Bilateral gynandromorphs

FROM the right-hand side, H E Schaef’s chicken looked like any normal cockerel, with a bright red comb and a wattle. But from the left you would think it was a hen: its body was slighter and had plainer markings.

Even its behaviour was decidedly confused. The creature attempted to mount the other hens in the yard, yet also laid small eggs itself.

When it died, Schaef decided to prepare the bird for his table. Once the bird had been plucked, it was obvious that the right half of the skeleton was much bigger than the left. When Schaef opened the abdomen to remove the gizzards, he found both a testis and an ovary with a partially formed egg.

Keen not to waste it, Schaef proceeded to roast and eat the chicken. But once the meat had been stripped off the bones, he preserved the skeleton and passed it on to his anatomist friend Madge Thurlow Macklin. She wrote up the story in the Journal of Experimental Zoology in 1923.

Today, we call these creatures “bilateral gynandromorphs”. Unlike hermaphrodites, whose gender-bending often begins and ends at the genitals, these animals are split across their whole bodies: male on one side, female on the other.

Their odd characteristics could explain some of the mysteries of sex, and how our bodies develop.

It’s impossible to say exactly how common they are. Michael Clinton at the University of Edinburgh in the UK estimates that 1 in 10 000 and 1 in 1 000 000 birds develop this way. Nobody knows what the equivalent figure would be for mammals.

Unsurprisingly, courtship for these animals sometimes presents difficulties.

Other animals largely seem to ignore it. This isolation is apparently common for gynandromorphs. Either they are quietly shunned, or actively attacked, by their peers.

For a long time, many assumed that the phenomenon was down to a genetic accident after conception.

At least, that was the theory. A few years ago, Clinton received a phone call that would cause him to reconsider this idea.

One of his colleagues had been visiting a chicken farm, and had found a gynandromorph that strongly resembled Schaef’s chimera bird. “He telephoned and asked if I was interested in getting it,” says Clinton. “Of course, I said yes.”

Soon, the team had found another two gynandromorphs, all of which showed the same mixed characteristics.

However, when Clinton screened the chickens’ genes, he found completely normal sex chromosomes across the whole chicken. In other words, the chicken was essentially formed of two, non-identical twins, fused down the centre.

That was a pretty startling result, but at first Clinton was just disappointed at having his idea proved wrong. “Like most scientists we thought we knew the answer before the experiment,” he says.

When an egg is formed, the cell is meant to discard half its chromosomes, in a bag of DNA called the “polar body”. However, in rare cases the egg may keep the polar body, as well as its own nucleus.

If both are fertilised, and the cell starts dividing, each side of the body will develop with its own genome, and its own gender.

This apparent accident may actually be a cunning evolutionary trick gone wrong.

Biologists have long known that the ratios of males and females within a population can switch depending on the environment.

During stressful times, mothers are more likely to give birth to females. They tend to be more likely to mate and pass on the mother’s DNA, even when times are tough.

Now suppose one of the mother’s eggs holds onto its polar body, and therefore has two nuclei. If the mother allows each one to be fertilised, she will have a half-male-half-female embryo.

The mother could then somehow reject the unwanted sex before laying the egg, neatly controlling her offspring’s sex.

However, in the rare case that the unwanted nucleus is not discarded, the result will be a gynandromorph. –BBC/Earth

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