Gender-Bending Chickens: Mixed, Not Scrambled
Fascinating “half-sider” birds are prompting a reassessment of our understanding of the evolution of sex determination
I’ll never forget the first time I saw a bilateral gynandromorph. I was a bird-crazy teenager reading my way through a stack of avicultural publications when I spied the strangest bird I’d ever seen on the cover of one magazine: an eclectus parrot that was very precisely divided down the middle: one side was rich scarlet and the other was brilliant emerald. Because eclectus parrots are sexually dimorphic — females are red and males are green — this remarkable bird was easily identifiable as being composed of both sexes, one on each side.
Even though this was the first time I’d ever seen a gynandromorph, these mysterious birds do pop up from time to time. For example, bird watchers occasionally run across them in the wild (see above photograph) and poultry farmers sometimes find them in their flocks: it is estimated that roughly one in 10,000 domestic chickens — another sexually dimorphic species — is a gynandromorph.
Bilateral gynandromorphs are mysterious because nothing like them has been seen in mammals, so they generate a lot of speculation whenever they turn up.
It was widely assumed that sexual development in birds and mammals follows basically the same trajectory. In almost all mammals, including humans, embryonic cells start off being “unisex”; indistinguishable regardless of which sex chromosomes they contain. Early in development, the sex-determining region of the Y chromosome (Sry) controls whether a testis or ovary forms: if Sry is present, testes grow, while ovaries develop in the absence of Sry. By the seventh week of embryonic life, the developing gonads begin secreting chemical messengers — hormones — that direct other cells to develop as either male or female.
Given this paradigm, scientists studying gynandromorphs were surprised to discover that sexual development in birds is dramatically different from most mammals: unlike mammals, individual chicken cells apparently “know” which sex they are at the time of fertilisation and they maintain their own male or female identities throughout life.
“We assumed that sex determination in birds would follow the mammal pattern,” reports Michael Clinton, a developmental biologist at the University of Edinburgh in Scotland. Because Dr Clinton has spent much of his scientific life scratching his head over why it was so difficult to find the avian Sry in chicken chromosomes, he was intrigued by these peculiar chickens and quickly assembled a team of scientists to study the birds.
Dr Clinton and his colleagues originally hypothesised that one side of bilaterally gynandromorphic birds would be a genetically normal female (or male) while the other side suffered some kind of chromosomal anomaly or mutation. In their journey to understand the molecular and cellular mechanisms that lead to a gynandromorph, these researchers made a fundamental and bizarre discovery about sexual development.
This story begins serendipitously enough. A few years ago, an employee in the poultry industry described to Dr Clinton some peculiar chickens on nearby farms. These rare chickens were bilateral gynandromorphs; half male and half female. Like my eclectus, these birds were neatly divided down the middle between their male and female sides, almost as if two individuals of opposite sexes had been stitched together.
These “half-siders,” as poultry farmers and aviculturists often refer to bilateral gynandromorphs, are rare, but have been seen in a number of avian families, ranging from finches to pigeons to parrots. Most recently, a wild gynandromorphic Northern Cardinal, Cardinalis cardinalis, was photographed on the East Coast of the United States almost exactly one year ago, re-igniting interest and speculation among online birders throughout the world (see featured image at top).
Like northern cardinals, domestic chickens are sexually dimorphic. So when Dr Clinton first saw these oddly lopsided chickens, he was immediately impressed by the birds’ striking appearance: the larger male side had white feathers, spurs, large wattles and breast muscles, whereas the smaller female side showed the characteristic dark colouring, small wattles, and the lack of spurs (Figure 1):
Dr Clinton’s team eventually obtained three of the unusual birds, which are referred to in their newly published paper as G1, G2 and G3. All three were ISA brown birds, a commercial breed with sex-linked plumage colour. ISA brown males are heterozygous for the dominant silver and recessive gold genes (Ss) so they have white plumage; females possess only the gold gene (s-) and thus have brown plumage.
Careful observations of the behaviours displayed by one of the team’s gynandromorphs, G1, which came to them already named “Sam” — Samantha for the right side and Samuel for the left — showed that it not only looked strange, but it was also a bit confused.
Although gynandromorphs are nearly always sterile, “Sam seemed to think it was male,” reports Dr Clinton. “But when we put it in with a couple of females I don’t think they were too sure.”
This confusion was also apparent when the birds’ gonads were examined: G1 contained a testis-like gonad on the left side, G2 contained an ovary-like gonad on the left side, and G3 contained a swollen testis-like structure on the left side (in contrast to G1 and G2, G3 appeared female on the left side and male on the right).
G1′s testis-like gonad was composed primarily of sperm containing seminiferous tubules, whereas the G2 ovary-like gonad was composed predominantly of large and small follicles. The gonad from G3 comprised a mixture of empty tubules and small follicularlike structures (ovo-testis).
While the researchers were observing their three odd chickens, they were also collecting tissue samples and examining the chromosomes to decipher the molecular mechanisms underlying bilateral gynandromorphs.
The team tested whether the chickens suffered from a genetic anomaly by examining the chromosomes found in tiny samples of blood as well as skin, breast muscle and wattles collected from both sides of each gynandromorph. They colour-coded the chromosomes in these cells with fluorescent dyes that stick to either the Z or W sex chromosomes and examined them with confocal microscopy.
Contrary to their prediction, they found that fluorescently-labelled cells collected from opposite sides of the same bird were either predominantly normal male or female cells, while the blood was a mixture of normal male and female cells. The gynandromorphs were in fact nearly perfect male:female chimæras comprised of normal female cells with ZW sex chromosomes on one side, whereas the cockerel side contained mostly normal male cells with male sex chromosomes, ZZ (Figure 2):
Because mammalian cells are strongly influenced by sex hormones after embryonic gonads begin to develop, the team wondered if this was also true for birds. They investigated whether sex differences between male and female cells exist independently of gonadal hormone influences or if individual cells assume the same sexual identity as their neighbors. To test this, they compared gene expression in male and female embryos during the developmental stages before the gonads form, and discovered that even at these early stages, male and female cells already “know” their sexual identity by showing sex-specific gene expression profiles (Figure 3):
Intrigued, the team then tested whether individual cells respond to the overwhelmingly male or female environment seen on each side of bilateral gynandromorphs — do male cells “conform” to a mostly female environment by assuming a female role, and vice versa? To examine this possibility, they created embryos containing chimæric gonads by embedding female cells in male host tissue and vice versa. They found that male donor cells embedded in female host cells didn’t take on female roles. Similarly, female donor cells embedded in male host cells didn’t assume male functions, either (Figure 4):
Based on these data, the team concluded that individual avian cells maintain their own sexual identity and were not able to switch sexual roles despite developmental signals from nearby cells: their gender orientation was fixed from their inception.
Since these data clearly establish the presence of both ZZ- and ZW-containing cells, the team realised that it is highly unlikely that gynandromorphs arise as a consequence of either a mutation or a loss of a sex chromosome at the two-cell stage of development, as they originally predicted. Thus the team proposed that bilateral gynandromorphs start at the very beginning: they result from the failure of the developing ovum to extrude a polar body during meiosis (refer to figure at right). When this abnormal ovum containing two pronuclei is fertilised (by two sperm, a situation known as polyspermy), it contains both a Z- and W-containing nucleus, which then give rise to each half of the whole bird — a bilateral gynandromorph. (Avian sperm only contain the Z sex chromosome, whilst a mature egg cell has either a Z or W sex chromosome.)
Further, because both sides of the bird are exposed to exactly the same hormones, individual cells respond to these chemical signals according to their own inherent chromosomal complement rather than simply following orders sent out by the gonads.
Based on their studies, the team proposed two different models of sexual development; one for birds and a second, contrasting model to account for most mammals (Figure 5):
Interestingly, birds aren’t the only exceptions to the mammalian model of sexual development. The mammal model also fails for some marsupials. Previous studies have shown that formation of mammary glands and scrotum in the wallaby, a marsupial, is independent of gonadal hormones [DOI: 10.1038/331716a0]. Additionally, the platypus (a monotreme) has a complex sex chromosome system that is unique among mammals [DOI: 10.1038/nature06936] documented so far, although like birds, they also lack the mammalian sex determining region (Sry). It would be interesting to identify whether there are platypus gynandromorphs and if so, to find out if each cell in the platypus has its own sex identity, as in birds.
Interestingly, sex determination in reptiles appears closer to the mammalian system but again, it’s complicated, and there are species where sex is dependent upon ambient temperature experienced during specific stages of embryonic development.
Of course, this makes me wonder if any bilateral gynandormorphic Maniraptoran fossils have ever been identified?
“The problem is, once people develop a hard and fast rule, it becomes the only game in town,” explained Dr Clinton. Certainly, Sam’s “tubes and plumbing” would suggest there is no universal rule for all vertebrates.
Zhao, D., McBride, D., Nandi, S., McQueen, H., McGrew, M., Hocking, P., Lewis, P., Sang, H., & Clinton, M. (2010). Somatic sex identity is cell autonomous in the chicken, Nature, 464 (7286): 237–242 | doi:10.1038/nature08852
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Originally published at scienceblogs.com on 12 March 2010.