Luis J. Villanueva-Rivera writes: > This paper answers the question: > > Warner, D. A., and R. Shine. 2008. The adaptive significance of > temperature-dependent sex determination in a reptile. Nature 451, > 566-568.
The Warner & Shine paper above resurrects the 30-year old Charnov-Bull model for temperature sex determination, an idea that I personally have never found to be exceptionally plausible, but for which Warner and Shine now claim experimental verification. Another paper of value on the subject is: > F. J. JANZEN* & P. C. PHILLIPS. 2006. Exploring the evolution of > environmental sex determination, especially in reptiles. Journal > of Evolutionary Biology, Volume 19, Issue 6 (p 1775-1784) This paper has the added advantage that it is open access. It's available at: http://www3.interscience.wiley.com/cgi-bin/fulltext/118631945/PDFSTART But the paper's true advantage is that I believe it to be a more equitable assessment of our current state of knowledge. In this mini review, the authors repeat my initial response to the question of the adaptive significance of temperature sex determination (TSD), "The simple answer is that we don't know," with this sentence: "Despite elegant theoretical work and several decades of empirical examination in this context, we are not obviously closer to a clear, general solution to the adaptive significance of TSD in amniote vertebrates." Among the amniote vertebrates, as I mentioned earlier, birds and mammals are universally chromosomally sex determined (CSD), although gender reversed, while the crocodilians are universally TSD. The additional paragraph that I meant to write in my first response but somehow forgot is that some turtles, tuataras and lizards are also TSD. What this means that TSD has been invented polyphyletically, as a recurrent independent invention, within otherwise CSD clades. The question then is: what does this recurrence mean? It may mean nothing at all. Parthenogenesis in lizards represents a cautionary tale. When parthenogenesis was first discovered in lizards 40 years ago, the discovery initially engendered a great deal of excitement, and eventually a large number of explanations, all proposing significant evolutionary and ecological advantages to the process. The listed advantages were generally centered on one of these three ideas: (1) the possession of hybrid genotypes, thus providing the bearer with a greater ecological "valence," capable of operating in a broader ecological context than either parental species (2) the reproductive potential of all-female populations, absent the "cost of males" and the two-fold (or greater) intrinsic rate of reproduction (3) the fidelity in transmission of genotypes from generation to generation With further experimental and field work, none of these advantages proved to be true. Parthenogenesis in lizards appears to be a form of reproductive incompetency, the result of the hybridization of two good sexual species, resulting in a form of outbreeding depression. If the parthenoforms have any advantage, it is that they can populate disturbed environments faster than their fully sexual congeners and can persist for time in that manner. A single individual of an all-female parthenogenetic "species" can found a population, thus there are parthenogenetic geckoes currently rapidly spreading across the Pacific because of increased human-generated transportation opportunities had not previously existed. Similarly, the arid deserts of the Americas, Australia and Asia are also home to a variety of parthenogenetic lizards, able to exist at population densities too low to support sexual populations. But under more clement conditions, the parthenoforms quickly disappear in competition with their fully sexual congeners, for all of the standard reasons. TSD may be the same sort of "invention," but with slightly better odds of long-term persistence. Gender is determined during embryogenesis by the ratio of androgen and estrogen hormones present at the time of development. Somehow, in the evolution of TSD, the signals taken from the information borne on the chromosomes has come to be swamped by the temperature of incubation, thus creating a TSD system. To the best of my knowledge, this sensitivity to temperature during development has only occurred in the amniote vertebrates. No other form of plant or animal life on this planet has allowed temperature to so dominate, although there are a great number of other environmental sex determinates in other plants and animals, but almost all of those alternatives actually make some good sense ecologically. Wirt Atmar
