With the frequency of gender fluidity in the news (often disparagingly), it’s helpful to read that it’s something that exists in many areas of the natural world.
“Nature’s dealing with conformity all the time in brutal ways and loving ways and all the rest of it,” Dr. Dreger said. “It doesn’t follow the human fantasy of everybody having to be normal. And humans don’t follow that ridiculous idea either.”
All serious butterfly collectors remember their first gynandromorph: a butterfly with a color and pattern that are distinctly male on one wing and female on the other.
Seeing one sparks wonder and curiosity. For the biologist Nipam H. Patel, the sighting offered a possible answer to a question he had been pondering for years: During embryonic and larval development, how do cells know where to stop and where to go?
He was sure that the delicate black outlines between male and female regions appearing on one wing — but not the other — identified a key facet of animal development.
“It immediately struck me that this was telling me something interesting about how the wing was being made,” said Dr. Patel, a biologist who now heads the Marine Biological Laboratory, a research institute in Woods Hole, Mass., affiliated with the University of Chicago.
The patterning on the gynandromorph’s wing shows that the body uses signaling centers to control where cells go during development and what tissues they become in creatures as diverse as butterflies and people, Dr. Patel said.
Gynandromorph butterflies and other half-male, half-female creatures, particularly birds, have fascinated both scientists and amateurs for centuries. The latest sensation was a half-red, half-taupe cardinal that became a regular visitor in the backyard of Shirley and Jeffrey Caldwell in Erie, Pa. Although the bird would have to be tested to confirm that it is a gynandromorph, its color division strongly suggests that it is, scientists say.
Split-sex creatures are not as unusual as they may seem when one discovery goes viral, as the cardinal’s did. It extends beyond birds and butterflies to other insects and crustaceans, like lobsters and crabs.
Scientists say these instances of split-sex animals and insects could offer clues to why some human diseases strike one sex more than the other.
Researchers thought they had figured out the genetics of birds and bees, but gynandromorphs suggest that there is more to learn, said Jennifer Marshall Graves, a distinguished professor at La Trobe University in Melbourne, Australia.
Mammals have X and Y chromosomes, birds and insects have Z and W, and some reptiles can change their sex depending on temperature, or a combination of temperature and sex chromosomes, she said.
It was believed that the sex of a bird was determined by a protein made by the DMRT1 gene, which would reach all the cells of the bird through the bloodstream, Dr. Graves said. But for two sides of the bird to share the same bloodstream but not the same sex, there must be more to the story.
Hormones can’t be the sole drivers of sex either, but they most likely play some role, said Arthur Arnold, a distinguished research professor at the University of California, Los Angeles. In a paper published in 2003 in PNAS, Dr. Arnold showed that in gynandromorphic zebra finches, brain cells on the female side were more masculine than comparable cells in a typical female.
How gynandromorphs are born at all still remains a mystery. For birds, the most likely explanation is that a female makes an unusual double-nucleus egg cell, one with a Z chromosome and one with a W chromosome, and each is fertilized by a Z sperm, making some cells ZZ and others ZW in the same individual, Dr. Arnold said.
“Although this happens regularly, it’s very rare,” he added. Gene editing is tricky in birds, so it has not been possible to experimentally induce this phenomenon in birds, and it’s not well understood, he said.
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