Female-biased sex ratios despite stable genetic sex determination across a climatic gradient in a marine fish

Adult sex ratio (ASR) is a fundamental parameter shaping population dynamics and evolutionary trajectories. In theory, ASR is governed by the sex determination (SD) system and sex-specific mortality. Yet, ASR is often hard to predict in natural populations because of ecological effects on its determinants. We address this ecological complexity by investigating the drivers of ASR across a steep climatic gradient in Scandinavian populations of the two-spotted goby Pomatoschistus flavescens. Demographic surveys accounting for over 25000 fish in 30 locations, spanning 10{degrees} of latitude, revealed a general female-bias of the ASR, ca. 75%. To assess the contribution of SD, we combined reduced representation sequencing of 180 adults with our novel long-read male genome assembly. We characterised the genetic architecture of an XX/XY SD system, and identified a male-specific duplicate from the TGF-{beta} signaling pathway, amhr2y, as a candidate sex-determining gene. Subsequent genotyping of eggs and juveniles showed consistently unbiased sex ratios across latitudes, implying that the female-biased arises at later life stages. Interpreted in the light of historical data, our findings allow us to reject environmental sex determination and sex-biased mortality as important drivers of ASR bias in this species. We propose instead that the female bias originates from sex-specific patterns of habitat distribution, which could result from ecological conditions interacting with the distinct reproductive strategies of females and males. Our results underscore the need to go beyond primary theoretical expectations to understand fundamental demographic parameters within their relevant ecological context.