BioEd Online

A study of sexually scrambled chickens suggests that sex in birds is determined in a radically different way from that in mammals.

Researchers studied three chickens that appeared to be literally half-male and half-female, and found that nearly every cell in their bodies — from wattle to toe — has an inherent sex identity. This cell-by-cell sex orientation contrasts sharply with the situation in mammals, in which organism-wide sex identity is established through hormones.

The confused fowl have upended a century-old rule, established for vertebrates, that all cells in an embryo start off sexually indifferent and remain so until a sex-determining gene directs the development of gonads into either ovaries or testes. The work appears in today's Nature, and may trigger a rethink of the evolution of sex determination¹.

Clucking confusing

Researchers were first alerted to the chickens by an employee in the poultry industry who spotted the unusual birds while visiting farms. A team of scientists eventually obtained three of the animals, which are known as gynandromorphs. The 'cockerel' side of these birds has white feathers, large wattles and breast muscles, whereas the smaller 'hen' side has characteristic dark colouring.

One gynandromorph was named Sam, for Samantha on the right and Samuel on the left. Like the other gynandromorphs, Sam was infertile and a bit confused. It seemed to think it was male, says Michael Clinton, a developmental biologist at the University of Edinburgh in Midlothian, UK who led the study. "But when we put it in with a couple of females I don't think they were too sure."

Gynandromorphs are striking because nothing like them has been seen in mammals. In almost all mammals, including humans, embryonic cells are initially sexually indistinguishable. During development, genetic factors trigger the formation of male or female gonads according to an animal's combination of sex chromosomes (XY for males and XX for females). The gonads then secrete hormones that direct other cells to develop as a certain sex.

"We assumed that sex determination in birds would follow the mammal pattern," Clinton says. Accordingly, the researchers thought that one side of the gynandromorphs would be a normal female (or male) and that the other side would have a some kind of chromosomal anomaly.

Instead, they found the chickens to be almost perfectly split between male and female. The hen half was, for the most part, made up of normal female cells with female chromosomes, whereas the cockerel side contained mostly normal male cells with male chromosomes. Because both sides were exposed to exactly the same hormones, the team realized that the cells must respond according to their own chromosomal complement rather than taking orders from the gonads.

Scrambled eggs

To test their hypothesis, the researchers created living embryos with chimaeric gonads by placing female cells in male tissue and vice versa. They found that female donor cells embedded in male cells didn't assume male functions. Similarly, male donor cells in an ovary-inducing environment didn't take on female roles. The team concluded that the cells couldn't switch sexual roles and that their orientation was fixed before they arrived.

Clinton says the work shows that chickens have a fundamentally different way of determining their sex from mammals: "Hormones do play some role, but nowhere near the extent seen in mammals." He suspects that the same rules apply to other species of bird, although gynandromorphs probably go unnoticed much of the time because the differences between the sexes aren't as pronounced.

Combined with work on songbirds, this study strongly suggests that birds follow a different developmental pattern from mammals, agrees behavioural neuroscientist Juli Wade at Michigan State University in East Lansing who works on sexual differentiation in the songs of zebra finches².

Birds aren't the only exception to the rule. The mammal model also fails with some marsupials and invertebrates like fruitflies. "The problem is, once people develop a hard and fast rule, it becomes the only game in town," Clinton says. Sam's "tubes and plumbing" would suggest there is no rule for all vertebrates.

Next, the researchers want to show by just how much these sex differences precede sex-hormone influences. "We believe the cells know they are male or female at fertilization," Clinton says.