Abstract Detail

Pacific Biogeography

Choi, Jae Young [1], Purugganan, Michael [1], Stacy, Elizabeth [2].

Divergent selection and gene flow shape incipient speciation of the riparian tree, Metrosideros polymorpha var. newellii, on Hawaii Island.

In trees, which are characterized by long life spans, high dispersability, and the slow evolution of reproductive isolating barriers, purported cases of speciation with gene flow are few. The hypervariable woody species complex, Metrosideros (Myrtaceae), dominates the native forests of the Hawaiian Islands and provides the opportunity to examine how selection across heterogeneous environments can lead to phenotypic divergence and the evolution of reproductive isolating barriers within continuous populations. Here, we examined incipient speciation within M. polymorpha on the youngest island, Hawaii (~0.7 MYO). The Hawaii Island-endemic riparian tree, M. polymorpha var. newellii (newellii) is a purported case of incipient sympatric speciation due to disruptive selection across a sharp forest-riparian ecotone. Earlier studies demonstrated that relative to its purported progenitor, M. polymorpha var. glaberrima (glaberrima), newellii is better adapted to the mechanical stress of rushing water and high light, two conditions characteristic of the riparian zone. We examined patterns of genome-wide variation among 42 trees representing both varieties from Hawaii Island plus outgroup populations from older islands. Results revealed a split between glaberrima and newellii likely within the geologic time frame of Hawaii Island and complex patterns of demography, including extensive admixture between lineages within Hawaii Island and between islands.  Introgression from populations on older islands, however, did not appear to contribute to the emergence of newellii.  Demographic modeling showed recurrent gene flow between glaberrima and newellii since their split from a common ancestor that contributed to the formation of localized genomic regions of higher divergence.  These genomic islands had elevated levels of both absolute and relative divergence and were enriched for population genetic signals associated with positive selection, especially in newellii.  These results suggest that the islands of divergence were driven by divergent selection on ancestral haplotypes likely associated with adaptation to the riparian environment or reproductive isolation.  Gene ontology-enrichment analysis was conducted for the 341 genes that overlapped with genomic islands of divergence and revealed genes significantly enriched for functions relating to DNA binding and transcriptional regulation and acetyltransferase activity. These results support riparian newellii as a case of incipient ecological speciation with gene flow in trees.

1 - New York University, Center for Genomics and Systems Biology, Dept. of Biology, 12 Waverly Place, New York, NY, 10003, USA
2 - University Of Nevada Las Vegas, School Of Life Sciences, 4505 South Maryland Parkway, Mail Stop: 404, Las Vegas, NV, 89154, United States

Keywords:
Hawaii
incipient speciation
Metrosideros
Population genomics
speciation with gene flow
trees.

Presentation Type: Colloquium Presentations
Session: CO03, Pacific Biogeography
Location: San Luis 2/Starr Pass
Date: Monday, July 29th, 2019
Time: 1:45 PM
Number: CO03001
Abstract ID:930
Candidate for Awards:Margaret Menzel Award