My third part of my paper began focusing on different phylogenetic changes. I was focusing on how sex determination mechanisms were always changing within a phylogeny. However, as I have worked on my paper and discussed my paper with peers, I have realized that the evolutionary transitions of sex determination mechanisms are actually what I am referring to rather than phylogeny. This is important because it connects better with my thesis. It is also important because it explains why mechanisms change within phylogeny rather than simply stating that they have changed.
As I have done research for transitions between sex determining mechanisms, I have found many possible reason for the transition from GSD to ESD. Many articles argue that all species began with the evolution of sex chromosomes and the ESD evolved from GSD [Schwantz, et al.]. I am still trying to find more research on the transition from GSD to ESD, as it seems to be more common. However I feel like there must be examples for a species to be able to transition back and forth between ESD and GSD.
Another aspect I have been researching is the costs of transitioning between sex determining mechanisms. One article argued that the cost is the unstable state of the mixture of ESD and GSD while transitioning [Uller, et al]. Even though a mixture of TSD and GSD does not happen often, it is possible for this mixture to exist [Schwatnz, et al]. This mixture occurs when sex determination starts as genotypic (XX/XY or ZZ/ZW). Then sex is determined in heterogametic individuals by incubation temperature. One reason that the mixture of sex determination mechanisms is rare is that it usually takes place during the rapid transition from GSD to TSD. Since this switch is rapid, the combined state of sex determining mechanisms is not present long, and therefore does not need to be stable [Uller, et al].
I have more research to do on the transitioning between ESD and GSD, so any source suggestions would be greatly appreciated.
Schwanz, L.E., Ezaz, T., Gurber, B., and Georges, A. 2013. Novel evolutionary pathways of sex-determining mechanisms. Journal of Evolutionary Biology.
Uller, T., Pen, I., Wapstra, E., Beukeboom, L.W., and Komdeur, J. 2007. The evolution of sex ratios and sex-determining systems. Trends in Ecology and Evolution 22(6): 292-297.