Both phenotypic and genotypic sex influence sex chromosome dosage compensation in a sex reversing lizard

Abstract

Studies of sex chromosome dosage compensation have historically focussed on therian mammals which have a conserved XY sex determination system. In contrast, lizards have sex determination systems that can differ between even closely related species that include XY and ZW systems and thermolabile systems where genetic and temperature interact to various degrees to determine sex. The eastern three-lined skink (Bassiana duperreyi) has a differentiated XY sex determination system, in which low temperature incubation during development can cause female to male sex reversal, producing XX males. This provides a unique opportunity to investigate how genotype and phenotype affect dosage compensation. We generated transcriptomes from brain and heart tissue of normal adult males and females, along with brain tissue of sex-reversed XX males. We observed partial dosage compensation between XX females and XY males in both brain and heart, with median gene expression from the X in normal males being 0.7 times that of normal females. Surprisingly, in brain of sex reversed XX males the median X chromosome output did not match that of either normal males or females, but instead was 0.89 times that of the normal XX female level. This suggests that not just genotype, but also sexual phenotype, influences gene dosage of the X chromosome. This has profound implications for our understanding of the evolution of dosage compensation.