Hybridisation and chloroplast capture between distinct Themeda triandra lineages in Australia.

Ecotypes are distinct populations within a species that 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 generate a reference genome for T. triandra and use whole genome sequencing for over 80 Themeda accessions to reconstruct the evolutionary history of T. triandra and related taxa. Organelle phylogenies confirm that Australia was colonized by T. triandra twice, with the division between ecotypes predating their arrival in Australia. The nuclear genome provides evidence of differences in the dominant ploidal level and gene-flow among the ecotypes. In northern Queensland there appears to be a hybrid zone between ecotypes with admixed nuclear genomes and shared chloroplast haplotypes. Conversely, in the cracking claypans of Western Australia, there is cytonuclear discordance with individuals possessing the coastal chloroplast and interior clade nuclear genome. This chloroplast capture is potentially a result of adaptive introgression, with selection detected in the rpoC2 gene which is associated with water use efficiency. The reason that T. triandra is the most widespread plant in Australia appears to be a result of distinct ecotypic genetic variation and genome duplication, with the importance of each depending on the geographic scale considered.

Widespread lateral gene transfer among grasses.

Lateral gene transfer (LGT) occurs in a broad range of prokaryotes and eukaryotes, occasionally promoting adaptation. LGT of functional nuclear genes has been reported among some plants, but systematic studies are needed to assess the frequency and facilitators of LGT. We scanned the genomes of a diverse set of 17 grass species that span more than 50 Ma of divergence and include major crops to identify grass-to-grass protein-coding LGT. We identified LGTs in 13 species, with significant variation in the amount each received. Rhizomatous species acquired statistically more genes, probably because this growth habit boosts opportunities for transfer into the germline. In addition, the amount of LGT increases with phylogenetic relatedness, which might reflect genomic compatibility among close relatives facilitating successful transfers. However, genetic exchanges among highly divergent species indicates that transfers can occur across almost the entire family. Overall, we showed that LGT is a widespread phenomenon in grasses that has moved functional genes across the grass family into domesticated and wild species alike. Successful LGTs appear to increase with both opportunity and compatibility.