Macrogeographic patterns of breeding system diversity in the Daphnia pulex group. I. Breeding systems of Canadian populations

Abstract

Efforts to deduce the evolutionary significance of sexual reproduction from the taxonomic and ecological distributions of asexuals rely on the presumption that reversions of asexual taxa to sexual reproduction are rare. There is growing evidence, however, that reversions may be likely as long as asexuals are diploid, and some sexual populations persist. However, few systems have been studied in sufficient detail to provide insight into the nature of interactions between the sexual and asexual components of a single taxon. The cladoceran crustaceans Daphnia pulex and D. puilcaria both show variation in breeding system with some populations reproducing by cyclic and others by obligate parthenogenesis. The present study used allozymic analysis to characterize the breeding systems of both species across Canada. Although F1 hybrids were prevalent between the species and invariably reproduced by obligate parthenogenesis, each parent species showed regional variation in its breeding system. D. pulex reproduces by obligate parthenogenesis in eastern Canada but switches to cyclic parthenogenesis in the west with an abrupt transition coincident with the forest/prairie ecotone. D. pulicaria shows a divergent pattern, with populations on the priaries reproducing by obligate parthenogenesis, while those in eastern Canada and the far west are cyclic parthenogens. The results make clear that breeding systems in both taxa tend to be uniform over large areas. The discordance in breeding system employed by each species at specific sites suggests that the patterns of breeding system variation are not linked to environmental factors, but are instead a consequence of the interplay between historical factors, such as dispersion patterns from glacial ref ugia and the sites of origin of mutations promoting the transition to asexuality.

Geographical cline in breeding systems and ploidy levels in European populations of Daphnia pulex

“…Asexual populations belonging to the D. pulex group in temperate North America fall into two genetically distinguishable groups (Hebert et a!., 1989a(Hebert et a!., , 1993. One of these groups consists of clones assigned to D. pulex s.s., while the other group represents F1 hybrids between this species and D. pulicaria.…”

“…The cladoceran which has attracted most attention in recent years is Daphnia pulex. Populations of this species in the USA and southern Canada are diploid and may be either cyclically or obligately parthenogenetic (Lynch, 1983;Hebert et at., , 1989aHebert et at., , 1993, while in the low Arctic regions of Canada diploid and polyploid obligate parthenogens predominate and in the high Arctic the great majority of animals are polyploid obligate parthenogens (Weider et at., 1987;Beaton & Hebert, 1988).…”

Genetic evidence of widespread dispersal in a parthenogenetic freshwater ostracod

“…Hebert & Payne, 1985;Havel et at., 1990). Those studies that have also included macrogeographical sampling indicate that the finescale divergence is only rarely accompanied by regional differentiation (Benzie, 1986;Schwartz & Hebert, 1987;Crease et at., 1990;Hebert et a!., 1993).…”

“…57 parthenogens (Benzie, 1986;Crease et al, 1990;Hebert et a!., 1993). These species almost invariably show a pattern of strong microgeographical gene frequency variation but with little additional variation over macrogeographical scales.…”

“…However, marked subdivision can occur between continents or major ecotones (e.g. Hebert, 1987;Hebert et a!., 1993). This microscale heterogeneity coupled with macroscale homogeneity has been linked to the combined effects of multiple founder events and limited gene flow among established local populations (Crease et a!., 1990;Boileau & Hebert, 1991).…”