| Department
of Biology The Pennsylvania State University 213 Mueller Laboratory University Park, PA, 16802 |
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Research Interests Introduction: NEW: Our research on coral reproduction was featured in News Focus Article published in the journal Science (14 Dec 2007): Vol. 318. no. 5857, pp. 1712 - 1717. DOI: 10.1126/science.318.5857.1712
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ENSO-mediated gene flow across the Eastern Pacific Barrier: The geographic isolation
of shallow-water tropical corals living in the eastern Pacific has stimulated
much interest in their origin and evolution. As elsewhere in tropical
seas, habitats formed by these corals harbor a rich diversity |
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foundation species of Caribbean reefs: Most reef connectivity research has been conducted on fish with highly developed larvae. However, larvae of the organisms that build the reef ecosystem, scleractinian corals, are much less able swimmers. We thus chose the foundation species of Caribbean reefs as a model organism for reef connectivity: the elkhorn coral Acropora palmata. A. palmata forms branching colonies that occur in shallow water. Similar 
to foundation species in other systems such as redwoods and seagrasses,
A. palmata has two modes of propagation. Colonies reproduce sexually by
releasing egg-sperm bundles into the water column in an annual synchronized
mass-spawning event. However, asexual reproduction by fragmentation (breakage
of branches) may dominate local recruitment. Both asexual and sexual means
of reproduction have consequences for the conservation genetics of this
imperiled species. Hence, we determined the genotypic diversity within local
populations of A. palmata, and the extent to which geographically isolated
populations are genetically similar throughout the Caribbean. Most population genetic analyses assume dispersal patterns have been stable for thousands of generations, thus they commonly reflect past colonization histories more than ongoing dispersal. Recently developed multi-locus genotyping approaches, however, have the demonstrated ability to detect both migration and population isolation over far shorter time scales. Using microsatellite markers and a combination of traditional F-statistics and Bayesian analyses, we showed that populations of Acropora palmata have experienced little or no recent genetic exchange between and asymmetrical migration within the western and eastern Caribbean. Puerto Rico was identified as an area of mixing between the two subregions (Baums et al. 2005). A phylogeographic break at the Mona Passage between Puerto Rico and Hispanola is congruent with results reported for coral reef fish. However, this is the first such break described for a scleractinian coral. |
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Clonal
reproduction in branching corals: While
clonal reproduction may mark an asexual strategy to secure resources, recent
work on plant populations suggests low genotypic diversity may also evidence
a failure of sexual recruitment that put such populations at high extinction
risk. This risk is increased in isolated populations. Using highly polymorphic
microsatellite markers, we showed that clonal structure varies with population
structure, i.e. the western Caribbean is dominated by asexual reproduction
patterns while the eastern Caribbean is more sexual (Baums et al. 2006).
We argue that the level of sexual recruitment has been consistently higher
in the eastern range of A. palmata than the west based on genetic and physical
larval dispersal models. Most stands of A. palmata in Florida, at the northern
limit of the species' distribution, were monoclonal. Several plant species
show a similar pattern of dominance of asexual reproduction at their range
limits. The low genotypic diversity in Florida indicates that the evolutionary
potential of the coral to adapt to environmental changes may be greatly
reduced in this area. Clonal reproduction is not restricted to branching corals. In collaboration with Nicola Foster and Pete Mumby (University of Exeter), we showed that Montastraea annularis can also reproduce asexually by fragmentation (Foster et al. 2007). These finding challenge the assumption that massive corals mainly reproduce through larval dispersal. A Caribbean-wide survey of asexual reproduction in this species in underway. |
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Bio-physical
and genetic models predict dispersal break: In
collaboration with physical and biological oceanographers at the University
of Miami (Drs Claire Paris and Laurent Cherubin), we combed results of molecular
work with models of larval dispersal based on the life history of the larvae
and physical forcing (wind, currents) (Baums et al. 2006). The goal is to
better understand the mechanisms that maintain the population structure
of A. palmata. The semipermeable barrier at the Mona Passage identified
by the genetic model may stem from a seasonal eddy that develops in the
Mona Passage during the spawning period. This interdisciplinary approach
to phylogeography and conservation genetics of marine organisms holds promise
to solve the problems of estimating migration over ecologically relevant
time scales. |
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Lack
of population genetic markers: Hard
corals present particular challenges for population genetic studies: they
frequently produce asexually, theycontain intracellular symbionts and mitochondrial
DNA is nearly invariant within coral species. Ad novo development and testing
of multiple single-copy microsatellite markers (Baums et al. 2005) was thus
a necessary first step. Through controlled breeding experiments it was confirmed
that the markers showed Mendelian inheritance, a critical assumption of
population genetic models. Symbiont-free spawn was collected during the
annual spawning event from marked colonies and crossed. Mendelian inheritance
and coral-specificity were proved by genotyping individual larvae and comparing
them to the parental genotypes. |
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Ongoing Research areas: 1. Co-evolution of coral hosts
and their zooxanthellate symbionts (with Todd LaJeunesse and Scott Santos) |
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 Any
opinions, findings, and conclusions or recommendations expressed in this
material are those of the author(s) and do not necessarily reflect the views
of the National Science Foundation. |
of associated species, and may contribute heavily to local economies via
fisheries and reef-related tourism. However, coral communities in the
eastern Pacific often occur where environmental conditions for reef growth
are marginal. These precarious conditions have spurred interest in how
reef corals persist in the eastern Pacific. Our goal is to assess one
potentially critical component of this persistence: the timing and extent
of demographic connections between coral populations in the eastern Pacific
and populations further west, in the heart of the equatorial Pacific.
To answer this question, we use novel ways of combining analyses to infer
the history of isolation and connectivity between populations. Results
from this research will not only answer long standing questions for marine
biogeographers, but will also provide critical information for the management
of coral reefs. This material is based upon work supported by the National
Science Foundation under Grant No. 0550294.