Pleistocene divergence in the absence of gene flow among populations of a viviparous reptile with intraspecific variation in sex determination

Polymorphisms can lead to speciation if there is differential mating
success among conspecifics divergent for a trait. Polymorphism for sex
determining system might be particularly expected to isolate gene pools,
given strong selection for the production of viable males and females and
the low success of heterogametic hybrids when sex chromosomes differ
(Haldane’s rule). We investigated this question using a rare example of a
species exhibiting polymorphism for sex determination: the viviparous snow
skink Carinascincus ocellatus. While a coparatively high elevation
population has entirely genotypic sex determination, in a lower elevation
population there is an additional environmental component to sex
determination. These systems also exhibit minor differences in sex-linked
genotypes. Using ‘Isolation with Migration’ analysis of neutral loci, we
estimated that these populations and their sex determining systems
diverged in the absence of gene flow, across multiple periods of
geographic proximity during Pleistocene glaciations. Our analysis suggests
that populations of C. ocellatus with divergent sex determining systems
are likely reproductively isolated, even though they are presently
underlined by only subtle DNA differences. Given the influence of
temperature on sex in one lineage, we also discuss the implications for
the persistence of this polymorphism under climate change.