MITOCHONDRIAL-DNA DIVERSITY IN AN APOMICTIC DAPHNIA COMPLEX FROM THE CANADIAN HIGH ARCTIC

Cyclic parthenogenesis is the ancestral mode of reproduction in the cl adoceran crustacean, Daphnia pulex, but some populations have made the transition to obligate parthenogenesis and this is the only mode of r eproduction known to occur in arctic populations. Melanism and polyplo idy are also common in arctic populations of this species. Prior alloz yme studies of arctic D. pulex revealed substantial levels of clonal d iversity on a regional scale. Clonal groupings based on cluster analys is of allozyme genotypes do not conform to groupings based on the pres ence/absence of melanin or on ploidy level. In order to further elucid ate genetic relationships among arctic D. pulex clones, mitochondrial DNA (mtDNA) variation was examined in 31 populations from two Canadian high-arctic sites. The data were also compared to a previous study of mtDNA variation in populations from a Canadian low-arctic site. Cladi stic analysis of restriction site variation of the entire mitochondria l genome and nucleotide sequence variation of the mitochondrial contro l region was used to construct genetic relationships among mitochondri al genotypes. Three distinct mitochondrial lineages were detected. One lineage was associated with diploid, nonmelanic clones and is the sam e as the lineage that is found in temperate populations of D. pulex. T he other two lineages (A and B) were associated with polyploid, melani c clones. Sequence divergence between the A and B lineages was 2.4%. S equence divergence between D. pulex and either of these two lineages e xceeded 3%. It is suggested that the melanic, polyploid clones are hyb rids between males of D. pulex (and/or a closely related congener, D. pulicaria) and females of either of two ancestral melanic species that have mitochondrial lineages A and B. Geographic patterns of mitochond rial diversity in 'melanic' lineage B support the hypothesis of an hig h-arctic refuge for the ancestral species during the last glacial peri od.