ABSTRACT
Water fleas of the genus Daphnia (Crustacea: Cladocera) are key components of freshwater ecosystems, being primary consumers of algae and predated by fish and as such, have been extensively studied in the fields of limnology (Peters & de Bernardi 1987) and general ecology (Kalff 2002). In addition to their important ecological role, their wide geographic distribution, phenotypic variation, and peculiar reproductive mode (facultative parthenogenesis) have rendered them important models in evolutionary studies of speciation and adaptation (Schwenk et al. 2004). Extensive genetic surveys have revealed that the once thought cosmopolitan Daphnia species represent complexes of cryptic species with limited geographical distribution (Colbourne et al. 1998; Taylor et al. 1998). Fifteen different complexes were identified upon first sequencing 32 members of the genus Daphnia at the 16S mitochondrial gene (Colbourne & Hebert 1996). In this chapter, I will focus on one of these complexes: the Daphnia pulex complex that includes two major clades (the Daphnia pulicaria and the Daphnia tenebrosa clades) divided into ten divergent mitochondrial lineages (Colbourne et al. 1998). In North America, diploid D. pulex show a longitudinal gradient in their mode of reproduction with obligately asexual populations in eastern (temperate, subarctic, and arctic) Canada, mixed populations in Ontario, and cyclically parthenogenetic populations in western Canada (Hebert & Crease 1983; Hebert et al. 1993). Transitions to obligate parthenogenesis are thought to incur due to meiosis-suppressor genes that disrupt meiosis in females but not in males (Innes & Hebert 1988). As a result, males carrying these genes spread the “asexuality genes” in a contagious fashion. Meiosis-suppressor genes are thought to have arisen in an eastern D. pulex lineage and, hence, may explain the geographical distribution of obligate parthenogenesis in this species (Paland et al. 2005). In addition, glacial advances and retreat have provoked, through isolation in refugia and secondary contacts, hybridization and polyploidizations of numerous lineages of the complex in subarctic and arctic areas (Dufresne & Hebert 1997). Sharp phylogeographical patterns have been found in these young arctic lineages (Weider et al. 1999a; Markova´ et al. 2007) suggesting that historical processes have contributed significantly to the current distribution of these lineages. The recent sequencing of the Daphnia pulex genome will provide additional tools that will greatly contribute to our understanding of speciation.
