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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
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).
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.
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.
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.
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.
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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