| The causes of speciation, or the formation of reproductive isolating barriers between diverging populations, are poorly understood for trees. The hypervariable, endemic Hawaiian tree species, Metrosideros polymorpha ('ohi'a) appears to be in the early stages of diversification into multiple lineages and as such allows investigation of the mechanisms that generate new species. On Hawai'i Island, 'ohi'a comprises four varieties that are distinguished through vegetative characters and show low, but significant neutral genetic divergence from each other, indicating partial separation of gene pools. They also specialize on different habitats suggesting that divergent natural selection is an important driver of divergence in 'ohi'a.;In this study, the strengths and stages of early-acting reproductive barriers among these four varieties on east Hawai'i Island were assessed through a one-way cross-pollination design using a single, high-elevation Mauna Loa population of var. polymorpha as the pollen recipient/maternal population. Each of 21 maternal trees received pollen from all four varieties of 'ohi'a (var. polymorpha serving as the control), each sampled from two separate populations. Reproductive isolation was examined as pollen tube density and length, fruit set, weeks to fruit maturation, timing of seed germination, and number of seeds germinated per fruit. It was predicted that reproductive isolation would increase with increasing neutral genetic and ecological distance between varieties.;Results revealed three contrasting patterns of modest, early reproductive isolation between the three other pollen-donor varieties and var. polymorpha. An early, post-zygotic barrier to reproduction was present in crosses with the riparian specialist, var. newellii, in the form of reduced fruit set. Pre- and post-zygotic barriers were revealed in crosses with the low-elevation, early-successional var. incana , possibly due to floral character divergence along an elevation gradient. Lastly, the reduced pollen tube density observed in crosses with late-successional var. glaberrima may reflect reinforcement of reproductive isolation between two hybridizing varieties to prevent the formation of less-fit hybrids. The barriers to reproduction observed in all three cross types are consistent with a model of ecological speciation. However, there was no clear relationship between the strength of reproductive isolation and ecological/genetic distance between varieties. Instead, a more complex story has emerged, indicating that there is more than one path for the evolution of reproductive barriers among diverging lineages of trees. |
