Hybridization during the adaptive radiation of Oxera (Lamiaceae) in New Caledonia: Is flower shape shift driven by introgression?

Recent genomic studies have suggested that hybridization may play a significant role in adaptive radiation, rapid speciation, and convergent evolution. The genus Oxera, a plant taxon thought to have diversified at its beginning through adaptive radiation in New Caledonia, provided an opportunity to investigate these processes. Within the robusta subclade of Oxera, characterized by bird-pollinated yellow-orange flowers, convergent evolution of flower shape is likely to have occurred. We aimed to elucidate the hybridization history of the robusta subclade by whole genome sequencing and MIG-seq data. Our analyses revealed an ancestral introgression from O. coriacea to O. sympatrica, whose flowers are remarkably similar to each other. Among the introgressed genomic regions, we identified several genes potentially involved in flower shape development. O. sympatrica and its sympatric sister species exhibit distinct flower shapes, and pollinator-mediated reproductive isolation presumed to be a major barrier between them. The ancestral introgression uncovered in this study may have driven the convergent evolution of flower shape in the robusta subclade and played a crucial role in the speciation process of O. sympatrica. These finding contribute to our understanding of the interplay between hybridization, adaptive radiation, and speciation process.