Genetic analysis of an ephemeral intraspecific hybrid zone in the hypervariable tree, Metrosideros polymorpha, on Hawai'i Island.

Intraspecific hybrid zones involving long-lived woody species are rare and can provide insights into the genetic basis of early-diverging traits in speciation. Within the landscape-dominant Hawaiian tree, Metrosideros polymorpha, are morphologically distinct successional varieties, incana and glaberrima, that dominate new and old lava flows, respectively, below 1200 me on volcanically active Hawai'i Island, with var. glaberrima also extending to higher elevations and bogs. Here, we use morphological measurements on 86 adult trees to document the presence of an incana-glaberrima hybrid zone on the 1855 Mauna Loa lava flow on east Hawai'i Island and parent-offspring analysis of 1311 greenhouse seedlings from 71 crosses involving 72 adults to estimate heritabilities and genetic correlations among vegetative traits. Both the variation in adult leaf pubescence at the site and the consistency between adult and offspring phenotypes suggest the presence of two hybrid classes, F1s and var. incana backcrosses, as would be expected on a relatively young lava flow. Nine nuclear microsatellite loci failed to distinguish parental and hybrid genotypes. All four leaf traits examined showed an additive genetic basis with moderate to strong heritabilities, and genetic correlations were stronger for the more range-restricted var. incana. The differences between varieties in trait values, heritabilities and genetic correlations, coupled with high genetic variation within but low genetic variation between varieties, are consistent with a multi-million-year history of alternating periods of disruptive selection in contrasting environments and admixture in ephemeral hybrid zones. Finally, the contrasting genetic architectures suggest different evolutionary trajectories of leaf traits in these forms.

Incipient radiation within the dominant Hawaiian tree Metrosideros polymorpha.

Although trees comprise a primary component of terrestrial species richness, the drivers and temporal scale of divergence in trees remain poorly understood. We examined the landscape-dominant tree, Metrosideros polymorpha, for variation at nine microsatellite loci across 23 populations on young Hawai'i Island, sampling each of the island's five varieties throughout its full geographic range. For four varieties, principal coordinate analysis revealed strong clustering of populations by variety across the 10 430 km(2) island, indicating partitioning of the species into multiple evolutionarily significant units. The single island-endemic form, riparian var. newellii, showed especially strong differentiation from other varieties despite occurring in sympatry with other varieties and likely evolved from a bog form on the oldest volcano, Kohala, within the past 500 000 years. Along with comparable riparian forms on other Pacific Islands, var. newellii appears to represent parallel incipient ecological speciation within Metrosideros. Greater genetic distance among the more common varieties on the oldest volcano and an inverse relationship between allelic diversity and substrate age appear consistent with colonization of Hawai'i Island by older, partially diverged varieties followed by increased hybridization among varieties on younger volcanoes. This study demonstrates that broad population-level sampling is required to uncover patterns of diversification within a ubiquitous and long-lived tree species. Hawaiian Metrosideros appears to be a case of incipient radiation in trees and thus should be useful for studies of divergence and the evolution of reproductive isolating barriers at the early stages of speciation.