Ecotype maintenance in Callicarpa subpubescens via local environmental adaptation amidst frequent hybridization and low pre- and post-mating barriers

Abstract Adaptive radiations frequently occur on oceanic islands, but the mechanisms behind the maintenance of species and ecotypes after such radiations remain poorly understood, especially regarding potential gene flow among sympatric congeners. Callicarpa subpubescens is endemic to the oceanic Ogasawara Islands, and it has been suggested that multiple ecotypes exist within the southern part of this group, the Hahajima Islands, each associated with a unique localized habitat. Here, we determined the habitat characteristics of each ecotype and the degree of pre- and post-mating isolation by investigating flowering time overlap cross compatibility, and gene flow among ecotypes using an EST-SSR marker analysis of adult trees and naturally pollinated seeds. Our study confirmed the presence of four distinct ecotypes on the Hahajima Islands, with one likely arising from hybridization. Each ecotype was confined to specific island habitats, and distinct leaf morphologies and plant heights suggested local adaptation. However, overlapping flowering times and successful artificial cross pollination indicated little pre- and post-mating isolation. Hybridization rates in adult trees and naturally pollinated seeds were 37.2% and 26.4%, respectively, and most hybrids were backcrosses with few first- or second-generation hybrids. The hybridization rates of each ecotype and paternal correlation indicated that the flowering synchrony and close spatial distribution of ecotypes contributed to hybridization among ecotypes. Overall, C. subpubescens likely maintains four distinct ecotypes in spite of frequent hybridization probably due to selection pressures that result in reversion to the original ecotype via backcrossing, since this is well-adapted to the local environment.