Adaptive diversification in Dubautia laxa (Asteraceae), a widespread member of the Hawaiian silversword alliance

Dubautia laxa Hook. & Arn., is a widespread member of the Hawaiian silversword alliance (Asteraceae), found on five of the six major islands of the Hawaiian Archipelago, and is one of the few taxa within the silversword alliance that occurs on several islands. Although the distribution of D. laxa is widespread, there are four recognized subspecies that have easily identifiable morphological traits and that are sympatric on the island of Oahu. This unusual distribution raises questions about the nature of species divergence at the population level and the eventual formation of new species. The distribution of D. laxa is further complicated because it is believed to be self-incompatible, requiring the dispersal of multiple unrelated propagules to establish a new population. The goal of this project was to untangle the complex evolutionary history of D. laxa , utilizing genetic data from nuclear microsatellites, nuclear coding and non-coding DNA sequences, and chloroplast DNA sequences. The genetic data resolved two evolutionary lineages within D. laxa, referred to as the hirsuta and laxa groups. The hirsuta group contained all populations of subsp. hirsuta and a single population of subsp. laxa. The laxa group contained all populations of subspp. bryanii and pseudoplantaginea, and all remaining populations of subsp. laxa. The hirsuta group is ancestral to the laxa group, contains a high level of chloroplast DNA diversity, and low levels of nuclear DNA diversity. The laxa group exhibited elevated rates of amino acid change within most populations and no genetic independence among subspecies, despite considerable morphological variation. Subspecies pseudoplantaginea was identified as the product of hybridization between D. laxa and a closely related species, D. plantaginea. All populations of subsp. laxa from the Maui Nui island group shared a common ancestry, suggesting a single colonization event. A total of three dispersal events among island groups were inferred, progressing from older to younger islands. This research provides a detailed understanding of population diversification, speciation, and long distance dispersal in a rapidly evolving Hawaiian plant.