April 17, 2017 - 21:19
TotM: Alligator mississippiensis TRPV4 and sex determination in reptiles
The XIXth century was characterized by the fight of workers all over the industrialized world for a myriad of rights that are now widely accepted. The fight is not over, and protests are still common, but the bulk of achievements and ideological advancements were made at that time. Analogously, it can be argued that the XXth century most important social fights (without ever forgetting the different fights for the civil rights of ethnic minorities) were related to feminism, sexuality and gender roles. The beginning of the previous century was marked by the fight over female suffrage, rightfully claiming that half of humanity cannot be excluded from political decisions. On the other hand, the last half of the century has witnessed the birth of the protests over the rights of sexual minorities, also known as LGBTIQA people.
The increased visibility of sexual minorities has changed the way society views sexuality and gender, revealing and praising a wide spectrum of sexual manifestations and identities. However, being the newest of all of these important fights, LGBTIQA rights face strong opposition from certain sectors. It is a common practice among these opposing forces to use pseudo-scientific arguments, claiming a strict binary separation of sex and gender (heteronormativity), and defining any other scenario as a d
eviation or even a pathology. One of the most common arguments is an oversimplification of the genetic mechanisms of sex determination in mammals, the idea that the presence or absence of a Y chromosome deterministically sets the sex of the individual, which is one and the same as gender identity, sexuality and sexual preference. Such is the “normal” state in animals and thus any deviation of that is considered contra natura.
Arguments like these not only ignore the diversity of human sexuality, but they are also ignoring what we know of animal sexuality as well. Chromosomal determination of sex is not universal in vertebrates, for instance. Sexual differentiation in many reptiles, such as crocodilians, depends of environmental parameters. In these animals, the temperature of the egg at certain embryological stage affects the probability of the embryo to develop as either male or female, otherwise completely lacking any genetic sexual determination system. The ecological implications of this model of sexual determination are huge, and it is expected that climate change may have tremendous consequences due to imbalancing the sexual on natural reptilian populations.
Due to the lack of genetic engineering tools on reptiles, the genetics and physiology of these animals is poorly understood compared to the rest of vertebrates. We know for sure that is the temperature of the gonad itself the determining factor, which implies that the regulation of the process is internal and does not depend on any hormonal or nervous input. By expression analysis, several important genes have been identified, including the ortholog of human TRPV4.
TRPV4 is a Ca2+ ion channel that is involved in a wide array of functions in mammals, including but not limited to osmoregulation in brain, skin damage, nociception and skeleton development. The channel is activated by several stimuli, including mechanical, chemical and osmotic factors. As we can observe in our tree of the month from the Alligator mississippiensis phylome, it is widely conserved in amniotes. This is not surprising given the relevance it has in human, but to have such an important protein in sex determination of crocodilians is unexpected nonetheless.
First picture: Photogram of the videoclip of "I want to break free", from the british music band Queen, 1984.
Second picture: An american Alligator, Alligator mississippiensis. Public domain picture.
Third picture: A crocodile hatching from the egg. Photography by Veronik80000. CC-BY-SA-4.0