Environmental Effects on Fish Sex Determination and Differentiation

Abstract

Environmental factors affect the sex ratio of many gonochoristic fish species. They can either determine sex or influence sex differentiation. Temperature is the most common environmental cue affecting sex but density, pH and hypoxia have also been shown to influence the sex ratio of fish species from very divergent orders. Differential growth or developmental rate is suggested to influence sex differentiation in sea bass. Studies in most fish species used domestic strains reared under controlled conditions. In tilapia and sea bass, domestic stocks and field-collected populations showed similar patterns of thermosensitivity under controlled conditions. Genetic variability of thermosensitivity is seen between populations but also between families within the same population. Furthermore, in the Nile tilapia progeny testing of wild male breeders has strongly suggested the existence of XX males in 2 different natural populations. Tilapia and Atlantic silverside studies have shown that temperature sensitivity is a heritable trait which can respond to directional (tilapia) or frequency dependent selection. In tilapia, transitional forms within a genetic sex determination (GSD) and environmental sex determination (ESD) continuum seem to exist. Temperature regulates the expression of the ovarian-aromatase cyp19a1 which is consistently inhibited in temperature masculinized gonads. Foxl2 issuppressed before cyp19a1. Recent in vitro studies have shown that foxl2 activates cyp19a1, suggesting that temperature acts directly on foxl2 or further upstream. Dmrt1 up-regulation is correlated with temperature-induced male phenotypes. Temperature through apoptosis or germ cell proliferation could be a critical threshold for male or female sex differentiation.

Climate change and pollution speed declines in zebrafish populations

“…Climatic instability, including the periodic onset of El Niño events and drier monsoon seasons (63) accompanying increasing temperatures in South East Asia, may therefore augment population impacts on zebrafish. Other environmental stressors likely to accompany projected climate change, including reduced nutrition, overcrowding, and, in aquatic environments, increasing acidification and reduced dissolved oxygen, may also generate male-skewed sex ratios in fish (25,26) and other taxa, including amphibians (22) and reptiles (51).…”

“…However, in lizards (17,18), fish (21), and some amphibians (22), temperature elevation tends to induce male development and male-biased sex ratios can dramatically reduce population fitness (23,24). Sexual differentiation in these animals is also strongly influenced by chemicals that disrupt hormone systems (25)(26)(27)(28). Some endocrine disrupting chemicals (EDCs) are highly potent pollutants, and exposure concentrations occurring in natural surface waters have been shown to affect reproductive development negatively in reptiles (29), amphibians (28), and fish (30,31).…”

Did sex chromosome turnover promote divergence of the major mammal groups?

“…Some reptile species have sex chromosome systems that can be overridden by temperature 28, 29; these dual systems are extremely labile, able to change rapidly to TSD systems and even between ZW and XY 30, 31. Similar GSD‐TSD transitions may be common in some fish and may also facilitate changes in sex determination 32, 33, 34.…”

Genetic interactions controlling sex and color establish the potential for sexual conflict in Lake Malawi cichlid fishes

“…However, modeling sex determination in the context of QTL haplotypes (Figure 3) allows us to explain sex in B82% of F 2 offspring. We cannot discount the effect of environmental influences on sex determination in fishes (Baroiller et al, 2009), and we accept that these effects in cichlids in particular are not well described. The conditions in which we conducted our intercross experiment were well controlled in terms of light, temperature and other physical water parameters.…”