ORIGINAL ARTICLE Stuart C. Killick Æ Darren J. Obbard Stuart A. West Æ Tom J. Little Parasitism and breeding system variation in North American populations of Daphnia pulex Received: 2 February 2007 / Accepted: 13 February 2007 / Published online: 20 March 2007 Ó The Ecological Society of Japan 2007 Abstract The Red Queen hypothesis proposes that frequency-dependent selection by parasites may be responsible for the evolutionary maintenance of sexual reproduction. We sought to determine whether para- sites could be responsible for variation in the occur- rence of sexual reproduction in 21 populations of Daphnia pulex (Crustacea; Cladocera) that previous studies have shown to consist of either cyclical par- thenogens, obligate parthenogens, or a mixture of both. We measured parasite prevalence over a four-week period (which essentially encompasses an entire season for the temporary snow-melt habitats we sampled) and regressed three different measures of sexuality against mean levels of parasite prevalence. Levels of parasitism were low and we found no relationship between levels of sexuality and mean parasite prevalence. Genetic variation with infection level was detected in 2 of the 21 populations, with several different clones showing signs of overparasitism or underparasitism. Overall, however, our results suggest that parasites are not a major source of selection in these populations and it thus seems unlikely they are responsible for maintain- ing breeding system variation across the study region. Keywords Geographical parthenogenesis Æ Red Queen hypothesis Æ Evolution of sex Æ Parthenogenesis Æ Natural selection Æ Infection Introduction The predominance of sexual reproduction among Eukaryotic taxa has long puzzled evolutionary biolo- gists, because the ‘‘twofold cost’’ associated with male production should put sexual taxa at a major disad- vantage when they come into competition with their asexual counterparts (Maynard Smith 1978). This di- lemma has received much attention over the past few decades and as a result a large number of genetic and ecological hypotheses have been proposed in an attempt to explain both the long-term and short-term mainte- nance of sex (Bell 1982; Kondrashov 1993; West et al. 1999). One of the most prominent ecological explanations for sex, the Parasite Red Queen hypothesis, proposes that frequency-dependent selection by parasites is responsible for preventing the spread of asexuality (Bell 1982). The Parasite Red Queen hypothesis works on the assumption that parasites will be selected to infect the commonest host genotypes (Hamilton et al. 1990), and sex is thought to be advantageous because of the ability of recombi- nation to create novel genotypes, thus limiting parasite adaptation to particular genotypic combinations. Con- sequently, in the longer term, sexual populations may have higher geometric mean fitness. The hypothesis also makes several predictions of how prevalence may vary between breeding systems, depending on the distribution of parasite abundance. For example, if parasite abun- dance varies little across populations, then sexuals are expected to bear lower parasite loads than their asexual counterparts. If, on the other hand, the risk of infection varies widely across populations asexuals are predicted to dominate in low-risk areas whereas sexuals are ex- pected to hold precedence in areas where parasites are more abundant (Lively 2001). Most studies have attempted to test the predictions of the Red Queen by: 1 examining the relationship between host genotype frequency and infection by parasites, a test for fre- quency-dependent selection (Lively et al. 1990; Kelley 1994; Vernon et al. 1996; Dybdahl and Lively 1998; Little and Ebert 1999); 2 comparing levels of infection in closely related sexual and asexual species (Moritz et al. 1991; Hanley et al. S. C. Killick Æ D. J. Obbard Æ S. A. West Æ T. J. Little (&) Institute of Evolutionary Biology, School of Biological Sciences, University of Edinburgh, West Mains Road, Edinburgh EH9 3JT, UK E-mail: tom.little@ed.ac.uk Ecol Res (2008) 23: 235–240 DOI 10.1007/s11284-007-0368-x