Thilina asks:
> I have a question on the temperature dependant sex determination of
> reptiles. What is the evolutionary or ecological advantage of such a
> phenomena, especially in reptiles when the females are produced under both
> low and high temperature extremities?
The simple answer is that no one knows.
You could speculate that a species population that moves too far out of its
temperature preferendum would begin to drop in fecundity. You could also
speculate that such a population might depend on being "rescued" by female
parthenogenesis, a process that is being reported in increasingly more reptilian
species each year, but all of this is just speculation.
What you can say is that temperature-determined sex determination is ancient and
an odd system to have persisted so long. Mammals are universally
chromosomally-determined, so their system must have evolved sometime after the
split of the mammal-like reptiles from the main amniote branch (~325 Ma).
Similarly, birds are also universally chromosomally-determined, but
gender-reversed, and they too are reptile descended, arising sometime in the
mid-Mesozoic (~190 Ma) from the theropod ("beast-footed") dinosaurs, strongly
suggesting that dinosaurs were similarly chromosomally determined.
Crocodiles and their relatives are the only remaining living animals of the
Crurotarsi, one of the two subclades of the archosaurs; the other clade is the
Ornithodira (dinosaurs, pterosaurs, and birds). One clade (Ornithodira) is
chromosomally determined, the other (Crurotarsi) temperature-determined.
Both of these clades lie within the Sauropsida, a sister clade to the
Therapsida, the "mammal-like reptiles," the group from which true mammals
descended. This split in clades is believed to have occurred in the
Carboniferous (~340 Ma), making chromosomal sex determination older in the
mammal-like reptiles than in the dinosaurs, but making both forms of sex
determination quite old.
Wirt Atmar
