# Genotype-by-environment interactive effects and conflict solving during gonadal sex differentiation of pejerrey Odontesthes bonariensis, a fish with dual genotypic/environmental sex determination

## TL;DR

This study explores how genetic and environmental factors interact to determine sex in a fish species, revealing that male development is easier to trigger and how conflicting signals are resolved.

## Contribution

The study provides new insights into how genotype-by-environment interactions influence sex determination and resolve conflicts in gonadal differentiation.

## Key findings

- Convergence or divergence of genotypic and environmental signals affects the timing and sensitivity of sex determination.
- Ovarian formation is the default state, while testis formation can be rapidly induced by strong environmental stimuli.
- The developmental features increase the likelihood of male development and help avoid ambiguous differentiation.

## Abstract

Genotypic (GSD) and environmental (ESD) sex determination coexist in many species of reptiles, fish, and amphibians. Inherited genotypic signals and environmental factors conceivably interact as pro-testis or pro-ovary signals during sex determination, but how such interactions affect gonadal sex differentiation in these species remains largely unexplained. This study uses a model gonochoristic fish with coexisting GSD and ESD, the pejerrey Odontesthes bonariensis, to examine how synergism and antagonism between sex genotype (XX/XY) and thermal (feminizing/masculinizing) regimes interactively affect environmental sensitiveness and the critical time of environmental sex determination as well as how genotype-by-environment conflicts are resolved.

We performed a series of controlled rearing experiments involving shift-once and shift-twice transfers of fish of known sex genotype (XX/XY) between feminizing and masculinizing temperatures at different stages of gonadal sex differentiation. Match/mismatch analysis of phenotypic (ovary/testis) and genotypic (absence/presence of the master sex determining gene amhy) sex was performed in juveniles to estimate sex reversal rates and the critical period of sex determination for each combination of sex genotype and thermal conditions.

The results show that convergence/divergence between genotypic and environmental signals advances/delays the critical time of sex determination and lowers/raises the degree of environmental sensitiveness, respectively, even when genotypic control is ultimately overridden. This study also provides evidence that ovarian formation is the default state regardless of genotypic sex but commitment to femaleness is a lengthy, passive process requiring absolute seclusion from environmental pro-male stimuli in the span of weeks. Testis formation, in turn, is the alternative state that can be imposed on this default, regardless of genotype, by an extremely short (range of hours) environmental stimulus of sufficient strength at any time before ovarian commitment. We argue that this combination of developmental features increases the likelihood of male development and at the same time may be crucial to avoid ambiguous differentiation under conflicting genotypic/environmental signals in GSD + ESD species.

Overall, the results reveal genotypic sex-dimorphic critical periods of sex determination, show that it is “easier” to make males in pejerrey, and provide clues to understand how GSD + ESD species may prevent discrepant sex determination/differentiation when genotype and environment diverge.

The online version contains supplementary material available at 10.1186/s13293-025-00768-7.

Genotypic (GSD) and environmental (ESD) sex determination coexist in many species of reptiles, fish, and amphibians. Inherited genotypic signals and environmental factors conceivably interact as pro-testis or pro-ovary signals during sex determination, but how such interactions affect gonadal sex differentiation in these species remains largely unexplained. We used a model gonochoristic fish with coexisting GSD and ESD, the pejerrey Odontesthes bonariensis, to examine how synergism and antagonism between sex genotype (XX/XY) and thermal (feminizing/masculinizing) regimes interactively affect environmental sensitiveness and the critical time of environmental sex determination and how genotype-by-environment conflicts are resolved. We show that convergence/divergence between genotypic and environmental signals advances/delays the critical time of sex determination and lowers/raises the degree of environmental sensitiveness, respectively, even when genotypic control is ultimately overridden. The results also evidence that ovarian formation is the default state regardless of genotypic sex and yet that commitment to femaleness is a lengthy, passive process requiring absolute seclusion from environmental pro-male stimuli. Testis formation, in turn, is the alternative state that can be imposed on this default by an extremely short environmental stimulus of sufficient strength at any time before ovarian commitment. We argue that this combination of developmental features is crucial to avoid ambiguous differentiation under conflicting genotypic/environmental signals. Overall, the results reveal genotypic sex-dimorphic critical periods of sex determination, show that it is “easier” to make males in pejerrey, and provide clues to understand how GSD + ESD species prevent discrepant sex determination/differentiation when genotype and environment diverge.

The online version contains supplementary material available at 10.1186/s13293-025-00768-7.

Convergence or divergence between genotypic and environmental signals advances or delays the critical time of sex determination and lowers or raises the degree of environmental sensitiveness, respectively, even when genotypic control is ultimately overridden.Ovarian formation is the default state regardless of genotypic sex but commitment to femaleness is a lengthy, passive process requiring absolute seclusion from environmental pro-male stimuli in the span of weeks.Testis formation is the alternative state that can be imposed on this default, regardless of genotype, by an extremely short (range of hours) environmental stimulus of sufficient strength at any time before ovarian commitment.This combination of features for female and male differentiation increases the likelihood of male development in pejerrey but may also be key to prevent discrepant gonadal differentiation under conflicting genotypic and environmental signals.

Convergence or divergence between genotypic and environmental signals advances or delays the critical time of sex determination and lowers or raises the degree of environmental sensitiveness, respectively, even when genotypic control is ultimately overridden.

Ovarian formation is the default state regardless of genotypic sex but commitment to femaleness is a lengthy, passive process requiring absolute seclusion from environmental pro-male stimuli in the span of weeks.

Testis formation is the alternative state that can be imposed on this default, regardless of genotype, by an extremely short (range of hours) environmental stimulus of sufficient strength at any time before ovarian commitment.

This combination of features for female and male differentiation increases the likelihood of male development in pejerrey but may also be key to prevent discrepant gonadal differentiation under conflicting genotypic and environmental signals.

The online version contains supplementary material available at 10.1186/s13293-025-00768-7.