My research program centers on plant evolution with an emphasis on speciation in trees. Hawai`i is world renowned as an evolutionary hotspot and the source of numerous textbook examples of adaptive and non-adaptive radiations.
I. Divergence and Speciation in Trees
There are an estimated 100,000 species of trees. Yet, little is known about how reproductive isolating barriers accumulate between diverging tree populations to generate new species. Studies in my lab are producing data on the strengths of, and interconnections among, neutral genetic distance, ecological distance, and reproductive isolation among taxa of Hawaiian Metrosideros. Results to date have: (1) established Hawaiian Metrosideros as a model for speciation studies in trees, (2) revealed initial insights into how adaptation along strong environmental gradients or ecotones leads to phenotypic divergence, the evolution of reproductive isolating barriers, and the gradual isolation of gene pools (speciation) in trees, and 3) suggested that postzygotic isolating barriers may evolve before prezygotic isolating barriers during speciation in widespread trees.
a) Local Adaptation and Divergence among Hawaiian Metrosideros taxa: This work comprises molecular, field and greenhouse work and has been funded since January 2009 by NSF Centers for Research Excellence in Science and Technology (CREST) grants. My work on CREST I uses molecular approaches along with field and greenhouse methods to contrast patterns of neutral and functional genetic variation among forms of Hawaiian Metrosideros. Together, these studies are yielding insight into the adaptive differences among Metrosideros’ many forms, the strengths of isolation of their gene pools, and insight into the evolutionary relationships among forms. Our extensive field observations on four Hawaiian Islands have led to the identification of 25 forms of Metrosideros and the recording of their geographic distributions. To date, we have genotyped at 9 microsatellite loci over 1,800 individuals from 25 purported taxa from across the main islands. Our results indicate that four of Hawai`i Island’s five named varieties of M. polymorpha are valid evolutionary units, that young Hawai`i Island was likely colonized by multiple older, partially diverged varieties that experienced increased hybridization on the island’s young volcanoes, and that the Hawai`i Island-endemic riparian variety (var. newellii) arose in sympatry from a bog form on the oldest volcano (Kohala) within the past 500,000 years (DeBoer & Stacy 2013; Stacy et al. 2014). This work has established Hawaiian Metrosideros as a rare case of incipient radiation in trees and thus as a useful model for studies of divergence and the evolution of reproductive isolating barriers at the early stages of speciation. Var. newellii has emerged as an especially interesting form, providing the opportunity to study incipient speciation within a long-lived tree driven by an extreme environment. Currently we are SNP-genotyping (at “single-copy” nuclear genes) 196 individuals of Pacific and Hawaiian Metrosideros to improve insights into the evolutionary history of the genus. Through a broader phylogenetic analysis based on a subset of these genes, we suggest an expansion of genus Metrosideros to include Carpolepis of New Caledonia and Tepualia of South America (Pillon et al. 2015).
Studies of ecological divergence within Hawaiian Metrosideros reveal strong evidence of differential local adaptation among taxa at the seedling stage, beginning with significant divergence between the two common successional varieties on Hawai`i Island in their use of both light and nitrogen (Morrison and Stacy 2014). This work demonstrated the presence of the classic plant life history trade-off of fast growth in high light and high survivorship in shade, but notably within a single species. These results suggest that this trade-off may evolve within a species through disruptive selection even in the presence of gene flow and implicate a role for Hawaii’s striking environmental heterogeneity in the emergence of at least two endemic forms of this woody genus. Four other studies of seedling-stage ecological divergence among taxa are ongoing: field and greenhouse studies (5 and 8 taxa, respectively) of divergence among O`ahu taxa – and field and greenhouse studies of divergence between two taxa along the forest-riparian ecotone and between two Hawai`i Island taxa along a steep elevation gradient.
Other related Metrosideros studies underway or completed by my lab group include: 1) Variation in floral morphology at the M. polymorpha experimental common garden in Volcano (Johansen and Stacy, in prep); 2) Variation among taxa in leaf micromorphology (Sur et al., in progress); and 3) A collaborative study on Metrosideros metabolomics with UH Hilo’s College of Pharmacy to characterize leaf chemical variation within and among taxa on Hawai`i Island and O`ahu (Clark et al., in progress).
b) Reproductive Isolation among Hawaiian Metrosideros taxa: Our studies of cross-fertility and hybrid fitness in Metrosideros on Hawai`i Island (within species) and O`ahu (within and between species) funded by an NSF CAREER grant are revealing a range of partial reproductive barriers among forms that vary in genetic distance. Results of one-way crosses to high-elevation M. polymorpha v. polymorpha as maternal trees on Hawai`i Island revealed three contrasting forms of modest, early-stage reproductive isolation in the three cross types, including reduced pollen tube density in crosses with the late-successional v. glaberrima, consistent with (but not conclusive of) reinforcement of reproductive isolation between two hybridizing varieties to prevent the formation of less-fit hybrids (Rhoades et al. a & b, in prep). Studies of post-zygotic barriers are underway.
Previous studies in my lab documented a young intraspecific hybrid zone between the two successional varieties, v. incana and glaberrima on a ~150-year-old lava flow, comprising parental, F1, and backcross-v. incana trees (based on analysis of adult morphology and parent-offspring analysis; Stacy et al., in review). Work in this hybrid zone revealed reduced hybrid fertility, especially of purported backcross trees, as the only significant barrier isolating these varieties (Stacy et al., in revision; Stacy and Powless, in prep.). The observed pattern of hybrid fitness is consistent with the evolution of partial isolation by negative epistasis in hybrid genomes (Bateson-Dobzhansky-Muller incompatibilities). Lastly, crosses involving five taxa on O`ahu, also comprising a range of pairwise genetic distances, are revealing extrinsic postzygotic isolation (reduced fitness of F1 hybrid seedlings in parental environments) to be a significant isolating barrier among taxa.
II. DNA Barcoding on Hawai`i Island
This Gordon & Betty Moore Foundation-sponsored project involved extensive field sampling and DNA sequence analysis of two insect groups and two flowering plant groups in Hawai`i to evaluate the utility of DNA barcoding for young species of plants and animals, and to identify additional DNA “barcoding genes” useful for these groups.
For the plant groups, we used Roche 454 pyrosequencing of pooled-species runs to develop single-nucleotide-polymorphism (SNP) markers from the transcriptome for DNA barcoding. In addition to identifying DNA barcodes (Pillon et al., 2013; Applications in Plant Sciences), we were able to reconstruct cytoplasmic-DNA-based and nuclear-DNA-based evolutionary relationships among populations and taxa within Hawai`i to reveal significantly greater coalescence times in nuclear genes (Pillon et al. 2013; BMC Evolutionary Biology), significant phylogenetic discordance among nuclear genes (Pillon et al. 2013; Molecular Phylogenetics and Evolution), and high-resolution insights into the colonization and hybridization patterns of these groups on Hawai`i Island (Pillon et al., in revision).
Four other recent studies reveal the maintenance of species boundaries in sympatric Cyrtandra species predominantly through postzygotic barriers (Johnson et al., in press), and a cryptic adaptive radiation of tropical trees in New Caledonia (Pillon et al., 2014, New Phytologist), and document new records within plant groups (Pillon et al., 2014, New Zealand Journal of Botany; Hopkins et al., in review, Kew Bulletin).