Illinois research team reveals ‘virgin birth’ in a crocodile

Champaign, Ill. — A female crocodile living in isolation for 16 years at a Costa Rican zoo laid a clutch of eggs, a common practice among captive reptiles, even those without mates. After three months of incubation, one egg contained “a fully formed stillborn baby crocodile,” a team of scientists from the Illinois Natural History found.

Mark Davis, a conservation biologist at INHS and co-author on the study, helped examine the crocodile fetus’ genomic makeup, ultimately discovering that the fetal genome resulted from reproduction without a male crocodile’s genetic contribution.

“This discovery speaks to the inferential strength of our modern genomics era,” Davis said. “We couldn’t perform this without the powerful genomic advancements made in the last decade, which allowed us to leverage a massive amount of genomic data, rendering these data irrefutable.”

Davis leads the Collaborative Ecological Genetics Lab at the University of Illinois, which uses conservation genomics to assess the evolution of modern biodiversity, monitor biodiversity via environmental DNA (eDNA).

“It’s wild, but the data and the science confirm that this is what happened,” Davis said. “There are animals out there in our world – even in Illinois – that are parthenogens, or can produce offspring without a mate, but this is the very first time that anyone has confirmed this in crocodilians.”

Davis is referring to the asexual, all-female silvery salamander which is found in Vermillion County and reproduces via kleptogenesis, a close kin of parthenogenesis. The nest was constructed in a Costa Rican Zoo in January 2018, but this discovery was postponed for two years due to a lengthy permitting process required to deliver biological samples of the fetus from Costa Rica into the United States.

And discovery would be delayed once again with the COVID-19 pandemic, causing science labs worldwide to close their doors temporarily, and the scientific discoveries to come to a halt.

Before the first DNA samples made it to U.S. soil, Davis admits he was originally skeptical. Parthenogenesis is a rare phenomenon that is thought to occur when a species faces challenging or unfavorable conditions, such as environmental stress or lack of mates.

Crocodiles’ sex is not determined by their chromosomes, but instead by the temperature of the environment at which they are incubated, which adds complexity and nuance to this phenomenon, Davis said.

“Not only is this the first time that this has happened in a crocodile, but it also has never been documented in a temperature-dependent sex determination (TSD) species,” Davis said.

It has been documented that global warming has disrupted sex ratios in TSD species, with warmer environments producing more female offspring, skewing sex ratios for turtles and crocodiles. Davis notes that it’s too early to tell if there’s a link between parthenogenesis in TSD species and a warmer climate.

“As we continue to see sex ratios shift in TSD species, they could possibly do this to survive, but that level of certainty is not available right now,” Davis said.

Parthenogens inherit their asexual ability from a common ancestor, which introduces a possibility that this phenomenon could have happened in pterosaurs and dinosaurs. However, without the ability to retrieve dinosaurs’ and pterosaurs’ DNA analysis, questions remain.

Nevertheless, the confirmation of parthenogenesis in crocodiles represents a seismic shift in our understanding of this phenomenon in vertebrates.

Tiffany Jolley is a communications specialists at the Prairie Research Institute.