Evolutionary diversification and geographical isolation in Dubautia laxa (Asteraceae), a widespread member of the Hawaiian silversword alliance.
ABSTRACT The Hawaiian silversword alliance (Asteraceae) is one the best examples of a plant adaptive radiation, exhibiting extensive morphological and ecological diversity. No research within this group has addressed the role of geographical isolation, independent of ecological adaptation, in contributing to taxonomic diversity. The aims of this study were to examine genetic differentiation among subspecies of Dubautia laxa (Asteraceae) to determine if allopatric or sympatric populations and subspecies form distinct genetic clusters to understand better the role of geography in diversification within the alliance.
Dubautia laxa is a widespread member of the Hawaiian silversword alliance, occurring on four of the five major islands of the Hawaiian archipelago, with four subspecies recognized on the basis of morphological, ecological and geographical variation. Nuclear microsatellites and plastid DNA sequence data were examined. Data were analysed using maximum-likelihood and Bayesian phylogenetic methodologies to identify unique evolutionary lineages.
Plastid DNA sequence data resolved two highly divergent lineages, recognized as the Laxa and Hirsuta groups, that are more similar to other members of the Hawaiian silversword alliance than they are to each other. The Laxa group is basal to the young island species of Dubautia, whereas the Hirsuta group forms a clade with the old island lineages of Dubautia and with Argyroxiphium. The divergence between the plastid groups is supported by Bayesian microsatellite clustering analyses, but the degree of nuclear differentiation is not as great. Clear genetic differentiation is only observed between allopatric populations, both within and among islands.
These results indicate that geographical separation has aided diversification in D. laxa, whereas ecologically associated morphological differences are not associated with neutral genetic differentiation. This suggests that, despite the stunning ecological adaptation observed, geography has also played an important role in the Hawaiian silversword alliance plant adaptive radiation.
Full-textDOI: · Available from: Mitchell E. Mcglaughlin, May 22, 2014
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ABSTRACT: Premise of the study: Geographic isolation, habitat shifts, and hybridization have contributed to the diversification of oceanic island floras. We investigated the contribution of these processes to the diversification of Pericallis, a genus endemic to Macaronesia. Methods: Data from the chloroplast psaI-accD and trnV-ndhC regions and the nuclear ribosomal internal transcribed spacer region (ITS) were sampled for multiple accessions of all taxa and used to establish phylogenetic hypotheses. Habitat preferences were optimized to investigate habitat shifts, and divergence times were estimated. Species nonmonophyly was investigated using Bayes factors. Key results: Much of the diversification in Pericallis has occurred recently, within the past 1.7 Ma. Three habitat shifts have occurred in the evolution of the genus. However, geographic isolation has played a greater role in its diversification. Novel allopatric patterns were revealed within some species, highlighting the significance of geographic isolation in the evolution of Pericallis. One species (P. appendiculata) that resolved as monophyletic in the ITS analysis was polyphyletic in the chloroplast analysis. Bayes factors provide strong support for the nonmonophyly of P. appendiculata haplotypes, and their phylogenetic placement suggests that ancient hybridization is responsible for the haplotype diversity observed. Conclusions: Multiple markers and extensive sampling provided new insights into the evolution of Pericallis. In contrast to previous studies, our results reveal a more significant role for allopatry than habitat shifts and new evidence for ancient hybridization in the evolution of Pericallis. Our study highlights the power of broad taxon sampling for unraveling diversity patterns and processes within oceanic island radiations.American Journal of Botany 04/2014; 101(101):1-15. DOI:10.3732/ajb.1300390 · 2.46 Impact Factor
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ABSTRACT: Ecological conditions, such as high habitat diversity and the absence of competitors, have been proposed as key determinants of the patterns of speciation observed in oceanic island floras. However, the relationship between plant traits and lineage diversification has received less attention. Here, we review 120 published phylogenetic and population genetic studies of three well-studied oceanic archipelagos (Canary Islands, Galápagos and Hawai‘i) to investigate potential associations between life history characters (growth form and fruit type) and patterns of diversification. The available data suggest that the phenotypic syndrome ‘herbaceous-dry fruited’ was predominant among ancestors of species-rich lineages, although the Hawaiian flora also shows a substantial proportion of ‘woody-fleshy fruited’ ancestors. Growth form, unlike fruit type, is shown to be a labile character strongly selected for woodiness, particularly in radiating lineages. Dry fruits, although representative of diverse dispersal modes and efficacies, are generally associated with a low incidence of inter-island colonization, and the analysis of population genetic data confirms strong genetic differentiation among islands for dry fruited species of radiating lineages. In contrast, fleshy fruited species of monotypic lineages typically show widespread distributions coupled with extensive gene flow among islands, which probably impedes speciation. Our analyses suggest that fruit types associated with limited evidence of dispersal promote recurrent speciation within lineages, although particular character states related to speciation appear to be context dependent. This study reinforces the idea that plant traits associated with island colonization and population persistence are, in addition to ecological conditions, important factors in understanding the patterns of diversification on islands. © 2013 The Linnean Society of London, Botanical Journal of the Linnean Society, 2013, ●●, ●●–●●.Botanical Journal of the Linnean Society 01/2013; in press(3). DOI:10.1111/boj.12127 · 2.70 Impact Factor
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ABSTRACT: Asymmetrical gene flow, which has frequently been documented in naturally occurring hybrid zones, can result from various genetic and demographic factors. Understanding these factors is important for determining the ecological conditions that permitted hybridization and the evolutionary potential inherent in hybrids. Here, we characterized morphological, nuclear, and chloroplast variation in a putative hybrid zone between Schiedea menziesii and S. salicaria, endemic Hawaiian species with contrasting breeding systems. Schiedea menziesii is hermaphroditic with moderate selfing; S. salicaria is gynodioecious and wind-pollinated, with partially selfing hermaphrodites and largely outcrossed females. We tested three hypotheses: 1) putative hybrids were derived from natural crosses between S. menziesii and S. salicaria, 2) gene flow via pollen is unidirectional from S. salicaria to S. menziesii and 3) in the hybrid zone, traits associated with wind pollination would be favored as a result of pollen-swamping by S. salicaria. Schiedea menziesii and S. salicaria have distinct morphologies and chloroplast genomes but are less differentiated at the nuclear loci. Hybrids are most similar to S. menziesii at chloroplast loci, exhibit nuclear allele frequencies in common with both parental species, and resemble S. salicaria in pollen production and pollen size, traits important to wind pollination. Additionally, unlike S. menziesii, the hybrid zone contains many females, suggesting that the nuclear gene responsible for male sterility in S. salicaria has been transferred to hybrid plants. Continued selection of nuclear genes in the hybrid zone may result in a population that resembles S. salicaria, but retains chloroplast lineage(s) of S. menziesii.PLoS ONE 09/2011; 6(9):e24845. DOI:10.1371/journal.pone.0024845 · 3.53 Impact Factor