Diversification of Hawaiian Cyrtandra
Cyrtandra (Gesneriaceae) is an Old-World genus of flowering plants comprised of over 800 species distributed throughout Southeast Asia and the Pacific Islands, including the Hawaiian Islands. Molecular phylogenetic studies suggest that Cyrtandra colonized the Hawaii shortly after its formation approximately 4.7mya, and then colonized younger islands through a stepping-stone model of dispersal, much like many other groups of flowering plants on Hawaii (the Silverswords, Psychotria, Hawaiian lobeliads, Schiedea, Silene, and Hesperomannia to name a few). As of this study, 60 species of Hawaiian Cyrtandra were recognized primarily on the basis of morphology. However, despite numerous attempts, morphological and molecular studies have not been able to resolve relationships for this group. Furthermore, complex evolutionary processes of hybridization and introgression, incomplete lineage sorting, and convergence are likely to play a role in the evolutionary history of Hawaiian Cyrtandra, contributing to further difficulty in resolving species relationships (see the plate from our paper below).
In our study, we utilized targeted enrichment, high-throughput sequencing, and modern phylogenomics tools to test a subset of Hawaiian Cyrtandra species and putative hybrids for species relationships and hybridization. We found high levels of incongruence between our concatenated-tree and species-tree, indicating high levels of incomplete lineage sorting. Our species-tree was well resolved and supported previous hypotheses of a stepping-stone model of dispersal from older to newer islands, along with some back-dispersals to older islands. Tests for introgression and hybridization showed gene flow between taxa, though not always the taxa we expected. This suggests that hybridization does play a role in this group of flowers, but a study incorporating more putative species and hybrids is necessary to better understand how hybridization has shaped the relationships within this group.