A game of two halves

Halfsider singleOne of the photographs in the Alan Greenwood collection depicts a chicken. This is hardly unusual in itself, seeing as Greenwood was director of the Poultry Research Centre. However, a closer look reveals that this is no ordinary chicken: it is coloured and shaped differently on each side: in fact, it looks more like two separate chickens stitched together. This chicken is a ‘halfsider’. Halfsiders – also called bilateral gynandromorphs –  are birds whose colour, plumage, size and even gender is different on each side. Halfsiders are most commonly encountered in budgerigars, although they can occur in other birds too, including domestic chickens.

The phenomenon which produces halfsiders is is actually part of a larger phenomenon of ‘mosaicism’. Mosaics are organisms of a ‘patchwork’ phenotype and/or genotype generally only found in domesticated species (although a wild gynandromorphic Northern Cardinal was discovered on the east coast of America in 2009). F.A.E Crew at the Institute of Animal Genetics was one of the first geneticists to write on gynandromorphy in birds, with Greenwood and colleagues continuing to study it at the Poultry Research Centre.

Halfsiders are so interesting for genetics because they contradict the  theory that sexual development in birds and mammals follows the same course, with the embryonic cells being ‘unisex’, the gender being determined at around seven weeks by signals sent by hormones. However, chicken cells apparently ‘know’ which sex they are at the time of fertilisation, and scientists have had to propose an entirely separate model for avian sexual development.

Early explanations for how halfsiders happen usually posited gene mutation or embryo damage in the early stages of cell division, or else the loss of a chromosome.. However, a study in 2010 (Zhao, McBride et al) found that halfsider chickens are in fact nearly perfect male:female chimeras comprised of normal female cells (with ZW chromosomes) on one side and normal male cells (with ZZ chromosomes) on the other side. Therefore, halfsiders appear to result from an abnormal ovum containing two pronuclei, fertilised by two sperm, which then results in both a Z and W chromosome–containing nucleus. As the cells develop, and are subject to exactly the same hormones, they respond according to their own chromosomes rather than signals being given out by the gonads, as with mammalian development. This in essence means that the two ‘halves’ develop entirely separately, at the same time.

Although the phenomenon producing halfsiders is genetic in origin, it is not heritable and so cannot be intentionally bred (in any case, gynandromorphs are nearly always sterile). It is estimated that 1 in 10,000 domestic chickens is a gynandromorph.

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