Hybridisation and chloroplast capture between ancient Themeda triandra ecotypes in Australia

Ecotypes are distinct populations within a species which are adapted to specific environmental conditions. Understanding how these ecotypes become established, and how they interact when reunited, is fundamental to elucidating how ecological adaptations are maintained. This study focuses on Themeda triandra, a dominant grassland species across Asia, Africa and Australia. It is the most widespread plant in Australia, where it has distinct ecotypes that are usually restricted to either wetter and cooler coastal regions or the drier and hotter interior. We use whole genome sequencing for over 80 Themeda accessions to reconstruct the evolutionary history of T. triandra and related taxa. A chloroplast phylogeny confirms that Australia was colonised by T. triandra twice, with the division between ecotypes predating their arrival in Australia. The nuclear genome provides evidence of gene-flow among the ecotypes, largely restricted to two geographic areas. In northern Queensland there appears to be a hybrid zone with admixed nuclear genomes and shared plastid haplotypes. Conversely, in the cracking claypans of Western Australia, there is cytonuclear discordance with individuals possessing the coastal plastid and interior clade nuclear genomes. This chloroplast capture is potentially a result of adaptive introgression, with selection detected in the rpoC2 gene which is associated with water use efficiency. A stable hybrid zone in the east, and the displacement of one ecotype in the west, highlights the unpredictable nature of hybrid zones, with repeated contacts between the same ecotypes producing different outcomes.