investigator at the Stowers Institute, became interested in the question of asexual reproduction, the scientific literature turned out to be mostly silent on the molecular details of parthenogenesis. So, he and his team decided to look for answers themselves. Graduate student, Aracely Lutes, isolated nuclei from whiptail oocytes about to enter meiosis and examined their contents.

Sexually reproducing lizards use a process called meiosis to halve the number of chromosomes from 46 to 23 in egg and sperm cells. When a sperm fuses with an egg during fertilization the combined number of chromosomes is back to normal ensuring that each somatic cell in the offspring’s body contains the correct number of chromosomes. In contrast, Lutes found, asexual lizards enter the reproductive process with 92 chromosomes resulting in eggs with twice the number of chromosomes compared to their cousins. Even without fertilization these eggs result in lizards with a full complement of chromosomes.

“It raises the intriguing possibility that females of some species may switch from sexual reproduction to parthenogenesis by regulating the chromosome content of oocytes entering meiosis,” says Bill Neaves, Ph.D., President Emeritus of the Stowers Institute and one of Baumann’s collaborators. “There are cases where females from sexually reproducing species, such as Komodo dragons, sharks and snakes reproduce parthenogenetically when mating options are limited. Switching from sexual reproduction to parthenogenesis may allow a female to pass her genes on to another generation until a male comes along later.”

The team’s research also revealed how celibate lizards maintain the genetic richness resulting from the original hybridization event between two sexual species. Instead of pairing homologous chromosomes—corresponding, but slightly different chromosomes from each parent—and swapping information, they pair identical sister chromosomes—those duplicated one extra time before entering meiosis—and swap information between them. But no matter how much they mix and match, the genetic information stays the same ensuring that nothing ever gets lost.

“Now that we know that chromosome duplication and identical chromosome pairing are central to parthenogenesis we are trying to understand the events that trigger the molecular switch from bisexual to unisexual reproduction” says Baumann.