Inter-archipelago dispersal, anagenetic evolution, and the origin of a rare, enigmatic plant genus on a remote oceanic archipelago.

Abstract

Island plants have long interested biologists because of their distinctive morphological features and their isolation on small land areas in vast oceans. Studies of insular endemics may include identifying their ancestors, tracing their dispersal to islands, and describing their evolution on islands, including characters adaptive to island life. Thamnoseris is a monospecific genus endemic to the Desventuradas Islands, Chile. Its origins and relationships are unresolved, given the challenges of getting to the islands and accessing plants there. Sequences from ITS of nrDNA and the complete chloroplast genome were employed to resolve phylogenetic relationships of Thamnoseris. Phylogenetic analyses of nuclear and chloroplast sequences showed Thamnoseris nested within or sister to Dendroseris, the largest endemic genus in the Juan Fernández Islands. Thamnoseris evolved from a common ancestor of all or most species of Dendroseris prior to the diversification of Dendroseris in the Juan Fernández archipelago. The ancestor of Thamnoseris dispersed to the Desventuradas archipelago, which consists of the islands San Ambrosio and San Félix, within the past 3 Ma (the age of San Ambrosio). This is the only known example of possible plant dispersa\l between the Juan Fernández and Desventuradas Islands. We also mention two less likely biogeographic scenarios for the origin of Thamnoseris, which has features not seen in Dendroseris: small capitula with yellow florets; style branches barely divergent; and basally swollen subtending involucral bracts, all features associated with selfing and reduced dispersal. Goats and rabbits (now removed) reduced T. lacerata, once very abundant on the Desventuradas Islands, to several plants, making it of extreme conservation concern and worthy of further study.