Transitions in sex determination mechanisms through parental antagonism

Parental antagonism (PA) occurs when the fitness effects of a gene depend on the parent from which it is inherited. Such genes may become enriched on sex chromosomes, due to their biased inheritance patterns. Although various sex determination (SD) genes exhibit parent-of-origin effects themselves, and between-parent conflict over offspring sex may affect SD, PA itself has not been considered as a driver of SD transitions. Here, I present a model to investigate the scope for transitions in SD mechanisms through PA. My model assumes an ancestral SD locus linked to a PA gene, as well as an autosomal PA gene in whose vicinity a novel SD gene arises. Transitions between functionally-homologous genes are found to depend on the fitness effects of both PA genes and their linkage to nearby SD genes. Transitions between male and female heterogamety by the invasion of a dominant SD gene are however nearly unconstrained. This also allows for back-and-forth dynamics where the ancestral SD and novel SD genes constantly evolve to be dominant over each other. These results further underline the malleability of SD mechanisms, and the need to consider parent-of-origin effects in driving transitions in SD, through proximate and/or ultimate means.