Biology Reference
In-Depth Information
P
OPULATION
S
TRUCTURE
Morphological and molecular data strongly support the existence of different franciscana
populations. Evidence of population structuring was first demonstrated through multivariate
analysis of morphometric data, which revealed the existence of two geographical forms: a
smaller form in the northern part of the species' range (north of 27
o
S) and a larger form in the
coastal waters of southern Brazil, Uruguay and Argentina (south of 32
o
S) (Pinedo, 1991,
1995). Body length of individuals from São Paulo (ca. 23
o
30'S-25
o
30'S) and Rio de Janeiro
(21
o
35'S-22
o
25'S) states were significantly different with animals from Rio de Janeiro being
larger (Ramos et al., 2002). Similar growth patterns were observed in cranial dimensions
(Ramos et al., 2002) as well as other body metrics (Barbato et al., 2008). These results
indicated that metric differences in body and skull variables were not clinal. Analyses of a
highly variable region of mitochondrial DNA (mtDNA) also supported these two geographic
forms (Secchi et al., 1998). Ott (2002) and Lázaro et al., (2004) compared the mtDNA of
franciscanas from Uruguay and Argentina with those published by Secchi et al., (1998).
These studies found support for the existence of a large southern population (composed of
animals from Rio Grande do Sul State in Brazil, Uruguay and Argentina) that is clearly
differentiated from animals in the waters of Rio de Janeiro. In addition, they revealed fixed
genetic differences between the populations that suggest essentially no effective genetic
exchange (Secchi et al., 1998; Ott, 2002; Lázaro et al., 2004). Ott's results also showed that
individuals inhabiting waters of the Paraná and São Paulo states belong to a genetically
distinct population. A pairwise analysis of haplotype distances between different geographic
locations showed increasing differentiation in the haplotype frequencies with increasing
geographic distance, following an isolation-by-distance pattern (Lázaro et al., 2004). These
authors and Ott (2002) also indicated that haplotypic frequencies of samples from Claromecó
(in Argentina) were significantly different from the rest of the southern population. Recent
results by Mendez et al., (2007), however, do not fit this model. Rather, they suggest that
ecological forces can be more relevant than geographic distance in determining population
structuring by regulating gene flow. Individuals from Claromecó are most similar to those
from coastal oceanic areas, including those from Uruguay, than with those from the estuary-
influenced Samborombon Bay. A similar pattern was observed in the Uruguayan coast (Costa
et al., 2008). The authors noticed that, among several sampled areas including oceanic and
estuarine coasts, the most genetically distinct individuals were those collected in the La Plata
estuary, regardless of the geographic distance. This suggests that a fine-scale structuring
occurs in areas with higher influence of the La Plata River estuary in both Argentina and
Uruguay. These areas are shallow, relatively enclosed and have high abundances of estuary-
dependent prey, which make them a suitable habitat for calving. In fact, telemetry studies
have demonstrated that individuals from Samborombon Bay are residents and have a very
restricted movement range of about 20 km (Bordino & Wells, 2005). Furthermore, Rodriguez
et al., (2002) found that individuals from this area prey upon different species when compared
to those from the coastal oceanic environments. If part of the population has evolved and is
mostly adapted to occupy an estuarial niche, intra-specific competition for resources is
minimized, representing an advantage for the species as a whole.
Search WWH ::
Custom Search