Biogeography of North Pacific Isoëtes (Isoëtaceae) inferred from nuclear and chloroplast DNA sequence data

Abstract

Recent advances in phylogenetics indicate that reticulate evolution has played an important role in the emergence of Isoetes species in the North Pacific region. However, the biogeographical origin of the North Pacific Isoetes species remains contentious. We present a fossilcalibrated phylogeny of species from the North Pacific region based on molecular data. Within this framework, we discuss their ancestral areas and biogeographical history. North Pacific Isoetes are divided into two clades: clade I, consisting of East Asian, Papua New Guinean, and Australian species, and clade II, consisting of West Beringian and western North American species. Within clade I, Australian Isoetes species were an early divergent group, and Papua New Guinea’s species form a sister clade to the East Asian species. Biogeographical reconstructions suggest an Australasian origin for the East Asian species that arose through long-distance dispersal during the late Oligocene. Within clade II, I. asiatica from West Beringia forms a clade with I. echinospora and I. muricata from Alaska. Western North America was the area of origin for the dispersal of Isoetes species to West Beringia via the Bering land bridge during the late Miocene. Our study identifies the biogeographic origin of the North Pacific Isoetes and suggests long-distance dispersal as the most likely explanation for their intercontinental distribution.