Mitchell E. McGlaughlin

Gene flow in Dubautia arborea and D. ciliolata: the roles of ecology and isolation by distance in maintaining species boundaries despite ongoing hybridization

The relative roles of gene flow and natural selection in maintaining species differentiation have been a subject of debate for some time. The traditional view is that gene flow constrains adaptive divergence and maintains species cohesiveness. Alternatively, ecological speciation posits that the reverse is true: that adaptive ecological differentiation constrains gene flow. In this study, we examine gene flow and population differentiation among populations of two species of the Hawaiian silversword alliance, Dubautia arborea and D. ciliolata. We compare divergence in putatively neutral microsatellite markers with divergence in leaf morphometric traits, which may be selectively important or physiologically linked to selectively important traits. Gene flow between populations was found to be significant in only one of the two species, D. arborea. Leaf morphometric differentiation between species was significant, though not among populations within species. No evidence of effective genetic introgression was observed between apparently ‘pure’ populations of these species. Gene flow as measured by microsatellites was not correlated with geographic distance between populations, but was correlated with the linear placement of the widest part of the leaf. Because these two species are interfertile, as demonstrated by the presence of active hybrid zone, the lack of genetic introgression and the maintenance of species boundaries may be associated with natural selection on differential habitat.

Hybrid Origin and Genomic Mosaicism of Dubautia scabra (Hawaiian Silversword Alliance; Asteraceae, Madiinae)

Abstract Incongruence among different estimates of species relationships in plants, from different molecules, cytogenetic data, biogeographic data, morphological/anatomical data or other sources, has been used frequently as an indication of introgression, hybrid species origin, or chloroplast (cp) capture. In plants, these incongruences are most often seen between data derived from the nuclear vs. the cp genomes and the nuclear markers used for comparison usually have been from the nuclear ribosomal (nr) internal transcribed spacer region (ITS). The amount of genomic material shared between introgressing species can be highly variable. In some of these cases, other nuclear genomic regions have moved between species without leaving a signature on the nrITS. An example of well-supported phylogenetic incongruence is the placement of Dubautia scabra (DC.) D. D. Keck in the Hawaiian silversword alliance (HSA); evolutionary hypotheses for D. scabra based on molecular as opposed to cytogenetic data are strongly discordant. In this paper, we test these two conflicting phylogenetic hypotheses regarding the evolutionary relationships of Dubautia scabra using evidence from six low-copy nuclear genes, as well as multiple chloroplast noncoding regions and nrITS. The nrITS region is also examined for the presence of multiple copy types. Incongruence between inferred relationships based on nuclear chromosomal arrangements and molecular phylogenetic data from chloroplast DNA and nrITS is resolved in favor of a hypothesis of ancient hybridization rather than cytogenetic homoplasy involving dysploidy. Most single-copy nuclear genes track histories of D. scabra compatible with cytogenetic data whereas chloroplast and nrITS data track a common, different history that appears to reflect hybridization with a chromosomally distinct lineage that also occurs on Maui Nui and Hawai‘i (the Big Island).

Genetic diversity following demographic recovery in the insular endemic plant Galium catalinense subspecies acrispum

Galium catalinense (Rubiaceae) is a perennial shrub consisting of two subspecies endemic to California’s Channel Islands: Galium catalinense subsp. catalinense on Santa Catalina Island, and G. catalinense subsp. acrispum, a state-endangered taxon on San Clemente Island. A long history of overgrazing by introduced herbivores has contributed to population declines in G. catalinense subsp. acrispum. We surveyed 12 populations throughout the taxon’s range for genetic variation using eight polymorphic microsatellite loci to determine the genetic impact of this demographic bottleneck. At the taxon level, 65 alleles were identified with an average of 8.1 alleles per locus, although many alleles were rare; the effective number of alleles per locus averaged 2.6. Expected heterozygosity was 0.550. Individual populations had between six and eight polymorphic loci, with expected heterozygosities ranging from 0.36 to 0.60, and effective numbers of alleles ranging from 1.8 to 3.5 per locus. Populations fell into three or four genetic clusters, depending on type of analysis, which may represent refugia where the populations persisted during intense herbivory. There is little evidence of genetic bottlenecks or substantial inbreeding within populations. These findings, coupled with indications of recent migration between populations, suggest that G. catalinense subsp. acrispum is currently unlikely to be endangered by genetic factors, but small population sizes make the taxon vulnerable to future loss of genetic diversity. Management strategies based on these genetic data, population sizes, and the spatial distribution of populations are discussed.

Evolutionary diversification and geographical isolation in Dubautia laxa (Asteraceae), a widespread member of the Hawaiian silversword alliance

BACKGROUND AND AIMS 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. METHODS 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. KEY RESULTS 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. CONCLUSIONS 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.

Population genetic diversity and species relationships in the genus Rhinanthus L. based on microsatellite markers

The genus Rhinanthus L. is complex, containing many taxonomically unresolved taxa. In this paper we studied genetic variation and species relationships in 15 populations of six Rhinanthus species from three sections. For this purpose, we developed new microsatellite primers for R. osiliensis and used them to investigate genetic variation in two narrow endemics (R. osiliensis, R. javorkae) and in four widespread species (R. rumelicus R. wagneri, R. angustifolius and R. minor). Species-specific private alleles were found in all species except R. osiliensis and R. angustifolius. The Bulgarian endemic R. javorkae showed the lowest genetic variation, followed by widespread R. minor and Estonian endemic R. osiliensis. Rhinanthus javorkae and R. minor were genetically most differentiated. Section Cleistolemus is weakly structured genetically, indicating close affinity between R. osiliensis, R. rumelicus, R. wagneri and R. angustifolius.

Isolation of microsatellite loci from endangered members of Lotus (Fabaceae) subgenus Syrmatium

Lotus subgenus Syrmatium is a group of 11 plant species that exhibit extensive ecological and morphological diversity throughout the California floristic province. Fifteen polymorphic microsatellite loci were isolated from two taxa, Lotus argophyllus var. adsurgens and L. dendroideus var. traskiae, and were screened for variability in 15 additional taxa within Lotus subgenus Syrmatium. Moderate levels of variability were observed with mean numbers of alleles per locus ranging from 1.3 to 7.3. The mean observed and expected heterozygosities ranged from 0.09 to 0.47 and 0.10 to 0.79, respectively. These new loci will be useful in conservation genetic and evolutionary studies within Lotus subgenus Syrmatium.

Characterization of microsatellite markers isolated from members of Oreocarya (Boraginaceae)

Genus Oreocarya (Boraginaceae) contains approximately 62 species, many rare or uncommon. The genus is endemic to western North America with a center of diversity in the Colorado Plateau and Great Basin of the U.S. Rarer members are often restricted to regionally uncommon soil types. Twelve polymorphic microsatellite loci were isolated from three taxa, Oreocarya bakeri, O. revealii, and O. paradoxa, and were screened for variability in 56 individuals of O. revealii and O. paradoxa. Cross amplification was assessed using 7 additional taxa. Diversity was observed to be high, with mean numbers of alleles per locus ranging from 4.0 to 15.5. Mean expected and observed heterozygosities ranged from 0.32 to 0.87 and 0.16 to 0.70, respectively. With its frequent development of soil restricted lineages, Oreocarya is an interesting system for investigations of edaphically associated rarity. The development of these new loci will aid evolutionary and conservation investigations in this system.

Microsatellite loci development for rare Colorado Sclerocactus (Cactaceae)

The genus Sclerocactus (Cactaceae) consists of 15 species, which have a confused taxonomic history due to morphological similarities and distribution overlap. Habitat loss and hybridization are of concern and have established cause for genetic investigations to further understand the genus and develop conservation strategies. Thirteen variable microsatellite loci were identified using S. glaucus and S. parviflorus and were tested in three additional species: S. wetlandicus, S. brevispinus and S. cloverii. The mean number of alleles per locus ranged from 2 to 11, with an average of 6.6. The observed and expected mean heterozygosity ranged from 0.00 to 0.90 and 0.26 to 0.90, respectively. These loci will aid in determining levels of hybridization, diversity and taxonomy of Sclerocactus.

Variable length chloroplast markers for population genetic studies in Acmispon (Fabaceae)

UNLABELLED PREMISE OF THE STUDY To estimate genetic structure, chloroplast loci containing length-variable regions were developed for two legumes, Acmispon argophyllus and A. dendroideus. • METHODS AND RESULTS Primers for 14 chloroplast loci containing repeat regions were developed from the chloroplast genome sequence of the legume Lotus japonicus and tested in Acmispon. Nine loci exhibited polymorphism in Acmispon, with up to six alleles per locus. Gene diversity ranged from 0 to 0.775 in A. argophyllus and 0.142 to 0.766 in A. dendroideus. The primers also amplified in other Acmispon species. Sequencing of the fragments revealed discordance between fragment sizes and underlying sequence for three loci containing complex repeat regions. • CONCLUSIONS Although genotypes were easily generated and sized, sequencing may be more informative of genetic variation in loci with complex repeat regions. These loci exhibit substantial variation and should be useful for understanding genetic structure associated with seed dispersal in Acmispon.

Microsatellite primers for the narrowly endemic shrub Eriogonum giganteum (Polygonaceae)

PREMISE OF THE STUDY Microsatellite primers were designed for Eriogonum giganteum var. formosum, an endemic shrub of San Clemente Island, to investigate population structure, genetic diversity, and demographic history. METHODS AND RESULTS Twelve polymorphic microsatellite loci were isolated from the California Channel Island endemic Eriogonum and were screened for variability. The primers amplified one to eight alleles in the target taxon. Many primers also amplified in conspecific and congeneric (E. arborescens, E. fasciculatum, E. grande, E. latifolium, and E. parvifolium) taxa and in the closely related Chorizanthe valida. The total number of alleles per locus for all taxa screened ranged from three to 24. CONCLUSIONS These primers will be useful for conservation genetic and evolutionary studies within the California Channel Island endemic Eriogonum.