ALLOZYME AND MTDNA VARIATION IN POPULATIONS OF THE DAPHNIA-PULEX COMPLEX FROM BOTH SIDES OF THE ROCKY-MOUNTAINS

Long-distance dispersal of diapausing eggs by migratory waterfowl is o ne factor thought to be responsible for the macrogeographical homogene ity of allozyme frequencies in species of the Daphnia pulex complex. I f so? populations on either side of the Rocky Mountains are expected t o be divergent because few major flyways cross them. To test this pred iction, Daphnia populations from lakes and ponds across eastern Oregon were surveyed for mtDNA and allozyme variation. The data were analyse d with previously collected data from populations in the midwest U.S. Phenetic analysis of the allozyme data clustered the populations into four discrete groups, which correspond to habitat: permanent lakes, po nds in the midwest, coastal and valley ponds in Oregon and sand dune p onds in Oregon, A recent taxonomic revision by Hebert suggests that th ese groups correspond to D. pulicaria, D. pulex, D, arenata and D. mel anica. respectively. Cladistic analysis of mtDNA variation revealed th e same groups except that mtDNA haplotypes from the D, pulex and D. pu licaria populations formed a single clade. All four species were signi ficantly subdivided with respect to allozyme markers, but there were n o clear differences between D. pulicaria populations on either side of the Rocky Mountains, suggesting that they are not a barrier to gene f low in this species, Whereas mtDNA differentiation among D. pulicaria populations was not significant, the pond-dwelling species, D. pulex a nd D. arenata, showed even greater differentiation for mtDNA than for allozymes. It is suggested that extinction/recolonization events occur more frequently in pond vs, lake habitats and have a greater impact o n the subdivision of mtDNA variation because of the haploid, maternal inheritance of the mitochondrial genome.