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Hydromedusa maximiliani is a vulnerable neotropical freshwater turtle endemic to mountainous regions …


Biology Articles » Genetics » Ecological Genetics » Estimating dispersal and gene flow in the neotropical freshwater turtle Hydromedusa maximiliani (Chelidae) by combining ecological and genetic methods » Discussion

Discussion
- Estimating dispersal and gene flow in the neotropical freshwater turtle Hydromedusa maximiliani (Chelidae) by combining ecological and genetic methods

The combination of ecological and biogeographical processes is an important mechanism for shaping the genetic structure of populations. Thus, life-traits (mating systems, dispersal ability), historical events (fragmentation, range expansion, colonization), and landscape matrix (mountain ridges, watersheds) may provide the background for understanding the geographic structure of genetic variation among populations (Templeton et al., 1995).

Nm estimates of more than one immigrant into the average deme per generation, as calculated for five loci (loci 1, 5, 7, 8, and 9), indicated that gene flow among populations inhabiting different rivers and streams was sufficient to deter population differentiation if they were at equilibrium between migration and random genetic drift (Wright, 1931; Slatkin, 1985, 1987). In contrast, Nm values for four loci (loci 2, 3, 4, and 10) were less than one, implying that the differentiation among populations could have occurred through random genetic drift alone (Slatkin, 1985, 1987). Overall, the Nm estimate of 0.589 indicated that the H. maximiliani populations inhabit different rivers.

Population structure bears an inverse relationship with the dispersal capability of organisms (Hartl and Clark, 1997), and the observed pattern of H. maximiliani population structure on a local spatial scale (Souza et al., 2002) is consistent with the known dispersal behavior of the species. Direct and indirect methods for estimating gene flow (Slatkin, 1987) in this species yielded similar results. Estimates of dispersal for H. maximiliani based on mark-recapture data (direct method) gathered during a 1993-1994 study (Souza and Abe, 1997b) showed that the movement of individuals was fairly restricted, with a mean daily displacement of two meters, suggesting that turtles from each river and stream within a drainage could be structured in as a metapopulation (Hanski and Simberloff, 1997). Indeed, individuals from which blood samples were taken for the present study in 1999 were found only a few meters from the site where they had been marked in 1993-1994.

The hypothesis of a metapopulation structure was confirmed by indirect (genetic) methods. The fact that several alleles were unique for rivers and streams within a drainage indicated that, because of the turtles limited dispersal, mating must be restricted to related individuals, which would facilitate the development of a local genetic structure. Furthermore, the molecular markers produced high FST values and low Nm values, indicating low gene flow among turtles inhabiting different rivers and streams. Templeton et al. (1990) found that mitochondrial DNA haplotypes in strictly aquatic salamanders were partitioned according to rivers within drainage, in a similar manner to the partitioning of RAPD variation for H. maximiliani. For organisms with a sedentary nature and low dispersal ability, such as snails, the genetic structuring of populations on small geographic scales is frequently detected (Johnson and Black, 1991; Arnaud et al., 1999, 2001), and could have originated from limited gene flow among populations.

Additional sampling of drainages and rivers and streams within drainage, as well as the use of mitochondrial and nuclear sequence markers will be necessary to estimate effective population sizes, population genealogies and other aspects fundamental to H. maximiliani conservation. These genetic analyses are important for the protection and management of this turtle species because such information could be useful in determining options for translocations. The genetic variation in this turtle species is structured according to the natural hierarchical system of rivers and streams within drainage, and limited gene flow is detected among populations from different rivers and streams. Thus, each watershed may harbor endemic populations, which suggests a strong degree of genetic structure and differentiation in the geographical range of the species.

 

ACKNOWLEDGEMENTS

The authors thank E.L.A. Monteiro-Filho and L.C. Duarte for helpful comments and criticisms on the manuscript, S. Hyslop for reviewing the English of the manuscript, and C.Y. Miyaki for assistance in the early stages of this study. The Instituto Florestal provided logistic support and P.P. Soares and A. John provided assistance in the field. J.R. Somera helped with the line drawings. This study was funded by the Fundação de Amparo à Pesquisa do Estado de São Paulo, Brazil (FAPESP, grant 00/00805-9) and Fundo de Apoio ao Ensino e à Pesquisa, Universidade Estadual de Campinas, São Paulo, Brazil (FAEP, grant 0023/00). FLS was the recipient of a post-doctoral fellowship from FAPESP (contract number 99/02761-0). SFR is partially supported by a research fellowship from the Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq), Brazil.

 


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