Sex or gender is inherently not easy. “Sex” is often used reductively and factually in political jargon, but the science of chromosomes is far from a simple dichotomy. In nature, some species change their sex with age. Like the White-throated Sparrow, some birds have four sexes, or genotypes. As described in "Current Biology," this species exhibited surprising chromosomal abnormalities during evolution that make mating more complex. Zonotrichia albicollis or white-throated sparrow has white and tan forms or subspecies. In contrast to tanned males, who are monogamous and care for their offspring, white males engage in promiscuity and neglect paternal care. Females also need to consider the trade-off between investment in parental care and mating effort.
White-and-brown striped birds usually mate with individuals of the opposite color. In the penguin, a gene mutation flipped a portion of chromosome 2, forming a 'supergene'. This new type of chromosome cannot exchange genetic material with its partner. As scientists discovered in the 1960s, this is because brown birds have two identical chromosomes, while white birds have one normal chromosome and one reverse chromosome. means The white-tan pair has a 50% chance of inheriting the inverted gene.
In the early 2000s, married researchers Ilaina Tuttle and Rusty Gonzer discovered that flip genes are not simple 'flips'. Rather, the genes have become even more messed up. They hypothesised that this gene is evidence that the species is developing two new chromosomes, four in total.
“These birds are an amazing system,” says Catherine Peichel, an evolutionary ecologist at the University of Berne, told Nature. “The process of sex-chromosome evolution tends to erase much of the evidence of how it happened, so being able to watch the process in action is a huge benefit.” Sadly, Elaina Tuttle passed in 2016, shortly after the publication of her incredible paper.
Their discoveries continue to fascinate us. "The bird behaves like it has four sexes," says Christopher Balakrishnan, an evolutionary biologist at the University of East Carolina in North Carolina who worked with Tuttle and Gonser. "Individuals can only mate with a quarter of the population. Few sexual systems have more than two sexes." But this mating dance is hard work. "The fact that we never see systems with four sexes suggests they are evolutionarily unstable and one of these alleles will eventually go extinct," Balakrishnan said. We are.”
It's not just sparrows that are polysexual. Microorganisms have seven sexes. What is certain is that sexuality in nature is diverse and complex. Like white-throated sparrows, "these birds contain a lot more information," Gonser says. "And I think Elaina would be happy if we tried to uncover her secret."