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 genotypes drawn from the same natural population, but differing in their breeding values for a polygenic trait, differed in their contribution of new mutational variation to that trait. We established >200 mutation-accumulation (MA) lines from four Drosophila serrata genotypes. Analysing >44,000 wing-size measurements, collected over 30 generations, we quantified mutational variance and mutational bias for size. Genotypes with the smallest and largest breeding values for size contributed similar (statistically indistinguishable) amounts of mutational variance. In contrast, the genotype with an intermediate breeding value exhibited remarkably low (statistically undetectable) mutational variance, low micro-environmental variance, and high line survival over time, consistent with limited mutational decay in fitness. The three genotypes with detectable mutational input showed declines in mean size over time, indicating a consistent mutational bias toward smaller size, as reported in other taxa. The magnitude of this bias appeared genotype dependent, with the MA populations founded from the larger ancestors declining nearly twice as fast as that founded from the smallest ancestor. 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.