Office Location: Romberg Tiburon Center Office Hours:to be arranged Office Phone: 415.338.3750 Email: sarahcoh@sfsu.edu	Courses Marine Ecology Molecular Methods in Ecology, Evolution, and Oceanography
	About the Instructor Education
	Research Marine population biology, evolution, population genetics, molecular evolution, conservation
	Department Biology Romberg Tiburon Center

Education
PhD University of Washington, Zoology
BA   Swarthmore College, Biology
 

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Research Interests

Adaptive change in response to environmental stress

We are looking at immunogenetic and detoxication genetic variation in natural fish populations subject to varying pollutant and parasite stress. The Major Histcompatibility Complex (MHC) is the extraordinarily variable genetic system that controls the adaptive immune system in vertebrates. This system is under selection related to disease resistance and in at least some cases, mate choice or reproductive success as well. One part of this system, the Class 2 antigen-binding receptor, contains the most polymorphic locus found in any organism thus far, and thus presents intriguing population genetic and molecular evolutionary puzzles about the generation and maintenance of very large numbers of alleles within populations and species.

Left: New Bedford Harbor, MA Superfund site, where Fundulus heteroclitus (right) show adaptation to extreme chemical contamination, bizarre parasite loads, and divergent immunogenetic profiles.

 

Some questions that we are interested in: How fast can adaptation to novel environmental stresses occur? What genetic mechanisms allow this to occur? What types of life histories most favor rapid genetic adaptation to stress rather than acclimation? Do immunogenetic loci show signals of local environmental degradation? This work is funded by the US EPA, the Hudson River Foundation, and Harvard College undergraduate research awards, and includes collaborations with scientists at Woods Hole Oceangraphic Institute, the EPA Atlantic Ecology Division, the University of Rhode Island, and the Virginia Institute of Marine Sciences.

Connections between marine populations

Current theory suggests that many organisms exist in a patchy network of populations that are loosely linked but may often have independent dynamics. These metapopulations may include both source populations that provide recruits to many other non-reproducing or non-reseeding populations (called sinks). A first step toward identification of source and sink populations is to combine demographic information with genetic markers to identify patterns of connectedness between local populations. Identification of source and sink populations will provide marine managers with the best possible information for designing fisheries regulations and reserves. Ongoing projects related to these issues are studies using microsatellite and Single Nucleotide Polymorphism (SNP) DNA variation in green sea urchins and lobsters in the Northwest Atlantic and Washington state. This collaborative work is funded by Sea Grant and the Marine Ecosystem Health Program at UC Davis.

Neuston nets towed behind fishing boats are used to collect lobster larvae at different developmental stages such as the stage IV postlarva shown below. Cohort sampling is used to look for population connections between nearshore and offshore deep canyon fishing areas shown in the 3-D representation of the Gulf of Maine and adjacent waters below. Deep canyons are an increasingly important resource for fishing fleets.

Evolution of recognition systems in colonial invertebrates

Invertebrates from sponges to protochordates have the ability to distinguish self from non-self, an ability we more frequently associate with the vertebrate immune system. Invertebrates across the phylogenetic spectrum show a dazzling diversity of mechanisms and uses for such recognition systems from pathogen recognition to competition for space in crowded marine habitats. These systems operate at molecular, cellular, tissue, and whole organism levels and involve specialized morphology and behavior. Studying these intriguing traits in invertebrates gives us an understanding of biological diversity in form and function and also a means to interpret fundamental processes in development and immunity in both invertebrates and vertebrates.

The speciose family of botryllid colonial ascidians shows behavioral variation in space competition apparently related to the evolution of their intraspecific highly variable genetic recognition system. Using techniques from molecular phylogenies and ecology to time-lapse video analysis of behavior, we have been studying the role of recognition systems in speciation and invasion success.

Identifying populations and species of introduced marine invertebrates

Recently, marine species with limited natural dispersal potential have been recognized as an important group of invaders with an added twist. These local dispersers may have an enhanced ability to evolve local adaptions to novel environments, making them a potentially yet more harmful group of invaders. Most phyla of marine invertebrates include short as well as long distance dispersers and many short dispersers, including ascidians, are now recognized as extremely successful global invaders. Many of these species are morphologically cryptic. Our work thus far includes molecular and behavioral characterization of introduced species, in collaboration with scientists in Japan and elsewhere around the world. Much more remains to be done in the lab and field worldwide identifying species and characterizing physiological and ecological tolerances and variability to understand invasion abilities and evolutionary processes in this beautiful and diverse group of invertebrates.

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Selected Publications

Cohen, S. 2002. MHC variation in natural populations of an estuarine fish: high levels of variation and relationship to severe environmental stress. Molecular Biology and Evolution 19 (11): 1870-1880.

Cohen, S. and D. Nacci. 2002. Effects of dioxin-like compound (DLC) contamination on an estuarine fish species: adaptive changes at specific loci. Conference proceedings, US/Vietnam Scientific Conference on Agent Orange/Dioxins, March 3-6, 2002, Hanoi, Vietnam. http://dir-apps.niehs.nih.gov/diox2002/search.cfm

Cohen, S. 2000. Botryllid ascidian invasions: genetic and behavioral evidence for multi-species invasions and character divergence following introductions. In, Proceedings of the First National Conference on Marine Bioinvasions, MIT Sea Grant.

Cohen, S., Saito, Y. and I. Weissman. 1998. Evolution of allorecognition in botryllid ascidians inferred from a molecular phylogeny. Evolution 52(3):746-756.

Cohen, S. 1996. The effects of contrasting modes of fertilization on levels of inbreeding in the marine invertebrate genus Corella., Evolution 50(5): 1896-1907.