It's a boy! Or is it? We live in an era of blurred gender lines, but for many reptiles this has always been a hot topic. In crocodilian as well as several turtle species, cranking up the thermostat a few degrees during a critical developmental window, called the thermo-sensitive period, will turn a whole clutch of eggs into males. Hormones also contribute to sex determination in these species, and exposure to female hormones – estrogens – can feminize alligators developing at a male-producing temperature. But while there is more than a decade of research on estrogen-mediated sex reversal, the mechanisms behind this phenomenon remain largely elusive. Knowing that estrogens can communicate with cells using either of two receptors (ESR1 or ESR2), a team of scientists based in South Carolina, at the lab of Louis Guillette Jr, wanted to know which of these two receptors is responsible for changing baby boy alligators into baby girls.
Guillette's team, led by Satomi Kohno, began by identifying two estrogen-like chemicals to differentially stimulate the estrogen receptors of the American alligator. The first chemical interacted only with ESR1 and the second chemical preferred ESR2. Armed with the tools needed to solve the sex-shifting conundrum, Kohno set out to raid alligator nests and collect freshly laid eggs for his gender-bending experiments. Back at the lab, he divided the eggs into four groups and left the first group to develop at a temperature that would produce female alligators. He put the other three groups at a warmer male-producing temperature, but he applied the ESR1-stimulating chemical to one group, to another he applied the ESR2-stimulating chemical and the last clutch received no chemicals. Kohno wanted to know which of the estrogen-like chemicals would feminize the developing gators. After allowing enough time for the gonads of the alligator embryos to develop, the team cracked the eggs, and the case!
Kohno compared the little gators from the egg groups given the ESR1- and ESR2-stimulating chemicals to see whether their developing gonads more closely resembled ovaries or testes. They found that eggs given the ESR1-stimulating chemical and incubated at a male temperature produced alligators with gonads that more closely resembled the appearance and gene expression patterns of ovaries, like those seen in the alligators that developed at the female temperature. Conversely, they found that the eggs given the ESR2-stimulating chemical shared the gonadal characteristics of the chemical-free male-producing group. This means that the sex reversal seen in male alligators exposed to estrogens during the thermo-sensitive period is mediated through ESR1, not ESR2. The switcheroo is through one, not two!