Studies on the causes and consequences of sex determination in turtles
Sex determination is a crucial decision of commitment of an individual's sexual fate into either a male or a female. In vertebrates, sex determination modes span a spectrum of extremes: genotypic sex determination (GSD), where sex is genetically determined, often due to the presence of sex chromosomes and temperature-dependent sex determination (TSD), where environmental temperature during a critical period of embryonic development establishes sexual fate. The evolution of this diversity in sex determining mechanisms is one that has defied scientific explanation for decades. An ideal system to demystify this conundrum includes turtle lineages, where both TSD and GSD co-occur.
This dissertation addresses two big questions on this topic: firstly, what are the underlying molecular causes of sex determination? Our knowledge of molecular components that could transduce the external temperature cue in TSD systems remains incomplete. Is sex determined by direct thermal responses of certain key elements in the gonadal network in TSD vertebrates? Are there epigenetic underpinnings regulating gene expression in response to temperature? Secondly, what are the consequences of possessing sex chromosomes? Upon acquiring a sex-determining factor, sex-linked sequences are hypothesized to evolve at a faster rate than autosomes owing to multiple factors, with no supporting evidence stemming from mainly from Drosophila and some mammals. However, any reptilian evidence to test this hypothesis is still forthcoming.
In Chapter 1, I analyze a time-series of embryonic transcriptomes of TSD and GSD turtles to characterize the composition of the transcriptional network that regulates male and female gonadal development, and test how it responds to temperature in TSD and GSD systems. In Chapter 2, I test for the involvement of epigenetic modification in TSD and whether it mediates the differential gene expression patterns detected via transcriptomics in Chapter 1. In Chapter 3, I test the effect of sex chromosome evolution on the molecular evolution of coding regions of genes by comparing genes that are sex-linked in some amniotes while being autosomal in others. Together, these chapters shed light on the genetic and epigenetic basis of a complex system (sexual phenotype), its environmental susceptibility (TSD v GSD), and evolution (divergence among species).