Genetic structure of Acropora palmata populations in the
John McManus, Iliana Baums, and Margaret Miller
December 15, 1999 – December 14, 2003
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
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.
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.
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.
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.
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,
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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