Sex determination is a developmental process by which bipotential gonads form either testes or ovaries. In vertebrates, sex can be determined genetically, or by environmental cues – usually by temperature during a critical sex determining window. The molecular mechanisms involved in temperature dependent sex determination (TSD) are poorly characterised. In some lizards (e.g. the Australian central bearded dragon – Pogona vitticeps) there are two sex determining switches: a genetic switch that is dependent on sex chromosome complement of an individual (either ZZ males, or ZW females), and a TSD switch that can override the genetic switch (resulting in ZZ females). This system provides a unique model for studying vertebrate sex determination.
Reduced representation bisulfite sequencing was used to generate single base pair resolution DNA methylation profiles of the three genotypic and phenotypic sexes of bearded dragon: 1) normal ZZ male; 2) normal ZW female; and 3) sex-reversed ZZ female. Regions of the genome that encompassed developmental genes (Hox gene clusters and brain development genes) were found to have higher DNA methylation in sex-reversed females compared to normal males and females. Additionally, DNA methylation over the transcription start site of genes involved in steroidogenesis and angiogenesis was found to be higher in sex-reversed females compared to normal males and females. The identification of these epigenetic signatures provides candidates for a temperature induced molecular pathway that can override the genetic switch, demonstrating that the female sex reversal pathway has different epigenetic signatures to that of normal males and females.