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Survey of five Australian avians finds numerous discordant individuals, including a genetically male bird that had laid an egg
When it comes to telling males and females apart, many bird species reject subtlety altogether. Roosters stand out thanks to their big, bright comb and ear-splitting “cock-a-doodle-doo.” Bachelor birds-of-paradise flaunt their vibrant plumage to attract more subdued females. And the male peacock’s feathered train is so ostentatious it famously threw even Charles Darwin for a loop.
But that’s not the case for all bird species. When males and females look pretty much the same, scientists must try harder—often using DNA testing—to separate the sexes. According to a new study of wild Australian birds, these methods may be leading to misidentification in cases where an individual’s gonads and outward appearance don’t align with the genetic sex determined by its chromosomes. As scientists report today in Biology Letters, this phenomenon—known as sex reversal—may be more common than anyone expected.
The discovery is likely to “raise some eyebrows” (or is it ruffle some feathers?), says Blanche Capel, a biologist at Duke University who wasn’t involved in the new work. Although sex determination is often viewed as a straightforward process, she explains, the reality is much more complicated.
In humans, individuals with XX chromosomes typically develop as female, whereas those with XY chromosomes are usually male. But Judith Mank, a zoologist at the University of British Columbia, notes it’s the genes carried on those chromosomes—not the chromosomes—that are the main players. The SRY gene on the Y chromosome, for example, kick-starts male development in mammals. Anyone missing this key gene will end up developing as female, even if they have XY chromosomes. “We think of sex chromosomes as being sex determining,” says Mank, who also wasn’t involved in the new research. “That’s not true.”
What’s more, it can matter how these genes are expressed on a cell-by-cell basis. In some species such as fruit flies, zebrafish, and chickens, individual cells have their own sexual identity based on the genes they happen to contain or express, rather than being influenced by the body’s overall hormone levels. When different cells contain different sets of chromosomes, this process can give rise to individuals called gynandromorphs, which exhibit both male and female characteristics.
Environmental factors can also complicate sex determination. Turtle eggs that incubate at cooler temperatures, for example, produce male hatchlings, whereas turning up the heat churns out females. Although rare in birds, this phenomenon has been seen in brush turkeys, which incubate their eggs in enormous mounds.
As a result, scientists “have known for a long time that there are other external factors that go into the development of sex characteristics in birds,” says study senior author Dominique Potvin, an animal ecologist at the University of the Sunshine Coast. Even so, it was unclear how often birds have the physical features of one sex but the genetic makeup of the other.
To find out, Potvin’s team dissected and examined the bodies of nearly 500 birds belonging to five common Australian species: the Australian magpie, laughing kookaburra, crested pigeon, rainbow lorikeet, and the scaly breasted lorikeet. All the animals had died after being admitted to wildlife hospitals in Queensland because of unrelated illness or injury. In addition to identifying the birds’ reproductive organs, researchers also tested their DNA to reveal their genetic sex.
The team was surprised to find sex-reversed individuals in all five species, at rates of 3% to 6%. Nearly all these discordant birds were genetically female but had male reproductive organs. However, the researchers also found a few genetic males with ovaries—including a genetically male kookaburra with a distended oviduct, indicating it had recently laid an egg.
The findings suggest sex reversal is more common in wild birds than previously thought, which may have implications for the conservation of threatened species. It’s often important to know how many males and females there are in a population, because sex ratios influence the group’s ability to reproduce, grow, and sustain itself. “If you’re going off the genetic sex ratio,” Potvin says, “you may end up being surprised.”
Capel, for her part, isn’t sure the rates of sex reversal found in the new study would be high enough to hamper a bird population’s ability to survive and reproduce. Even so, the findings suggest researchers’ DNA-based methods of identifying bird sex in the wild are less accurate than once thought. In addition, knowing wild birds’ baseline rates of sex reversal could help scientists gauge when rates are unnaturally high, possibly because of endocrine-disrupting chemicals in the environment.
- Did you hear about those other birdies?
“Now that we know discordance occurs, the next big question will be, what is driving this discordance in birds?” says Clare Holleley, an environmental biologist at Australia’s Commonwealth Scientific and Industrial Research Organization who wasn’t involved in the new work. “Is it chemicals, is it environmental stress, or some other factor that can alter developmental trajectories?”
Potvin hopes these findings will inspire scientists in other regions of the world to study sex reversal in a wider range of bird species, as well as investigate the brains of discordant individuals. Potvin’s work primarily focuses on birdsong, so she wonders how sex reversal might impact vocalization—especially in species where only the males can carry a tune. “We know a lot about the physiology,” she says. “We don’t know how it impacts the individuals and especially the populations as a whole.”
https://www.science.org/content/article/sex-reversal-surprisingly-common-birds-new-study-suggests
