Reply To Lazaridis And Reich: Robust Model-Based Inference Of Male-Biased Admixture During Bronze Age Migration From The Pontic-Caspian Steppe

Abstract

Comparing the sex-specifically inherited X chromosome to the autosomes in ancient genetic samples, we (1) studied sex-specific admixture for two prehistoric migrations. For each migration, we used several admixture estimation procedures, including ADMIXTURE model-based clustering (2), comparing X-chromosomal and autosomal ancestry in contemporaneous Central Europeans, interpreting greater admixture from the migrating population on the autosomes as male-biased migration. For migration into late Neolithic/Bronze Age Central Europeans (BA) from the Pontic-Caspian steppe (SP), we inferred male-biased admixture at 5-14 males per migrating female. Lazaridis & Reich (3) contest this male-biased migration claim. For simulated individuals, they claim that ADMIXTURE provides biased X-chromosomal ancestry estimates. They argue that if the bias is taken into account, then X-chromosomal steppe ancestry is similar to our autosomal ancestry estimate, and that hence, steppe male and female contributions are similar. We conduct simulations of ancient and modern data under a range of conditions. We conclude that our inference of male-biased Pontic-Caspian steppe migration, seen using ADMIXTURE, STRUCTURE, mechanistic simulations, and X/autosomal FST, is robust. Our analysis further illuminates the impact of small haploid reference samples on ADMIXTURE; we look forward to refining sex-specific migration estimates as larger, higher-coverage ancient samples become available.