Genome scans reveal high levels of gene flow in Hawaiian Pittosporum

Species radiations in the Hawaiian Islands have resulted in closely related yet morphologically and often ecologically distinct species, many of which have highly restricted distributions. Eleven species of Pittosporum (Pittosporaceae) are recognized in the Hawaiian Islands, yet it remains unclear whether the morphological variation within Hawaiian Pittosporum is due to hybridization, genetic polymorphism, phenotypic plasticity, or some combination of these processes. In this study of Hawaiian Pittosporum we used a genome-wide approach employing 626 Amplified Fragment Length Polymorphisms (AFLPs) to test species boundaries and elucidate gene flow within and among species. Parsimony trees were constructed to identify clades of individuals that correspond to species, island affinity, and/or sexual system. Network analysis was used to identify the number of lineages in the dataset; results showed a lack of resolution due to reticulation. Based on Bayesian assignment tests, Hawaiian Pittosporum exhibited high levels of gene flow. The interspecific allelic exchange and reticulation caused non-hierarchical relationships between species and a lack of reproductive isolation rendered species boundaries permeable. This study highlights the impact of hybridization on species persistence and conservation as well as on general patterns of Hawaiian biota.