Male or Female? How Evolution Rewired Sex Determination in One Frog Species

Early in development, many animals pick a team — male or female — based on their genetics, and, with time, acquire the characteristics to match. New research from the Marine Biological Laboratory (MBL) explores how one species of frog evolved its own distinctive genetic system for determining sex.

The study, published in PLOS Genetics, reconstructs the evolutionary emergence in frogs of a gene, dm-w, whose ancestor is a key regulator of sexual differentiation in humans and many other vertebrates. The study reveals how the function of one gene can change over time in dramatically different ways, depending on whether it is in a male or a female individual.

"Other than survival, what’s more important than reproduction from an evolutionary perspective? You need to be able to pass on your genes,” said senior author Ben Evans, an MBL Whitman Center Fellow and an evolutionary geneticist at McMaster University. “Yet, the orchestration of fertility and sexual differentiation can evolve incredibly quickly, despite the potentially devastating risks of change."

The study is the result of a decade-long collaboration between Evans’ group and Marko Horb’s team at MBL’s National Xenopus Resource (NXR), a hub for frog-related research, which Horb, an MBL senior scientist, directs.

Rewinding evolution

In the frog species Xenopus laevis, a gene called dm-w prompts female development. Without its influence, the frogs become male. Evans and Horb set out to reconstruct the evolution of this gene, which arose recently in these frogs. At the outset, they knew dm-w derives from the ancestral gene dmrt1, which regulates the development of male sexual characteristics in humans and many other animals. The question was how.

An unusual event about 20 million years ago, when one of X. laevis’s ancestors acquired double its normal genetic material, set up the conditions for dm-w to emerge. This duplication event endowed X. laevis with two copies of essentially every gene.

Consequently, that ancestral frog acquired two versions of the gene dmrt1 — dmrt1.S and dmrt1.L. To explore how they changed over time and in each sex, the researchers methodically shut down both S and L in X. laevis. They also inactivated dmrt1 in X. tropicalis, a related frog that stood in for X. laevis’s ancestor.

This approach revealed a striking divergence in functional evolution, with each version following a different trajectory in males versus females.

In females, the L version of dmrt1 kept the same role. X. tropicalis females, the proxies for the ancestral form, did not produce eggs when dmrt1 was shut off. Likewise, female X. laevis lacking dmrt1.L looked normal at first, but inside them, researchers found yellow fat instead of eggs. "Normally, when you open up a female, you see eggs everywhere," Evans says.

A different scenario played out in males. Shutting down dmrt1 had no effect on X. tropicalis. However, the loss of dmrt1.L in X. laevis led to abnormal and drastically reduced sperm— indicating this gene had assumed a new role.

“Dmrt1.S, meanwhile, remained nonessential for sperm production, like its predecessor dmrt1. But while dmrt1 had been necessary for egg production, X. laevis no longer needed the descendant S version to make eggs,” Evans says.

One of my favorite parts of the study is that the same gene can have radically different functions, depending on its context – specifically, whether that gene is in a male or a female individual,” Evans said.

This change, when the S version became unnecessary for female fertility, opened the door to something more dramatic, the researchers argue. Now, this gene had the freedom to evolve without harming an animal’s prospects for reproduction. In time, dmrt1.S itself was partially duplicated, giving rise to a new gene: dm-w, which then became the X. laevis’ new master switch for sex.

A genetic tipping point

The change to dmrt1.S in females pushed sexual determination past a tipping point, Evans said. “By becoming nonessential, a copy of this gene was able to hijack the entire system that determines whether an individual develops into a female or a male.”

This study would not have been possible without the NXR, according to Evans, who said he could not have raised Xenopus for these experiments in his own lab. Instead, he visited MBL two or three times a year for extended periods. Horb and his team produced the genetically modified frogs, cared for them, and collaborated with Evans on the experiments.

"We are a platform to help researchers develop their work, so they don’t have to worry about husbandry or animal care," Horb said of the NXR. "That's what we specialize in."

This research was funded by the National Institutes of Health (Office of Research Infrastructure Programs and National Institute of Child Health and Human Development) and the Natural Science and Engineering Research Council of Canada.

Citation:

Lindsey M. Kukoly, et al. (2026) Sex-specific functional evolution of Dmrt1 in African clawed frogs (Xenopus), and the importance of genetic tipping points in developmental biology. PLOS Genetics, DOI: 10.1371/journal.pgen.1011992

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The Marine Biological Laboratory (MBL) is dedicated to scientific discovery – exploring fundamental biology, understanding marine biodiversity and the environment, and informing the human condition through research and education. Founded in Woods Hole, Massachusetts in 1888, the MBL is a private, nonprofit institution and an affiliate of the University of Chicago.