Microsatellite Gene Diversity Analysis in Landlocked Arctic Char from Maine

—Using six microsatellite loci, we characterized the 12 remaining populations of Arctic char Salvelinus alpinus naturally occurring in Maine. More specifically, we challenged the hypotheses based on previous analyses with other markers that (1) Arctic char from Floods Pond (known locally as silver char) represent a distinct evolutionary lineage and (2) all other Arctic char populations from Maine belong to the same evolutionary lineage and therefore do not require individual consideration for conservation. The high level of polymorphism observed at microsatellite loci in this study contrasted sharply with the extremely low levels of variation previously reported at other markers. Analyses confirmed that all lakes possess genetically distinct populations among which gene flow is restricted and on which other evolutionary forces may act independently, enhancing their genetic divergence. However, hierarchical gene diversity, population clustering, and population assignment analyses all indicated that the populations from different drainages did not originate from genetically distinct ancestral population assemblages. Our results thus contradict previous conclusions, as we found that the Arctic char from Floods Pond likely did not originate from a distinct evolutionary lineage. Secondly, although all Maine Arctic char appear to belong to a single evolutionary lineage, sufficient divergence was found to reject the hypothesis that all other populations should be considered as genetically equivalent for conservation. We discuss the implications of these findings for the management and protection of these unique Arctic char populations. The Arctic char Salvelinus alpinus has a holarctic distribution and exhibits a complex pattern of variability in morphology, coloration, ecology, and life history traits (reviewed in Behnke 1972; Johnson 1980). The contemporary distribution, diversity, and population structure of Arctic char were molded by repeated glacial advances and retreats during the Pleistocene Epoch, as has been the case with other northern temperate freshwater fishes (Bernatchez and Wilson 1998). Recent phylogeographic surveys of mitochondrial DNA (mtDNA) variation revealed that in North America these historic events have led to the evolution of four major lineages of Arctic char that are largely allopatric in distribution (Wilson et al. 1996; Brunner et al. 2001). One of these, the Laurentian lineage, likely dispersed from a now submerged Atlantic coastal * Corresponding author: [email protected] Received November 1, 2001; accepted April 14, 2002 refuge and is mainly composed of landlocked populations that became isolated from the sea during the last glacial retreat. The present distribution of the Laurentian lineage includes populations from southeastern Quebec, New Brunswick, and the northeastern United States (Wilson et al. 1996; Brunner et al. 2001). Based on unique phenotypes and morphological differences from Arctic char in Labrador and Newfoundland, these landlocked populations have been recognized as a distinct subspecies, S. alpinus oquassa (Qadri 1974). Landlocked Arctic char are intolerant of environmental disturbance, and consequently many populations have become extinct due to either competition from human-introduced fish or human-induced environmental changes (reviewed in Kircheis 1989). Maine is now the only region in the United States that still has relict populations of the Laurentian lineage of Arctic char, which are found in 12 lakes scattered in three major watersheds. Based on its unique breeding color and oth1107 ARCTIC CHAR MICROSATELLITE GENE DIVERSITY TABLE 1.—Sample locations with their abbreviations, drainages of origin, areas, and sample sizes. Asterisks indicate populations not included in gene diversity analysis due to small sample size. Water body Abbreviation Drainage Latitude (N) Longitude (W) Area (ha) Sample size Gardner Lake Pushineer Pond Big Black Pond Big Reed Pond Debouillie Pond GAL PU BB BR DP St. John St. John St. John St. John St. John 468589 468589 468599 468219 468589 688539 688589 688509 698039 688519 117 22 59 36 106 15 3* 10 5* 23 Floods Pond Green Lake Bald Mountain Pond Penobscot Lake FP GL BMP PL Union Union Penobscot Penobscot 448459 448409 458159 458569 688309 688329 698449 708139 260 1,196 461 408 39 6* 28 24 Rainbow Lake Wadleigh Pond Wassataquoik Lake RA WA WAS Penobscot Penobscot Penobscot 458499 468219 468019 698079 698439 688579 666 62 71 43 30 30 er external phenotypic traits, the Arctic char population of Floods Pond has been recognized as a distinct entity referred to as silver char or Sunapee (Kircheis 1989). Other populations, known locally as blueback char, are all located in remote unpopulated areas of the state. Floods Pond, however, is the main source for domestic water in an area of very rapid human population growth, and consequently the long-term survival of the population in this lake may be threatened by the periodic lowering of water levels (Kircheis 1989). Recent successes with artificial spawning habitats in Floods Pond have mitigated some of these concerns (F. W. Kircheis, unpublished data). Conservationists regularly use the amount of genetic divergence between populations as a major criterion for determining population uniqueness and thus protection regimes (e.g., Moritz 1994; Bernatchez 1995; Waples 1995; Petit et al. 1998; Crandall et al. 2000). However, there is limited information on the genetic relationships among Maine Arctic char populations. A previous allozyme survey reported seven slightly polymorphic enzyme-coding loci but it failed to differentiate between four Maine populations, including Floods Pond (Kornfield et al. 1981). An analysis of restriction fragment length polymorphisms (RFLPs) in mtDNA showed predominantly similar results, with the exception of a unique, slightly divergent haplotype found only in Floods Pond (Kornfield and Kircheis 1994). This led the authors to conclude that the preservation of the Floods Pond silver char was warranted on genetic grounds and that all other populations should be viewed as a genetically cohesive group. Highly reduced allozyme and mtDNA polymorphism within evolutionary lineages is, however, a common feature of Arctic char throughout its range (Brunner et al. 1998; Osinov and Pavlov 1998) and hampers the use of those markers in characterizing the genetic structure of populations beyond the scale of glacial races. Therefore, it may be hazardous to equate the apparent homogeneity observed at these markers with the lack of genetic distinction among populations that are physically isolated. Microsatellites are a class of highly polymorphic nuclear loci that are receiving increasing attention (Estoup and Angers 1998). The usefulness of these markers for addressing fine-scale population structure has recently been demonstrated in members of the genus Salvelinus, such as the brook char S. fontinalis (Angers and Bernatchez 1998; Castric et al. 2001), the Dolly Varden S. malma, and the bull trout S. confluentus (Taylor et al. 2001). The use of microsatellites in Arctic char has also resolved population structure on small geographic scales where other markers had failed to detect significant partitioning of genetic diversity (Bernatchez et al. 1998; Brunner et al. 1998; Primmer et al. 1999). In this study, we performed a microsatellite gene diversity analysis among all known relict populations of lacustrine Arctic char from Maine to elucidate whether they are composed of two evolutionary lineages, one unique to Floods Pond and the other represented by all other populations. Secondly, we tested the conclusion of Kornfield and Kircheis (1994) that with the exception of the Arctic char in Floods Pond, all of the populations in Maine are genetically indistinguishable.