During the Deepwater Horizon oil spill, vast quantities of a chemical dispersant Corexit 9500 were applied in remediation efforts. In addition to the acute toxicity, it is essential to evaluate Corexit further with a broader scope of long‐term sublethal endocrine endpoints. The American alligator ( Alligator mississippiensis ) is an excellent organism for such an endeavor. It exhibits temperature‐dependent sex determination, in which egg incubation temperatures during a thermosensitive period (TSP) in embryonic development determine the sex of embryos. Estrogen signals play a critical role in this process. For example, a single exposure to exogenous estrogen during the TSP overrides the effects of temperature and leads to skewed sex ratios. At a concentration of 100 ppm, Corexit significantly induced transcriptional activity of both alligator nuclear estrogen receptors 1 and 2 in vitro in reporter gene assays. To investigate the estrogenic effects of Corexit on gonadal development, alligator eggs were exposed to Corexit at environmentally relevant concentrations (0.25, 2.5 and 25 ppm) before the TSP in ovo. Exposure to Corexit at 0.25 and 25 ppm significantly delayed hatching and growth. Corexit exposure at any treatment level did not affect sex ratios or testicular mRNA abundance as measured at 1‐week post‐hatching, suggesting that the combination of Corexit components did not synergize enough to induce ovarian development in ovo. These results point to a need for further investigations on individual and combined components of Corexit to understand better their long‐term effects on the development and reproductive health of alligators and other coastal aquatic wildlife.

All crocodilians and many turtles exhibit temperature-dependent sex determination where the temperature of the incubated egg, during a thermo-sensitive period (TSP), determines the sex of the offspring. Estrogens play a critical role in sex determination in crocodilians and turtles, as it likely does in most nonmammalian vertebrates. Indeed, administration of estrogens during the TSP induces male to female sex reversal at a male-producing temperature (MPT). However, it is not clear how estrogens override the influence of temperature during sex determination in these species. Most vertebrates have 2 forms of nuclear estrogen receptor (ESR): ESR1 (ERα) and ESR2 (ERβ). However, there is no direct evidence concerning which ESR is involved in sex determination, because a specific agonist or antagonist for each ESR has not been tested in nonmammalian species. We identified specific pharmaceutical agonists for each ESR using an in vitro transactivation assay employing American alligator ESR1 and ESR2; these were 4,4',4''-(4-propyl-[1H]-pyrazole-1,3,5-triyl)trisphenol (PPT) and 7-bromo-2-(4-hydroxyphenyl)-1,3-benzoxazol-5-ol (WAY 200070), respectively. Alligator eggs were exposed to PPT or WAY 200070 at a MPT just before the TSP, and their sex was examined at the last stage of embryonic development. Estradiol-17β and PPT, but not WAY 200070, induced sex reversal at a MPT. PPT-exposed embryos exposed to the highest dose (5.0 μg/g egg weight) exhibited enlargement and advanced differentiation of the Müllerian duct. These results indicate that ESR1 is likely the principal ESR involved in sex reversal as well as embryonic Müllerian duct survival and growth in American alligators.

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 …