As controversy about eligibility requirements for women’s boxing plagues the Olympics, many cling to what they learned in high school biology: inheriting XY chromosomes makes a male; XX makes a female. However, this explanation for sex inheritance lacks nuance and does not account for the genetic diversity actually seen in human sex determination. Conversations about sex as a spectrum are often ignored by the general public, but what does the science say? Let’s look to sex-changing fish to see what we can learn.
Over 400 fish species display mutable sex characteristics: some exist in hermaphroditic states, self-fertilizing to reproduce, while others undergo sex changes during their lifetime. Some fish can even switch their sex back and forth. Sex determination of fish can therefore be mutable depending on the context. For example, in some fish species, a female will transform from to male when the alpha male in the community dies or disappears. This sex change allows the female-turned-male fish to take over as alpha, mating with females in the community to increase its inclusive fitness, an evolutionary biology term that measures how many offspring an organism produces. In other situations, environmental factors affect sex development and determination. For example, high environmental temperatures alter the development of stem cells in Japanese rice fish, allowing XX embryos, which typically develop female sex characteristics, to develop testes. Altogether, fish demonstrate just how complicated and diverse sex determination can be.
You may be thinking, sure, fish might be complicated, but humans are simple. That isn’t so. For example, some humans inherit a mutation in a gene called 5 alpha reductase, an enzyme that converts testosterone into a stronger form, called dihydrotestosterone (DHT). Individuals who inherit this mutated gene are born with underdeveloped sex organs and are often assigned female at birth and raised female until puberty. However, at puberty, the massive flux of testosterone overcomes the body, enabling the development of a penis and testes. The question becomes: how do we define sex on a binary, when someone raised as female their whole life suddenly grows a penis?
Sex determination and its definition are somewhat messy, but one thing is clear: life of all forms is as fascinating as it is beautiful.
These studies were led by Laura Casas and Fran Saborido-Rey at the Institute of Marine Research, Mateus Contar Adolfi with Manfred Schartl at the University of Wuerzburg, and R.L.P. Nascimento with U. Barroso Jr. at the Federal University of Bahia.
Managing Correspondent: Nina Benites
Press Article:
Fish are the sex-switching masters of the animal kingdom (BBC Earth)
5-alpha reductase deficiency: how it affects hormones (VeryWell)
Research Article:
Environmental Cues and Mechanisms Underpinning Sex Change in Fish (Sex Development)
Increase of cortisol levels after temperature stress activates dmrt1a causing female-to-male sex reversal and reduced germ cell number in medaka (Molecular Reproduction and Development)
Gender identity in patients with 5 alpha reductase deficiency raised as females (Journal of Pediatric Urology)
Image Credit: WikiMedia
