Detailed Research Information

Detailed project information for
Study Plan Number 02084

Branch : Aquatic Ecology Branch

Study Plan Number : 02084

Study Title : Genetic population structure among diamondback terrapins (Malaclemys terrapin) in Chesapeake Bay and a comparison among representative populations throughout the species’ range

Starting Date : 05/01/2002

Completion Date : 09/30/2007

Principal Investigator(s) : King, Timothy L.

Primary PI : King, Timothy L.

Telephone Number : (304) 724-4450

Email Address : tlking@usgs.gov

SIS Number :

Primary Program Element :

Second Program Element :

Status : Active

Abstract : BACKGROUND
Due to the consequences of human population expansion in the twentieth century, aquatic habitat is disappearing at an unprecedented pace. Perhaps no habitat (and its associated species) illustrates this trend more profoundly than United States coastal wetland areas. At least one-third of the threatened and endangered species of the U.S. reside in wetland areas (Murdock 1994). Brackish coastal wetlands are being developed, and otherwise degraded, and destroyed at an accelerating rate as a direct and indirect effect of urbanization.

Diamondback terrapins (Malaclemys terrapin) exist as geographic populations that occupy brackish waters along North America’s Atlantic and Gulf coasts (Ernst et al. 1994). Living and breeding in salt marshes and tidal tributaries, the terrapin is the only North American turtle that lives exclusively in brackish water. Diamondback terrapins once supported a multi-million dollar restaurant trade, before overharvest and habitat degradation severely impacted populations. Currently the terrapin is listed as a Species of Special Concern in several states that have conducted inventory and monitoring efforts. The status of the species is unknown in those other provinces. Effective conservation and restoration plans require clearly definable units of management. Despite nearly continuous distribution along a narrow strip of brackish coastal waters that forms a rather continuous habitat from Cape Cod to southern Texas, terrapin populations exhibit extensive geographic variation in morphological characters resulting in the recognition of seven subspecies (Ernst et al. 1994). This distribution, and the observed morphological differentiation seem contradictory suggesting that the distribution may not be continuous and that the distribution may in fact best be described as consisting of as small often isolated populations. The classical metapopulation view details the balance between extinctions and recolonizations of local populations that allows long-term persistence of the metapopulation (Levins 1969). In reality, the local populations of a metapopulation occur in habitat patches that are immersed in a complex mosaic of other habitat patches, corridors and boundaries. The most obvious effects of this landscape structure are on individual movement patterns among patches and, consequently, on patch-recolonization probabilities. Clearly, individual movement is the most important unifying element in both metapopulation dynamics and landscape ecology (Saunders et al. 1991, Wiens 1992).

No genetic information exists on the fine-scale population structure, levels of gene flow, or relatedness of any geographic population of the terrapin. To develop management strategies for maintaining evolutionarily significant terrapin lineages that will ensure long-term population stability and reduce the need for protection through regulator processes, a thorough understanding of the evolutionary relationships (e.g., gene exchange) among geographically proximate and distal populations are essential.

Molecular genetics has recently achieved an important place in contemporary conservation biology as it has proven to be a robust tool for identifying reproductive isolation among populations, permitting the delineation of management units and allowing assessment of conservation priorities from an evolutionary perspective. Codominantly inherited genetic markers are required to fully assess population structure, gene flow, kinship, introgression, hybridization and speciation.

Contemporary nuclear DNA-based approaches to terrapin conservation may identify significant levels of genetic variation because repetitive DNA lack functional constraints, thus, allowing rapid accumulation of differentiation in DNA sequences. The development of tandemly repeated DNA simple sequence motif markers, or microsatellites (Tautz 1989; Queller et al. 1993), provides an ideal tool to investigate the degree of relatedness among species exhibiting small effective population sizes because of the hyper-polymorphism observed. Due to the relatively high mutation rate observed in microsatellite DNA markers, the markers have been remarkably successful at identifying recently diverged lineages (King et al. In preparation). Although the reason for ubiquitous occurrence of microsatellites in eukaryotic genomes has not been resolved (Tautz 1989), they have been lauded as the most useful class of nuclear marker available (Queller et al. 1993). The highly allelic Mendelian markers are amplified using the Polymerase Chain Reaction (PCR), a characteristic that allows the survey of a large number of markers from a minimally-invasive tissue sample. Sequence homology of regions flanking the microsatellite DNA marker has also permitted use of these markers in closely related species (Schlötter et al. 1991; Gotelli et al. 1994; Scribner et al. 1996).
OBJECTIVES
Currently little is known of the population structure of diamondback terrapins throughout the range. This study should provide the first delineation of fine-scale population structure for this species that will assist managers in defining the appropriate units of management within the Chesapeake Bay and among the geographic populations surveyed throughout the range. A suite of polymorphic microsatellite DNA markers have been developed for assessing levels and patterns of genetic diversity in diamondback terrapins. These markers have already been surveyed in over 200 turtles to date. Peer-reviewed publications describing the population structure observed within the Chesapeake Bay and among all geographic populations will be generated and disseminated to all interested parties.

Thus, the objectives of this research initiative are to work with resource management agencies to:

1) define the genetic population structure among multiple geographic collections of diamondback terrapins from within Chesapeake Bay;

2) delineate management units and evolutionarily significant lineages among geographic populations of diamondback terrapins collected throughout the species range;

3) provide a highly successful predictive model for assigning individual terrapins of unknown origin to their natal geographic population; and

4) promptly report the management implications of each aspect of this research to all the state and federal stakeholders.
HYPOTHESES TO BE TESTED
1) Diamondback terrapins inhabiting Chesapeake Bay comprise a single homogeneous population.

2) Diamondback terrapins inhabiting the Atlantic coast of the U.S. comprise a single homogeneous population.

3) Diamondback terrapins inhabiting the entire range comprise a single homogeneous population.

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Branch :	Aquatic Ecology Branch
Study Plan Number :	02084
Study Title :	Genetic population structure among diamondback terrapins (Malaclemys terrapin) in Chesapeake Bay and a comparison among representative populations throughout the species’ range
Starting Date :	05/01/2002
Completion Date :	09/30/2007
Principal Investigator(s) :	King, Timothy L.
Primary PI :	King, Timothy L.
Telephone Number :	(304) 724-4450
Email Address :	tlking@usgs.gov
SIS Number :
Primary Program Element :
Second Program Element :
Status :	Active
Abstract :	BACKGROUND Due to the consequences of human population expansion in the twentieth century, aquatic habitat is disappearing at an unprecedented pace. Perhaps no habitat (and its associated species) illustrates this trend more profoundly than United States coastal wetland areas. At least one-third of the threatened and endangered species of the U.S. reside in wetland areas (Murdock 1994). Brackish coastal wetlands are being developed, and otherwise degraded, and destroyed at an accelerating rate as a direct and indirect effect of urbanization. Diamondback terrapins (Malaclemys terrapin) exist as geographic populations that occupy brackish waters along North America’s Atlantic and Gulf coasts (Ernst et al. 1994). Living and breeding in salt marshes and tidal tributaries, the terrapin is the only North American turtle that lives exclusively in brackish water. Diamondback terrapins once supported a multi-million dollar restaurant trade, before overharvest and habitat degradation severely impacted populations. Currently the terrapin is listed as a Species of Special Concern in several states that have conducted inventory and monitoring efforts. The status of the species is unknown in those other provinces. Effective conservation and restoration plans require clearly definable units of management. Despite nearly continuous distribution along a narrow strip of brackish coastal waters that forms a rather continuous habitat from Cape Cod to southern Texas, terrapin populations exhibit extensive geographic variation in morphological characters resulting in the recognition of seven subspecies (Ernst et al. 1994). This distribution, and the observed morphological differentiation seem contradictory suggesting that the distribution may not be continuous and that the distribution may in fact best be described as consisting of as small often isolated populations. The classical metapopulation view details the balance between extinctions and recolonizations of local populations that allows long-term persistence of the metapopulation (Levins 1969). In reality, the local populations of a metapopulation occur in habitat patches that are immersed in a complex mosaic of other habitat patches, corridors and boundaries. The most obvious effects of this landscape structure are on individual movement patterns among patches and, consequently, on patch-recolonization probabilities. Clearly, individual movement is the most important unifying element in both metapopulation dynamics and landscape ecology (Saunders et al. 1991, Wiens 1992). No genetic information exists on the fine-scale population structure, levels of gene flow, or relatedness of any geographic population of the terrapin. To develop management strategies for maintaining evolutionarily significant terrapin lineages that will ensure long-term population stability and reduce the need for protection through regulator processes, a thorough understanding of the evolutionary relationships (e.g., gene exchange) among geographically proximate and distal populations are essential. Molecular genetics has recently achieved an important place in contemporary conservation biology as it has proven to be a robust tool for identifying reproductive isolation among populations, permitting the delineation of management units and allowing assessment of conservation priorities from an evolutionary perspective. Codominantly inherited genetic markers are required to fully assess population structure, gene flow, kinship, introgression, hybridization and speciation. Contemporary nuclear DNA-based approaches to terrapin conservation may identify significant levels of genetic variation because repetitive DNA lack functional constraints, thus, allowing rapid accumulation of differentiation in DNA sequences. The development of tandemly repeated DNA simple sequence motif markers, or microsatellites (Tautz 1989; Queller et al. 1993), provides an ideal tool to investigate the degree of relatedness among species exhibiting small effective population sizes because of the hyper-polymorphism observed. Due to the relatively high mutation rate observed in microsatellite DNA markers, the markers have been remarkably successful at identifying recently diverged lineages (King et al. In preparation). Although the reason for ubiquitous occurrence of microsatellites in eukaryotic genomes has not been resolved (Tautz 1989), they have been lauded as the most useful class of nuclear marker available (Queller et al. 1993). The highly allelic Mendelian markers are amplified using the Polymerase Chain Reaction (PCR), a characteristic that allows the survey of a large number of markers from a minimally-invasive tissue sample. Sequence homology of regions flanking the microsatellite DNA marker has also permitted use of these markers in closely related species (Schlötter et al. 1991; Gotelli et al. 1994; Scribner et al. 1996). OBJECTIVES Currently little is known of the population structure of diamondback terrapins throughout the range. This study should provide the first delineation of fine-scale population structure for this species that will assist managers in defining the appropriate units of management within the Chesapeake Bay and among the geographic populations surveyed throughout the range. A suite of polymorphic microsatellite DNA markers have been developed for assessing levels and patterns of genetic diversity in diamondback terrapins. These markers have already been surveyed in over 200 turtles to date. Peer-reviewed publications describing the population structure observed within the Chesapeake Bay and among all geographic populations will be generated and disseminated to all interested parties. Thus, the objectives of this research initiative are to work with resource management agencies to: 1) define the genetic population structure among multiple geographic collections of diamondback terrapins from within Chesapeake Bay; 2) delineate management units and evolutionarily significant lineages among geographic populations of diamondback terrapins collected throughout the species range; 3) provide a highly successful predictive model for assigning individual terrapins of unknown origin to their natal geographic population; and 4) promptly report the management implications of each aspect of this research to all the state and federal stakeholders. HYPOTHESES TO BE TESTED 1) Diamondback terrapins inhabiting Chesapeake Bay comprise a single homogeneous population. 2) Diamondback terrapins inhabiting the Atlantic coast of the U.S. comprise a single homogeneous population. 3) Diamondback terrapins inhabiting the entire range comprise a single homogeneous population.
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