Week 9: Evolution not phylogeny

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.

Week 8: Application instead of advantages versus disadvantages

The second part of my paper focused on the advantages and disadvantages of sex determination. However, with my change in thesis, I have realized that this section is more important to explain under which conditions genetic sex determination is used and which conditions environmental sex determination is used. This is important in order to understand why species use the mechanism. Many times, the mechanism of determination is used because it is better for one environment than another.

For example, genetic sex determination is used in conditions which can lead to the enhancement of individual fitness under a negative frequency-dependent scenario [Mank, et al]. For example, Eels and salmon have been shown to show this reoccurring trend that when there is no worry for a possible shortage of partners, than GSD can be advantageous. [Mank, et al].

On the other hand, environmental sex determination is used in conditions which the environment is highly variable [DeSoto, et al]. This can be shown in the plant W. radicans as they can adjust and still survive in both high and low temperatures [DeSoto, et al]. Another example is that reptiles and amphibians are more likely to have ESD because they are more likely to encounter variable environments than birds and mammals [Janzen]. ESD is not only advantageous to variable environments but it is also advantageous to patchy environments [DeSoto, et al]. Patchy environments are those in which females perform better than males on some patches and vice versa [DeSoto, et al].

These two examples are important in understanding when each sex determination mechanism is most likely to be used. This is important to eventually understand what causes the transitions between the two different mechanisms.

Mank, J.E., D.E.L. Promislow, J.C. Avise. 2004. Evolution of alternative sex-determining mechanisms in teleost fishes. Biological Journal of the Linnean Society 87: 83-93.

DeSoto, L., L.G. Quintanilla, M. Méndez. 2008. Environmental sex determination in ferns: effects of nutrient availability and individual density in Woodwardia radicans. Journal of Ecology 96: 1319-1327.

Janzen, F.J., and P.C. Phillips. 2006. Exploring the evolution of environmental sex determination, especially in reptiles. European Society For Evolutionary Biology 19: 1775-1784.

Week 7 blog: One mechanism is not better than another

Through the research I did this week, I realized that my thesis was not looking at the correct information. I was trying to find out which method of sex determination, environmental or genetic, was the most advantageous. However, this week I realized than neither can be better for all species. It is all dependent on the conditions that the species live in.

This was made clear by many sources, but the most effective source so far is by Schwanz, et al. This source shows how even in one species, the mechanisms of sex determination vary. Sex determination mechanisms can transition between environmental and genetic rapidly and without crossing any major fitness values. This is most commonly shown when species transition from genetic sex determination to environmental sex determination. The mechanisms transition when there is a major increase or decrease in the threshold (usually temperature). This threshold can be defined as “the minimum regulatory signal required to shift the balance between the competing signals directing the opposing male and female development programmes” [Quinn, et al]. This can be shown when extreme temperatures cause a loss in the W allele and lead to an entire population of ZZ individuals whose sex is determined largely by incubation temperature. Another example is during intermediate temperatures where the Y allele is lost it leads to an entire population of XX individuals [Schwantz, et al].

This source, as well as a few others, led to my new thesis which states:

There are many different ways sex is determined across the phylogenetic tree. These include both genetic sex determination and environmental sex determination. Although we do not know how these different sex-determining mechanisms have evolved, each of these mechanisms is essential for different species. ESD is important for species in variable environments, GSD is important for species which need stability to produce viable offspring. However, the most important factor when studying sex determination is the frequent transitions between these two mechanisms even within a species.

Hopefully this will lead to more depth in my paper, so that it is not simply an explanation of sex determination mechanisms.

Schwanz, L.E., Ezaz, T., Gurber, B., and Georges, A. 2013. Novel evolutionary pathways of sex-determining mechanisms. Journal of Evolutionary Biology.

Quinn, A.E., Sarre, S.D., Ezaz, T., Marshall Graves, J.A., and Georges, A. 2011. Evolutionary transitions between mechanisms of sex determination in vertebrates. Biology Letters 7: 443-448.

The struggle bus of senior sem

Well, this past week has been a struggle to work on my senior seminar topic/paper because I was blessed with a concussion. However, being back on my game now, I was able to do a lot of research this morning. I found one paper in specific that will be very helpful to my topic. This paper is called “Relic thermosensitive gene expression in a turtle with genotypic sex determination” by Nicole Valenzuela. This paper will be important because it talks about the evolution of sex determination while comparing genotypic and temperature dependent sex determination. I also found some review articles that I hope to find some good articles from their sources. I am starting to feel more comfortable with my outline as well. I will be introducing the topic by explaining the evolution of sex determination. Then I will explain genetic sex determination and environmental sex determination separately. Then my argument will then be comparing the two and discussing why both forms of sex determination still evolve. I am still working on my exact thesis, and I hope to narrow down the topic and wording today. All suggestions on what I have worked on so far or advice on articles are greatly appreciated. 

Environmental Dependent Sex Determination Chapter

For most species, keeping a balanced sex ratio is important to ensure reproductive success. But how do species control for this? One way that some species control for sex-ratio is through sex determination, more specifically, sex determination based on environmental factors [Rhen et al. 2011]. One example is temperature-dependent sex determination (TSD). In TSD, sex determination is determined after fertilization by the temperature of the embryo during development [McGaugh and Janzen 2011]. One study has even identified two genes, MALAT1 and C16ORF62, as being responsible for determining sex through temperature [Chojnowski and Braun 2012].

But how is temperature-dependent sex determination evolutionarily advantageous? Are there disadvantages to waiting until after fertilization to determine the sex? How do extreme environmental conditions affect the sex ratio in species from season to season? There is also evidence of mixed sex determination of both genetic sex determination and environmental sex determination [Grossen et al. 2010]. If this is possible, and could take care of issues due to environmental changes, why don’t all species have the ability to use both genetic and environmental sex determination?

These are all questions that have been addressed in different studies, however it will be interesting to take all aspects into consideration and combine the arguments into one paper, to compare the advantages versus disadvantages of environmental dependent sex determination.

 

D. Crews, et al. “Segregating Variation For Temperature-Dependent Sex Determination In A Lizard.” Heredity 106.4 (2011): 649-660. Academic Search Premier. Web. 22 Sept. 2013.

Choinowski, Jena L., and Edward L. Braun. “An Unbiased Approach To Identify Genes Involved In Development In A Turtle With Temperature Dependent Sex Determination.” BMC Genomics 13.1 (2012): 308-319.  Academic Search Premier. Web. 22 Sept. 2013.

Grossen, Christine, Samuel Neuenschwander, and Nicolas Perrin. “Temperature-Dependent Turnovers In Sex-Determination Mechanisms: A Quantitative Model.” Evolution 65.1 (2011): 64-78.  Academic Search Premier. Web, 22 Sept. 2013.

McGaugh, S. E., and F. J. Janzen. “Effective Heritability Of Targets Of Sex-Ratio Selection Under Environmental Sex Determination.” Journal of Evolutionary Biology 24.4 (2011): 784-794. Academic Search Premier. Wed. 22 Sept. 2013.

Infertility Chapter

In order for any species to continue to exist, reproduction must occur so that as some die off, others come to life. If sex is so essential to the survival of species, than why does infertility occur? One reason for infertility between species is to allow the preservation of species and control the formation of new species [Steiner and Ryder 2013]. One gene that causes sterility in offspring of different species is the Prdm9 gene which can be seen in mules as well as other vertebrate species [Seiner and Ryder 2013]. This gene, also known as Hst1, arrests meiosis in come males causing this sterilization [Flachs, et al. 2012].   

With this topic, I would like to learn more about how this gene evolved. I would include in my chapter the importance of the preservation of species. I would also include the dangerous effects that could occur if there were no preservation of species.

Steiner, Cynthia C., and Oliver A. Ryder. “Characterization Of Prdm9 In Equids And Sterility In Mules.” PLoS ONE 8.4 (2013): 1-7. Academic Search Premier. Web. 22 Sept. 2013.

Zdenek Trachtulec, et al. “Interallelic And Intergenic Imcompatibilities Of The Prdm9 (Hst1) Gene In Mouse Hybrid Sterility.” PLoS Genetics 8.11 (2012): 1-10. Academic Search Premier. Web. 22 Sept. 2013.

Senior Sem topic idea take 2

As I have thought long and hard about what to write my senior sem chapter on, I have been struggling with the issues of too broad and too narrow. After reading about exterior sex determination in chapter 11/12 of Judsons book, I have become very interested in sex determination. I have found articles that propose temperature, epigenetics, and effects of gene expression as ways to control sex determination.