American alligators incubating their eggs at 33oC (91.4oF) will produce mostly male babies, while incubation temperatures below 30oC (86oF) result in mostly females. In Scientific Reports this week, researchers report that this so-called temperature-dependent sex determination is linked to a thermosensitive protein called TRPV4.
In most vertebrate species, sex is determined genetically (XX and XY in humans, for example, and ZW and ZZ in birds). But for many reptiles, varying environmental cues – such as incubation temperatures during a critical temperature sensitive period – result in different sexual outcomes for the developing embryos. Various mechanisms of thermal detection have been reported in the past, but researchers have yet to figure out how incubation temperature during the temperature sensitive period triggers sex determination.
To investigate, an international team led by Taisen Iguchi of the Okazaki Institute for Integrative Bioscience conducted a series of experiments with Alligator mississippiensis eggs collected at the Lake Woodruff National Wildlife Refuge in Florida in June of 2011 to 2013. Back in the lab, the eggs were incubated in damp sphagnum moss at either 33.5oC (92.3oF) or 30.0oC (86oF). The team focused on the TRPV4 ion channel, which is known to be activated by moderate heat (27 to 35oC; 80.6 to 95oF) in mammals.
It found that the TRPV4 channel is present within the still-developing alligator gonads inside the egg. The protein responds to warm temperatures, and it activates cell signaling by triggering the influx of calcium ions. When the team used drugs to experimentally inhibit TPRV4 thermosensor function in eggs, this impacted the genes that are important for male development, resulting in partial feminization even at male-producing temperatures.
This is the first experimental demonstration of a link between a well-described thermo-sensory mechanism – the TRPV4 channel – and regulation of temperature-dependent sex determination, the authors write. But while TRPV4 channel activity significantly influences the sex determination pathway of male gonads at a molecular level, it has little effect on the differentiation of ovaries – making it just one part of an elusive and much larger thermosensitive trigger mechanism.