Background check: mutational input to size variation depends on ancestor's breeding value

The phenotypic effects of mutations often depend on the genetic background, yet general patterns remain poorly resolved. Here, we tested whether same-population individuals (genotypes) with different breeding values for a polygenic trait differed in their contribution of new mutational variation to that trait. We analysed more than 44 000 wing-size measurements over 30 generations of mutation-drift evolution in four Drosophila serrata genotypes to quantify genotype-specific mutational variance and mutational bias for size. The smallest and largest breeding value genotypes contributed similar mutational variance. By contrast, the genotype associated with average size contributed effectively no mutational variance, had reduced micro-environmental variance, and high line survival, consistent with no mutational decay in fitness. Genotypes with detectable mutational input exhibited declines in mean size over time, indicating a consistent mutational bias towards smaller size. This bias appeared genotype dependent, nearly half as strong in the genotypes associated with smallest size. Together, these results demonstrate substantial heterogeneity in mutational properties among genotypes within a single natural population where the trait value spans a relatively narrow range. Such genotype-specific mutational input is expected to shape both the standing genetic variance and the evolutionary trajectory of polygenic traits.