Oral Paper

         Biogeography

Biogeography and Evolution of the Iconic and Endangered Mascarene Hibiscus

Presenting Author
Brock Mashburn
Description
The Mascarene Archipelago comprises three volcanic islands in the Indian Ocean east of Madagascar: Rodrigues, Mauritius and Réunion. Although recent studies are beginning to address long-standing questions about the origins of the Mascarene flora and biological connectivity across the islands, comparatively little research has focused on understanding the evolutionary and biogeographic histories of Mascarene plant groups. One emblematic group of plants whose evolutionary and biogeographic history in the Mascarenes is poorly known is Hibiscus section Lilibiscus. The group currently contains four described species in the Mascarenes, all of which are critically endangered: Hibiscus boryanus (Mauritius and Réunion), H. fragilis (Mauritius and Réunion), H. genevii (Mauritius), and H. liliiflorus (Rodrigues). The goals of this study were to: 1) assess the monophyly of sect. Lilibiscus in the Mascarenes and place them in relationship with other section Lilibiscus species that occur across the world; 2) assess species boundaries, particularly for species that occur in more than one island; and 3) understand biogeographic patterns within the Mascarenes, particularly how island ages and topological complexity may have contributed to patterns of colonization and speciation. We sampled 312 individuals from the four species on all three islands, including all known wild individuals in both Rodrigues and Mauritius. We also collected samples of outgroup species from across the worldwide range of sect. Lilibiscus. We genotyped these samples using a 2bRADseq genotyping approach and constructed a maximum-likelihood phylogeny of the group. Results showed that the Mascarene species of sect. Lilibiscus form a monophyletic group that is nested within a clade of species from the geographically distant Fijian Archipelago, suggesting that the Mascarene species may have originated via a dispersal event from Fiji. For the two species that occur in both Mauritius and Réunion, we found convincing genetic and morphological evidence to divide each species into two single-island endemics, resulting in one new species in each of Mauritius and Réunion and a total of six species in the Mascarenes instead of the currently described four. We found that diversification followed the progression rule, where the species endemic to the oldest island (H. liliiflorus in Rodrigues) is sister to the remainder of the species, with the other species arising through what appears to be single dispersal events from Rodrigues to the next-oldest island (Mauritius), and from there to the youngest island (Réunion). The species in Mauritius and Réunion may have diverged through adaptation to environmental variation, as they occur in different habitats (i.e., wet and dry habitats), which may have been facilitated by topographical characters unique to each island. These patterns of dispersal and speciation provide a framework for understanding the evolution of the unique biodiversity of the Mascarene Archipelago.