Evidence for Variation in the Genetic Basis of Sex Determination in Brook Stickleback (Culaea inconstans)

Abstract

The genetic basis of sex determination is typically conserved within species if not within broader lineages. For example, within the stickleback family (Gasterosteidae), AmhY has been identified as a master sex-determination (MSD) gene in multiple species across two genera. By contrast, the existence of within-species variability in the genetic basis of sex determination is not frequently observed but provides an opportunity to understand the evolution and turnover of sex determination systems. In this study, we investigated the consistency with which AmhY is involved in sex determination across 610 individuals from five brook stickleback (Culaea inconstans) populations. We designed a PCR-restriction enzyme assay to identify the presence of AmhY in each individual and recorded sexual morphology in each individual in the field at the time of capture. We found that the genetic sex (presence/absence of AmhY) did not match the field-determined phenotypic sex in up to 44% of individuals within a population. This variation in the genetic basis of sex determination in brook stickleback suggests that the mechanism of sex determination in this species is likely more complex than thought when AmhY was first implicated and may still be evolving. Such within-species variation provides an opportunity to further investigate how and why transitions in sex-determination mechanisms occur.

FIGURE 1

Map showing the locations of brook stickleback populations in Alberta, Canada, and Washington, USA, sampled for this study. The pink shading in the inset map shows the area displayed in the larger‐scale map. The map was made with QGIS using Natural Earth Data.

FIGURE 2

Image of the PCR‐restriction enzyme AmhY assay results for two samples from Goldeye Lake collected in 2024. The sample on the left is a male (♂), visualizing its digested (left) and undigested (right) PCR fragments. The sample on the right is a female (♀), visualizing its digested (left) and undigested (right) PCR fragments. The AmhY gene copy does not have a BsaA1 restriction site and shows up as a 230 bp fragment in the digested PCR product (e.g., in the male depicted here).