Exposure of zebrafish to elevated temperature induces sex ratio shifts and alterations in the testicular epigenome of unexposed offspring

Accumulating evidence shows that environmental changes can affect population sex ratios through epigenetic regulation of gene expression in species where sex depends on both genetic and environmental cues. Sometimes, altered sex ratios persist in the next generation even when the environmental cue is no longer present (a multi-generational effect). However, evidence of transgenerational effects (i.e., beyond the first non-exposed generation), which tend to be paternally transmitted, is scarce and a matter of debate. Here, we used the AB strain of zebrafish, where sex depends on both genetic and environmental influences, to study possible multi- (to the F-1) and transgenerational (to the F-2) effects of elevated temperature during the critical period of sex differentiation. From eight initial different families, five were selected in order to capture sufficient variation between the sex ratio of the control group (28 degrees C) and the group exposed to elevated (35 degrees C) temperature only at the parental (P) generation. Results showed a consistent increase in the proportion of males in the P generation in all five families as a result of heat treatment. Sex ratios were then determined in the F-1 and F-2 offspring derived from both above groups, which were all raised at 28 degrees C. A persisting male-skewed sex ratio in the 35 degrees C-derived, unexposed offspring of the F-1 generation was observed in three families, denoting family-dependent multigenerational effects. However, no transgenerational effects were observed in the F-2 generation of any family. DNA methylation was also assessed in the testis of P, F-1 and F-2 males derived from exposed and non-exposed fathers and grandfathers. DNA methylation was significantly decreased only in the testis of the 35 degrees C-derived males in the F-1 generation but not of the F-2 generation and, surprisingly, neither in the 35 degrees C-exposed males of the P generation. Taken together, our results show great interfamily variation, not only in sex ratio response to elevated temperature, but also on its multigenerational effects, denoting a strong influence of genetics. Alterations in the testicular epigenome in F-1 males calls for attention to possible, previously unnoticed, effects of temperature in the unexposed offspring of heat-exposed parents in a global warming scenario.