Link to the Polystichum Homepage

David Barrington

Dr. David BarringtonProfessor of Plant Biology
Director of the Pringle Herbarium (UVM's plant systematics and conservation collection resource)

Ph. D. 1975, Harvard University

Email: David.Barrington@uvm.edu

Phone: 802-656-0431

Office: 225 Marsh Life Science Building

Research Area: Plant Systematics and Evolution

Courses Taught:
Exploring Biology (BCOR 12)
Systematics & Phylogeny, the summer version (PBIO 109)
Biology of Ferns (PBIO 209)
Botany Field Trip - Plant Diversity in Costa Rica, January of odd years (PBIO 232);
Tropical Plant Systematics (PBIO 241)

Mailing Address:
Plant Biology Department
Marsh Life Science Building
109 Carrigan Drive
University of Vermont
Burlington, VT 05405-0086 USA


THE BARRINGTON LAB AT THE UNIVERSITY OF VERMONT

Summary of Research Program

My graduate students Erin Sigel, Monique McHenry, and I are studying an array of problems in systematics and evolution, but they have in common questions of phylogeny and speciation in the context of historical biogeography and ecology. Though we usually work on ferns, we have been known to work on flowering plants as well.  All of our projects are based in genetic analysis of the plants: our data sets include various chloroplast and nuclear DNA sequences and isozyme analysis.  We are currently developing several low-copy nuclear genes to include in our studies.  At the same time, we are deeply interested in the evolution of plant structures, so we have developed a number of tools for representing plant structural features and analyzing them phylogenetically.  A generalized question that applies to all of our questions is: what are the likely historical and ecological determinants of the genetic profile encountered in a species complex, as constrained by the geographic distribution of the taxa we are studying. In all of this work, we find the geological context for our study to be of interest.  A continuing theme in the lab is the understanding of hybridization and polyploidy in speciation histories. We hope to take advantage of molecular techniques to identify the phylogenetic origins of hybrid progenitors in cases where the hybrid progenitors are unknown.

We have three undergraduates working in our lab at the moment: Dan Koenemann, Morgan Moeglein, and Julian Sbardella.

Cathy Paris works closely with us in developing our research projects; she has her own website here.


SYSTEMATICS OF FERNS: POLYSTICHUM

For some years I have been interested in the fern genus Polystichum, which includes well-known ferns such as the sword fern of the American northwestern forests and Christmas fern, widely distributed in the eastern United States. The European holly fern Polystichum aculeatum is a well-known species, and the Eastern Asian Polystichum tsus-simense is a commonly sold as a houseplant. Mere interest in a perversely difficult group originally attracted me to the genus, but as I have worked with the group, its appeal to me as a system for understanding problems in speciation, hybridization, and phylogeny has steadily grown. Much more detail on the genus Polystichum is available at the Polystichum webpage, which I maintain.


I can now identify four major research directions in my work on the genus Polystichum.

1. Circumscribing the phylogeny of Polystichum and its allied genera with molecular character sets

I remain most interested in the evolutionary history of Polystichum in the American tropics.  My early work on the morpholoy and chromosome behavior
Polystichum tripteron in Japan
of species and hybrids established a species concept and a sense of the morphological diversity in Latin America.  Isozyme variation and meiotic chromosomal behavior proved to be powerful in testing hypotheses for hybridization and polyploidy (e.g. Barrington 1990), but little sense of the evolutionary history of divergence was to be gained from these datasets.  About 1992 it became clear that further progress with Polystichum in tropical America would necessitate developing a basic phylogeny for the whole genus. Given the size and distribution of the genus (about 260 species worldwide, with endemic centers in China, Mexico, Africa, the austral regions of South America, New Zealand, and Australia, and minor regions elsewhere), this is an intimidating task. No consistent infrageneric classification based on a worldwide analysis exists, though there is an abundance of subgeneric taxa. 

Working with undergraduate Damon Little, now a curator at the New york Botanical Garden, I used a combination of molecular (rbcL) and morphological characters to develop a working phylogeny for Polystichum and its allies.  This work (Little and Barrington, 2003) established a primitive idea of the relations and history of the genus.   My work with master's candidate Heather Driscoll (now on the staff of the Berkeley Herbarium) on the origin of the three Hawaiian species of Polystichum (Driscoll and Barrington, 2007) was our next step in developing a world phylogeny (See this tree).  This phylogeny reveals that species from the same geographic region are more likely to be related than are species with similar morphology. The most surprising and enlightening example of this pattern was discovering that the allegiance of the very similar P. lemmonii and P. mohrioides (sometimes treated as one species!) was to species from their own region (western North America and austral South America and the Antarctic Islands respectively) rather than to each other
bonseyi
Polystichum bonseyi, Hawai'i
. 

At this point, I was fortunate to be approached by two different groups of Chinese researchers working on the polystichums there.   What ensued can only be called an Asian explosion, as we have since worked together to elaborate multisequence phylogenies rich in species from China, the world center of diversity for the genus (Lu et al., 2007; Li et al., 2008).  Dr. Li Chun-xiang (who is at the moment a visiting scholar here) and I are now collaborating on building a phylogeny from a much-expanded dataset (the target is four sequences for 130 species representing all endemic centers in the genus).

This array of work leads me to suggest that the significant clades in the genus are in general confined to endemic centers -- about a dozen of them -- in regions already known for their diversity in other groups. (See map of Polystichum diversity by region here.) Three regions in Eastern Asia and the Andes emerge as the most diverse centers for Polystichum in the world.


2. Exploring pattern and process of divergence using molecular and biogeographic contexts in tropical America

The most important result of the world-level molecular analysis for me has been our demonstration that the New World
bonapartii pinnae
Polystichum bonapartii pinnae, from Ecuador
tropical species we sampled constitute a monophyletic group.    Working principally with Ph.D. candidate Monique McHenry, I am interpreting geographic and elevational distribution of the tropical American species in the light of our molecular phylogeny to pursue questions about the isolating mechanisms that have operated in yielding the great diversity of species in the region (over 40 species in the continental neotropics, over 30 in the West Indies).  We continue to be interested in morphological evolution -- rapid changes in leaf dissection during recent evolution in the Andes are particularly striking, but we also have potential insights into the impact of exploring the tropical alpine on the morphology of these plants.  We have been fortunate in our collaborators here -- Klaus Mehltreter, Marcus Lehnert, Michael Kessler, Mike Sundue, and Jürgens Kluge have all sent substantial sets of plants.  We are also working with Joao Condack, who excited about the diversity of Brazilian Polystichum and has made great progress in locating Polystichum populations in southeastern Brazil.  We are happy to help anyone in Latin America with identification, to the best of our current ability. 

The potential to use all these tools together to discover the patterns of ecological and geographic isolation that lead to the diversity in these montane regions is tantalizing; it inspires all we currently do with our work in the lab.

3. Understanding the edges between species as they are complicated by hybridization and polyploidy

Working in New England and tropical America, I have used a combination of isozyme markers, morphometric characters, and chromosome analysis to expand upon the classic works on hybridization and polyploidy on the genus contributed by Manton (1950) and Wagner (1963). It is clear that in the montane regions of the American tropics, just as in the north-temperate, Polystichum is involved in an array of reticulate (hybrid) interactions. My tropical work, which centers on the Talamanca range of Costa Rica and Panama, reveals that hybrids are common and that allopolyploidy, the origin of species through hybridization and doubling of chromosomes, is a contributor to species diversity (three of 12 Costa Rican species are polyploid).  A spin-off of this work is my analysis of the composition of the fern flora above treeline in Costa Rica for a book on the páramos of that country (Barrington, 2005).

It has become clear from this work that a new assault on the role of polyploidy in the origin of species is relevant to discovering the general patterns of evolution in Polystichum. Consequently, our current goal is to identify nuclear genes that we can clone, allowing us to recover multiple genomes from polyploids and trace their individual phylogenetic relationships.  This has turned out to be a difficult task, since so little has been done with nuclear genes in the ferns. 

We have found, looking at the low-copy nuclear genes LEAFY and PGIc, that intron variation in these genes is nowhere near as great as in the angiosperms, where these markers have been quite useful.  What little variation we have encountered may be allelic diversity, though other explanations for the patterns we are seeing are possible.  Our current goal is to sequence as much of PGIc as possible, with the idea of locating the variation in the exons responsible for the allozymes that have proved so powerful as stable species markers in the genus.  In this work, both Monique McHenry and undergraduate Morgan Moeglein are the principals. 

4. Allied nomenclatural and floristic work on Polystichum.

In the process of developing my interest in Polystichum I have of necessity sought to control the immense nomenclatural problems associated with naming species in the genus and providing floristic treatments. I maintain a virtually complete nomenclatural database for the genus, which I am happy to use as a basis for answering any questions colleagues have about names in the genus. 


SYSTEMATICS OF FERNS: DRYOPTERIS CAMPYLOPTERA and ALLIES
Dryopteris dilatata Dryopteris dilatata  in the British Isles

Erin Sigel, a masters' student in my lab, is revealing the pattern of polyphyletic origin to the polyploid species complex including our northeastern mountainwoodfern Dryopteris campyloptera and its European counterpart Dryopteris dilatata.   For her master's work, Erin hopes to determine for the number of origination events for D. campyloptera and D. dilatata; identify the present populations of D. expansa most genetically similar to the expansa genomes in D. campyloptera and D. dilatata, identify the populations of the D.intermedia-aggregate species that are most genetically similar to the intermedia genomes in D. campyloptera and D. dilatata genotypes; infer the historical geographic location of the diploid progenitor populations; and finally compare genetic divergence among populations of each tetraploid with genetic divergence between the two tetraploid taxa as a basis for assessing divergence of the two.
    Some indication of the answers to these questions lies in the multisequence chloroplast-sequence dataset that Erin has already assembled, but the greater variation available from amplified-fragment-length polymorphisms (AFLPs) holds promise of a fine-resolution answer to the questions addressing population-level history and number of species origins in this classic problem of species polyphyly. 


GENETIC STRUCTURE AND HISTORICAL BIOGEOGRAPHY OF FERNS: MATTEUCCIA STRUTHIOPTERIS (THE OSTRICH FERN)
Matteuccia fiddleheads fiddleheads
of
Matteuccia
struthiopteris

Our work on the Quaternary Biogeography of New England (Paris and Barrington, 2007) has led us to open a new line of research, into explaining the historical origin of the distribution of genetic diversity in local ferns -- casting the problem in the context of a classic duality in population genetics.  We are studying genetic variation in the fern commonly collected as a food (the fiddlehead fern, Matteuccia struthiopteris) with the idea of tracing its patterns of genetic diversity.   We proceed with two possibilities in mind -- that either the large, more continuous populations in river-bottom communities harbor the greater diversity or the small, peripheral, more isolated populations in patchy habitats of smaller watersheds harbor the greater diversity.   In the context of history, has the diversity been sustained by the large central populations or newly elaborated in the more periperhal populations.   Or, to render the problem more complex, are the now-peripheral populations remnants of past central populations harboring a museum of the diversity archived in that former large system?  

Our work towards this goal is with AFLPs -- and undergraduates Dan Koenemann and Jacqueline Maisonpierre are pursuing this interesting problem.  This work is funded by CSREES funding through the Vermont Agricultural Experiment Station.  


PAST STUDENTS AND THEIR PROJECTS

Julie Dragon --- Carex systematics and biogeography

Sonja Schmitz --- Lathyrus japonicus historical biogeography


RECENT, FAVORITE, AND FORTHCOMING PUBLICATIONS

Note: linked publications are to PDFs