Repeatable phenotypic but not genetic response to selection on body size in the black soldier fly
Polygenic traits are expected to show high genetic redundancy and therefore low repeatability in the genomic response to selection. We tested this prediction by selecting for large body size in the black soldier fly (Hermetia illucens). Over three replicate experiments selected for large body size, we found a strong and repeatable phenotypic response, with a mean 15% increase in body size. Selected lines also increased in larval growth rate (+19%) and average protein content (+14%), suggesting that selection on large body size does not result in strong trade-offs. In contrast to the predictability of the phenotypic response across replicates, whole genome sequencing identified a highly polygenic and non-repeatable genomic response. We identified 120, 301 and 157 outlier genomic regions in the three replicates, but high redundancy with only four shared regions. Among 12 candidate genes found in these regions, the insulin-like receptor gene (HiInR) was confirmed as regulating larval growth using a CRISPR knockout experiment. In summary, polygenic quantitative traits show high genetic redundancy, even where the phenotypic response to selection is highly repeatable.