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Our second specific objective is to determine the extent to which populations in single rivers are genetically isolated from adjacent populations and whether gene flow occurs primarily up- and downstream or between adjacent rivers. Ongoing investigations into the phylogeography of other tenrec species (e.g., Olson et al., submitted) have uncovered extensive mitochondrial DNA variation at the intraspecific level. These studies suggest that wide-ranging species of Microgale have experienced a historical population subdivision along a north-south axis, resulting in at least one instance of allopatric speciation (Olson and Goodman, in prep.) as well as morphological divergence in many cases. The zone of secondary contact between many of these north-south phylogroups occurs within the known range of Limnogale. Given the pervasiveness of this pattern (observed in 12 species of Microgale to date), the question of phylogeographic partitioning within Limnogale becomes even more compelling. A reasonable a priori hypothesis is that Limnogale, whose movements and dispersal are presumably primarily limited by the eastward-flowing drainages of the island's eastern rainforests, should display an even more pronounced pattern of phylogeographic subdivision along a north-south axis. As such, the potential for allopatric speciation, already uncovered in some "cryptic" species of shrew tenrecs (Olson et al., submitted), may be high for Limnogale.
Molecular analyses will include parsimony, maximum-likelihood and bayesian tree inference methods to examine phylogeographic patterns and analysis of molecular variance (AMOVA) to assess hierarchical patterns in the distribution of genetic diversity (Avise, 2000). Specific hypotheses of population structure and population history with respect to the geographic distribution of mtDNA haplotypes will be tested using nested clade analysis (Posada and Crandall 2001). Specifically, we are interested in the extent to which populations from individual rivers are genetically isolated from adjacent rivers/populations and whether physiographic and/or ecogeographic factors (e.g., proximity of adjacent rivers, intervening habitat types, etc.) may be involved in promoting or preventing gene flow between populations. |
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