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 ReCONNECT

Project Title

3.1. Genetic structure of Acropora palmata populations in the Caribbean
 

Key InvestigatorS John McManus, Iliana Baums, and Margaret Miller
Project Duration December 15, 1999 December 14, 2003
 
OBJECTIVE(S)

This project contributes to the status review of Acropora palmata, a candidate species for the Endangered species list. We seek to determine the genotypic diversity within local populations of this coral, and the extent to which geographically isolated populations are genetically similar, information that will be essential for future conservation and recovery efforts.

Marine populations are often thought to be well connected via long-distance dispersal of planktonic larval stages. Genetic markers can be used to assess the extent to which geographically isolated populations are connected by larval dispersal.  Such genetic estimates of population connectivity rely critically on estimating the frequency of genotypes within different populations.  These frequencies can be skewed by local asexual reproduction of a few dominant genotypes in species that are capable of clonal reproduction. Ecological studies suggest clonal reproduction by fragmentation may be extensive in A. palmata (Highsmith 1982), however genetic work on other clonal cnidarians suggests that such direct observations cannot be extrapolated to infer local genotypic diversity (McFadden 1997).  Thus, any study of population connectivity in clonal organisms must necessarily include estimates of genotypic diversity within populations.

Knowledge of Acropora population structure is essential for assessing its degree of threat and deciding on appropriate conservation measures.  If, for example, we find very low genetic diversity (i.e. few genets) within individual stands and/or across the region, it might suggest that its status is under much greater threat than would be judged from its overall abundance.
 

SUMMARY

Polymorphic markers were developed and tested. We used gametes in the development process to preclude contamination caused by zooxanthellate symbionts that are present in adult tissues.

The high amount of polymorphism of the markers makes it unlikely that two none-clonemates would share identical genotypes. Thus, clonal identity of samples can be ascertained. Clonal structure varies both within and across regions. Notably, two adjacent reefs in the Key Largo area of Florida (Horseshoe and Little Grecian reefs, 3.3 km distance) showed very little clonal variation. We analyzed 22 colonies on Horseshoe Reef all of which are clonemates (or ramets). That means that all 22 colonies share one genotype and are one genetic individual (called a genet) (Fig 1 a). The same pattern was found at Little Grecian reef where 24 colonies were analyzed (Fig 2 b). Again, all belonged to the same genet. However, this genet was different from the one found at Horseshoe Reef. In other words, even though over 30 colonies of Acropora palmata grow on either reef, they might represent as little as two distinct individuals demonstrating that the effective population size of this coral might be overestimated by just counting the number of colonies present. In contrast, Sand Island reef located close by (<15km distance) showed high clonal variation with 19 out of 47 (=40%) collected colonies being unique genotypes (Fig 1 c). 

The markers developed here have the potential to unravel the genotypic diversity of a major reef building coral species suspected of highly clonal population structures. Preliminary results indicate a wide range of genotypic diversities both locally and across the Caribbean.

Preliminary analysis of population structure in the Caribbean indicates that A. palmata populations show little but significant substructure. Thus, larval exchange may be limited between certain reefs. If this holds true after the number of samples analyzed has increased it would indicate that A. palmata populations would have to be managed on a local scale.
 

PRESENTATION(S)

Baums, I.B. (2002) Genetic status of Acropora populations in the Caribbean. Presentation as an Invited Speaker at the Caribbean Acropora Workshop: Potential Application of the U.S. Endangered Species Act as a Conservation Strategy, Miami, Florida.

Baums, I.B., Hellberg, M.E., and Miller, M.W. (2002) Preliminary data on the genotypic diversity of Acropora palmata (Scleractinia: Acroporidae). Presentation at the International Society for Reef Studies 2002 meeting, Cambridge, UK

 


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