Testis transcriptome evolution among hominids

Sağlıcan, Ekin
The difference in the relative testis size between humans and their closest extant relatives is remarkable. Relative testis size of humans is more similar to that of gorillas than that of chimpanzees, although chimpanzees are phylogenetically closer relatives of humans. The relative testis size of chimpanzees is larger than those of both humans and gorillas; moreover, it is more similar to that of a more distant relative: the macaque. These differences in testis sizes are thought to be related with the mating behaviour of these species and to have evolved convergently. Specifically, species with single-male mating, humans and gorillas, have relatively small testes, and species with multi-male mating, chimpanzees and macaques, have large testes. This thesis includes a total of 8 RNA-seq and microarray datasets containing testis transcriptome data of 10 different species; namely, human, chimpanzee, gorilla, macaque, marmoset, mouse of two different species, rat, platypus and opossum. I conduct comparative meta-analyses using these datasets. First, I show that genes showing differential expression in testis between humans and chimpanzees have different levels of correlation with the testis transcriptomes of gorilla and macaques. As in the relationship between testis sizes among these species, this analysis reveals signs of convergent evolution of whole testis gene expression, with higher transcriptome similarity between humans and gorillas, and higher similarity between chimpanzees and macaques. One possible reason that can explain the divergence in testis transcriptome profiles of these species is the relative contribution of cell types present in testis. In the second part of the study, I used genes expressed in isolated cell types of mouse testis to detect the relative contribution of cell types found in the testes of the species used in the analysis. The results of this analysis is consistent with the previous findings: The testis transcriptome profiles of the species with single-male mating behaviour has higher contribution from pre-meiotic and somatic cell types, however the testis transcriptome profiles of the species with multi-male mating behaviour has higher contribution from meiotic and post-meiotic cell types. The proportion of the cell types present in the testis is expected to be changing with development. I therefore tested the hypothesis that single-male species have more immature testes compared to those of multi-male species. Indeed, calculating the levels of correlation of the whole testis transcriptome profiles of different species with testis transcriptome profiles of mice or macaques at different stages of maturation, I found a similar trend with cell type analysis: Single-male species' testis transcriptome profiles similar to those of immature mice and immature macaques. I then clustered all common genes present in all the datasets into four groups based on their expression profiles. Two of the clusters (about 53% of the genes) showed either increasing or decreasing gene expression profiles in the mouse and macaque testis development datasets. The same genes distinguished single- and multi-male species' profiles as well, indicating that convergent evolution of whole testis transcriptome profiles affects a large proportion of the transcriptome. To conclude, although a relationship between mating behaviour and testis size was known among hominids, whether such a relationship was also present at the transcriptome level was not known. My work shows that whole testis transcriptomes are affected by cell type proportions and these evolve convergently according to the mating behaviour of species.
Citation Formats
E. Sağlıcan, “Testis transcriptome evolution among hominids,” M.S. - Master of Science, Middle East Technical University, 2018.