The contribution of new mutations to phenotypic variation, and the consequences of this variation for individual fitness, are fundamental concepts for understanding genetic variation and adaptation. Here, we investigated how mutation influenced variation in a complex trait in zebrafish, Danio rerio. Typical of many ecologically relevant traits in ectotherms, swimming speed in fish is temperature-dependent, with evidence of adaptive evolution of thermal performance. We chemically induced novel germline point mutations in males and measured sprint speed in their sons at six temperatures (between 16°C and 34°C). Heterozygous mutational effects on speed were strongly positively correlated among temperatures, resulting in statistical support for only a single axis of mutational variation, reflecting temperature-independent variation in speed (faster-slower mode). These results suggest pleiotropic effects on speed across different temperatures, however, spurious correlations arise via linkage, or heterogeneity in mutation number when mutations have consistent directional effects on each trait. Here, mutation did not change mean speed, indicating no directional bias in mutational effects. The results contribute to emerging evidence that mutations may predominantly have synergistic cross-environment effects, in contrast to conditionally neutral or antagonistic effects which underpin thermal adaptation. We discuss several aspects of experimental design that may affect resolution of mutations with non-synergistic effects.

Individual fish from two treatments were swum in a flume using a step velocity test to assess their sprint speed, Usprint. The file contains 3479 records of Usprint for 594 individuals from 100 families, belonging to either Mutant or Control treatments. Analyses were conducted excluding individuals more than 3SD from the mean of their respective temperature, treatment, flume and block means.