Last week, the butterfly keepers at the Natural History Museum in London came across an incredibly rare phenomenon: a lateral gynandromorph. This butterfly is both male and female, and the division is bilateral, with the left half being male and the right female. This occurrence is almost certainly the result of chance improper separation of the sex chromosomes during the first division following fertilisation, hence its rarity. As well as being a great find for the lepidopterists at the NHM (and I would heartily encourage anyone even remotely interested in the diversity of animal form to visit the 9 million or so butterflies and moths at the museum), it illustrates wonderfully well that there is more than one developmental route to sexual identity.
The question of what makes males and females different from one another has an extremely long history (but I know better than to speculate on such differences and so will here refer only to their developmental basis), and has naturally been the subject of intense investigation by biologists. There are essentially two models of how this is achieved during somatic development: either through the concerted action of hormones emanating from a sex-specific gonad, or else through an inherently genetic mechanism where each cell knows what sex it is; as with so much else in development, the crucial distinction is between a non-cell autonomous and a cell autonomous process.
Mammals fall into the former category, with male phenotype specified by a genetic cascade beginning with the expression in the gonad primordium of the Sry gene, which sits on the Y chromosome. Remarkably, experimental introduction of the Sry gene can cause a female mouse embryo to develop as a male. This mode (if not the exact same genetic components) had been widely assumed to operate in most vertebrates. However, work published last year using elegant grafting experiments with transgenic GFP chickens, showed that in contrast, bird somatic cells know their sex. The motivation for this work was the presence of three lateral gynandromorphs amongst the flocks of the Roslin Institute in Edinburgh. Like the butterfly at the NHM, these curious creatures are male on one side and female on the other, and studying them showed that, contrary to expectations, this was not the result of aneuploidy, but rather was the result of them being genuine male/female chimaeras: one side composed largely of genetically normal male cells, and the other of genetically normal female cells. Their different responses to the hormones circulating during development were therefore cell-autonomous – like butterflies, the chick soma knows what sex it is. Thus, despite its rarity in nature (of the 9 million butterflies and moths at the NHM, only 200 are gynandromorphs, though only the new one is split down the middle), gynandromorphy can tell us fundamental things about the way animals determine their sexual identity. It also makes amateur lepidoptery an even more rewarding passtime.
Zhao et al. (2010) Somatic sex identity is cell autonomous in the chicken. Nature 464: 237-242.
Kashimada & Koopman (2010) Sry: the master switch in mammalian sex determination. Development 137: 3921-3930.