Thesis Defence: "The evolution of reproductive isolation within an endemic Hawaiian tree species (Metrosideros polymorpha) across environmental extremes" by Alicia Rhoades (EPSCoR Hawaii ECOGEM team)
The causes of speciation, or the formation of reproductive isolating barriers between diverging populations, are poorly known 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 tree lineages.
Title: Genotypic and reproductive analyses of species barriers in two closely related Hawaiian picture-winged Drosophila, D. sproati and D. murphyi
The process of speciation and the mechanisms that contribute to reproductive isolation are a complex and contentious area of study. Incomplete reproductive isolation is common in many species though the degree to which species can hybridize yet remain biologically distinct is largely understudied. The Hawaiian Drosophilidae is comprised of nearly 1000 species and is a well-known system for the study of reproductive isolation. The purpose of this study was to investigate the mechanisms that form and maintain reproductive isolation between two evolutionarily young and little studied species, D. sproati and D. murphyi. Mechanisms of prezygotic and postzygotic reproductive isolation act to reduce gene flow between populations and are most important where species are sympatric. D. sproati and D. murphyi are found both in sympatric locations where the two species are common and in other locations where there is limited or no geographical overlap between the two species. This variation in the geographical distribution of these species may affect the chances that D. sproati and D. murphyi will interact creating a mosaic of hybridization across the Island of Hawaii. The level of hybridization and introgression between D. sproati and D. murphyi was examined with genetic analyses of 8 microsatellite loci. Courtship and reproductive experiments were conducted to investigate the degree of pre- and postzygotic barriers to reproduction. Structure and NewHybrids analyses of microsatellites suggest evidence of asymmetrical hybridization and introgression primarily in areas of sympatry. Pairwise Fst estimates indicate that D. sproati and D. murphyi are genetically distinct but corroborates evidence of hybridization at sympatric sites. Results from courtship and reproductive analyses also show a trend of asymmetrical hybridization. D. murphyi and D. sproati provide a unique opportunity to corroborate our understanding of the patterns and processes that shape diversification.
Julie Gaertner provides an overview of her internship experience with Big Island Invasive Species Committee (BIISC) and presenting her research in creating distribution and change detection maps of Falcataria moluccana (albizia) using remote sensing techniques. The purpose of this internship was to provide a platform to work collaboratively between UH-Hilo TCBES graduate program and a government agency, Big Island Invasive Species Committee (BIISC) to fulfill the requirements of M.S. Degree and gain valuable work experience on Hawaii Island.Mapping the distribution of the highly invasive tree Falcataria fit well with BIISC’s mission of control of invasive species. The presentation will highlight: 1) the overall internship experience and outcomes, 2) research mapping the spatial distribution of Falcataria across the Puna and South Hilo districts of Hawaii Island, 3) change detection map of Falcataria to calculate the rate of growth by examining historical aerial photos and satellite imagery, and 4) using the change detection data to build a model to project the rate of Falcataria canopy spread across the landscape. High-resolution satellite imagery (2009 Worldview-2) and geospatial and remote sensing software (ArcGIS and ENVI) were used to classify vegetation using unique spectral signatures to visualize the spatial distribution of Falcataria. Graduate adviser: Jonathan Price, and committee members Flint Hughes and Jan Schipper.