Combined use of SMM and non-SMM methods to infer fine structure and evolutionary history of closely related brook charr (Salvelinus fontinalis, Salmonidae) populations from microsatellites

Abstract

In this paper, we characterized microsatellite and mtDNA variation among 779 individuals representing 26 brook chart (Salvelinus fontinalis Mitchill) populations from La Mauricie National Park (LMNP) to test the general hypothesis that microsatellite analysis may provide greater resolution than mtDNA to infer the evolutionary history of closely related and geographically proximate populations. We also assessed the relative efficiency of stepwise mutation model (SMM) and non-SMM genetic information, used separately and together, to depict population relationships and to quantify the divergence and the partitioning of genetic variance among populations. Based on restriction fragment length polymorphism (RFLP) and single-strand conformation polymorphism sequence data, LMNP displayed a disjunct mtDNA genetic composition compared to other northeastern North American populations. However, these populations formed a homogeneous group, suggesting that they originated from a different glacial refuge. The screening of five microsatellites revealed high levels of diversity, strongly partitioned among populations, based on both allelic and molecular variance (F(ST) = 37.04%; Φ(ST) = 45.96%). The use of chord distance was more reliable than (δμ)2 distance in resolving population tree topology using the neighbor-joining (NJ) algorithm. However, the construction of a hybrid tree obtained using the NJ-D(CE) topology and the least-squares method branch length estimation from (δμ)2 distances appeared to better reflect both the relationships and the divergence of populations. This tree was composed of seven population lineages characterized by different genetic compositions and the presence of unique alleles. These lineages coincided only partially with drainages, and, consequently, genetic variance was more strongly partitioned among them than among drainages. The magnitude of (δμ)2 distance suggested that LMNP has been recolonized by seven ancestral populations, already isolated prior to the beginning of the last Wisconsinian glacial period, and that differences among lineages could be as old as the refuge from which they originated. This supports the view that microsatellite polymorphism holds the potential for analysis of the evolutionary history of populations over short temporal and geographical scales. These results also suggest that the use of microsatellites in conservation genetics may change our views about the problems of temporal and geographical scales to be considered in order to optimize conservation strategies.