Biased gene retention in the face of massive nuclear introgression obscures species relationships

Abstract

Phylogenomic analyses are recovering previously hidden histories of hybridization, revealing the genomic consequences of these events on the architecture of extant genomes. We exploit a suite of genomic resources to show that introgressive hybridization occurred between close relatives of Arabidopsis, impacting our understanding of species relationships in the group. The composition of introgressed and retained genes indicates that selection against incompatible cytonuclear and nuclear-nuclear interactions likely acted during introgression, while neutral processes also contributed to genome composition through the retention of ancient haplotype blocks. We also developed a divergence-based test to distinguish donor from recipient lineages without the requirement of additional taxon-sampling. Finally, to our great surprise, we find that cytonuclear discordance appears to have arisen via extensive nuclear, rather than cytoplasmic, introgression, meaning that most of the genome was displaced during introgression, while only a small proportion of native alleles were retained.