The Ramsey lab combines molecular analyses, greenhouse studies and field experiments to address classical questions about speciation and adaptation in flowering plants. As is traditional in our area of research, we work on a diversity of non-model plant taxa selected on the basis of natural history rather than tractability to genetic analysis. Ongoing research projects are outlined below.
WILD YARROW. Achillea borealis (Asteraceae) is a complex of ecological races occurring in diverse climatic and edaphic conditions throughout North America. The species is notorious for its taxonomic complexity, incidence of polyploidy and profound degree of local adaptation. Our studies have three foci. (1) Characterization of historical relationships in the circumpolar Achillea millefolium aggregate using DNA sequence data (cpDNA, single copy nuclear genes) and molecular markers (AFLPs, microsatellites). (2) Evaluation of autopolyploidy as mechanism of adaptive divergence and incipient speciation, based on comparisons of naturally-occurring polyploids (“established” polyploid populations) vs. neopolyploids (spontaneous polyploid mutants screened from diploid progenitors). (3) Analysis of the phenotypic and genetic basis of climatic and edaphic adaptation, inferred from classical approaches (segregation of traits in hybrid pedigrees) and modern methods (candidate genes).
ENGLISH IVY. Ivy (Hedera spp, Araliaceae) is a woody vine that is native to Eurasia but cultivated worldwide for its attractive foliage, hardiness and ease of propagation. Ivy is also a serious pest in forest environments along the Atlantic Seaboard and Pacific Coast of North America. Despite its horticultural value and ecological impacts, many aspects of ivy phylogeny, biogeography and invasion biology are poorly understood. Our work involves four major projects. (1) Characterization of Hedera phylogeny based on DNA sequence data (cpDNA, single copy nuclear genes) and molecular markers (microsatellites). (2) Analysis of the taxonomic composition and population genetic structure of invasive ivy populations in North America using cpDNA and flow cytometry. (3) Evaluation of species-differences in invasiveness, inferred from field observations and experiments. (4) Characterization of fruit dispersal and seedling recruitment in cultivated and naturalized ivy populations.
CREOSOTE BUSH. Creosote Bush (Larrea tridentata, Zygophyllaceae) is a dominant shrub across the Mojave, Sonoran and Chihuahuan deserts of the southwestern U.S. and northern Mexico. This taxonomic species is comprised of diploid, tetraploid and hexaploid “races” that replace each other along a longitudinal gradient. We are working on three projects in this system. (1) Characterization of phylogeography and population structure of L. tridentata in comparison to its South American relatives, based on DNA sequence data (cpDNA, single copy nuclear genes). (2) Analysis of pre-zygotic barriers (habitat isolation, phenological isolation) and post-zygotic barriers (triploid block, sterility) isolating diploid and autopolyploid races of L. tridentata, as inferred from field observation and experiments. (3) Evolutionary rates of host specialization by Larrea herbivores and pollinators.
(a) Geographic distribution of the ecological races of Achillea borealis. Wild yarrow ecotypes are distinguished by a variety of morphological, phenological and life-history traits. (b) Phylogenetic relationships of the circumpolar A. millefolium aggregate inferred from cpDNA (five noncoding regions). North American populations have a peculiar genetic structure (single major haplotype, numerous minor haplotypes) that suggests recent demographic expansion. (c) Phylogenetic relationships inferred from AFLPs (1,013 fragments).
(Left) Leaf morphology of natural Hedera species, as grown in a common greenhouse environment. Taxonomic identification of naturalized populations is complicated by phenotypic plasticity and the occurrence of cultivars. (Right) Partial haplotype table based on noncoding cpDNA (twelve regions). The majority of North American invasives have haplotypes identical to European species (H. helix and H. hibernica) and distinct from Mediterranean and East Asian species.
(a) Photographs of Larrea tridentata (“creosote bush”), a dominant shrub in the southwestern U.S. and northern Mexico. (b) Spatial distribution of diploid, tetraploid and hexaploid populations (colored icons) in relation to traditionally-ascribed geographic ranges (colored backgrounds).
Cytotype distribution corresponds to the Chihuahuan (2x), Sonoran (4x) and Mojave (6x) deserts, with regional cytotype co-occurrence in some areas. (c) Creosote phylogeography inferred from noncoding cpDNA. The North American creosotes are monophyletic, closely-related to the South American Larrea divaricata, and have undergone a recent demographic expansion.
