Genetic differentiation in geographically close populations of the water rat Nectomys squamipes (Rodentia, Sigmodontinae) from the Brazilian Atlantic Forest

Abstract

We examined the genetic structure and the effects of a bottleneck in populations of the water rat Nectomys squamipes, a primary host of Schistosoma mansoni. Eight microsatellite loci were studied in 7 populations from the Sumidouro region of the Brazilian state of Rio de Janeiro. Our data, covering a four-year period during which a bottleneck occurred, revealed substantial variation (6-31 alleles per locus) and high levels of both observed (0.718-0.789) and expected (0.748-0.832) heterozygosity. Most populations were in Hardy-Weinberg equilibrium without linkage disequilibrium between loci. Overall average genetic differentiation between populations (estimated with the F ST (θ) and R ST (ρ) analogues was 0.037 for θ and 0.060 for ρ. There was significant allelic and genotypic differentiation between populations, especially in pairwise comparisons that included the most geographically isolated population. Direct migration estimates showed a low rate of migration, indicating that infected N. squamipes populations had a limited ability to spread S. mansoni. When the pre-and post-bottleneck populations were compared there was no detectable reduction in heterozygosity or allele number, although a significant excess of heterozygosity was detected in the post-bottleneck population.

Population structure and genetic variability of mainland and insular populations of the Neotropical water rat, Nectomys squamipes (Rodentia, Sigmodontinae)

“…Another reason for populations not being in equilibrium could be a recent population expansion. The isolation by distance trend (although not significant) found in a considerably smaller area (Maroja et al, 2003a) is in accordance with this hypothesis because equilibrium is first reached between nearby populations (Crow and Aoki, 1984). Although it has been suggested that population expansions result in low genetic variability (Slatkin, 1993), there is empirical evidence that fast expansions may occur without significant loss of variability (Zenger et al, 2003).…”

“…A six-year capture-recapture study of N. squamipes in a small area (3 km 2 ) detected only 10 migrants (mostly males) out of 162 recaptured individuals with all the displacements occurring between adjacent sites (LS Maroja, personal observation). Microsatellite data, when analyzed in this population, showed significant, although minor, differentiation between collection sites (Maroja et al, 2003a). Based on these results and theoretical models, wider areas would be expected to exhibit a higher degree of differentiation among sites.…”

“…Patton et al, 1996;Loxterman et al, 1998). To date, the only published report of a population study using microsatellites is that of Maroja et al (2003a) who studied N. squamipes in a restricted area. The lack of taxonomic problems with this rodent along with its well-researched ecology and wide and welldefined geographic range make N. squamipes a good candidate for studies of population dynamics.…”

“…Another important reason for studying the population genetics of N. squamipes is that it is a natural primary host for Schistosoma mansoni. Although it is apparently unaffected by this parasite, infected N. squamipes can potentially spread S. mansoni to uninfected areas Maroja et al 2003a).…”

Small mammal populations of an agroecosystem in the Atlantic Forest domain, southeastern Brazil

“…The data analyzed in this paper are part of a long-term research project on the population dynamics of the water rat, Nectomys squamipes, aimed to provide an ecological knowledge basis for the implementation of schistosomiasis control programs, emphasizing the role of the water rat as a wild reservoir. A previous five-year capture-mark-recapture study of small mammals conducted in this area generated data on their general ecology, population dynamics, population genetics, reproduction and schistosomiasis parasitism (D'Andrea et al, 1999;Gentile and Fernandez, 1999;D'Andrea et al, 2000;Gentile et al, 2000;Maroja et al, 2003).…”