Detecting adaptive trait loci in nonmodel systems: divergence or admixture mapping?

Mapping adaptive trait loci (ATL) underlying ecological divergence is an essential step towards understanding the processes that generate phenotypic diversity. Technological advances have made it possible to sequence exomes in nonmodel systems, providing an efficient means of analysing functional genetic variants. Divergence scans of genetic markers for outlier loci, or 'divergence mapping', have been used to map locally adapted genes, but this approach is likely to be underpowered when background divergence is elevated. Genotype-phenotype association tests in admixed populations, or 'admixture mapping', may provide a useful approach for mapping locally adapted loci when neutral divergence is high. To determine the power and limits of divergence mapping, we simulated exomes containing a single ATL across two parental populations of varying neutral divergence, estimated divergence and quantified the power to identify the ATL. We found that divergence mapping had very high power when background FST is <0.2, but decreased dramatically above this level. To evaluate the utility of admixture mapping, we simulated exomes from admixed populations, then simulated phenotypes, conducted genotype-phenotype association tests and found that even two generations of random mating after admixture could provide high mapping power in scenarios where pure divergence mapping was ineffective (FST = 0.35). Moreover, admixture mapping had high power across all levels of divergence after 20 generations since admixture. Together with high-throughput exome sequencing, admixture mapping could be used to map ATL in systems such as Heliconius butterflies or Gryllus crickets when experimental design and analytical approach are chosen accordingly.