PRIMARY RESEARCH PAPER Does genetic diversity reduce intraspecific competition in rotifer populations? Nohemi Dimas-Flores • Manuel Serra • Marı ´a Jose ´ Carmona Received: 10 August 2012 / Revised: 23 October 2012 / Accepted: 29 October 2012 Ó Springer Science+Business Media Dordrecht 2012 Abstract As a result of reduced intraspecific com- petition, genetically diverse populations may have higher relative fitness than genetically uniform pop- ulations. To test this hypothesis, we compared poly- clonal (i.e., genetically diverse) versus monoclonal (i.e., composed of a single clonal genotype) experi- mental populations of the rotifer Brachionus plicatilis (Mu ¨ ller, 1786) growing separately and in competition. We estimated the following fitness components: intrinsic growth rate; carrying capacity; proportion of sexual females; diapausing egg production per sexual female and total egg production. Polyclonal populations showed similar dynamics to monoclonal populations and no statistical difference between their fitness components was detected. Therefore, results do not support the hypothesis that genetically diverse populations reduce competition through diversifica- tion in resource use. Instead, results suggest that B. plicatilis is a generalist consumer whose polyphagy does not depend on genetic differences, but on the broad diet of each genotype. However, clones showed significant differences in almost all fitness components demonstrating among-clone variation in life-history traits. We found a trade-off between sexual ratio and carrying capacity, highlighting the cost of sex in cyclical parthenogenetic rotifers. We discuss the mechanisms that could maintain the observed among-clone genetic variation in natural populations, and speculate on results implication for sex mainte- nance in rotifers. Keywords Monoclonal versus polyclonal populations  Fitness  Brachionus plicatilis  Life-history traits  Maintenance of sex Introduction Genetic diversity may have a positive effect on the average fitness of a population by reducing intraspecific competition through an increase in niche breadth (Bell, 1985; Cheplick, 1992; Tagg et al., 2005a). This reduction in intraspecific competition would be expected if the genotypes composing a genetically diverse population differ in resource use, leading to a more complete utilization of resources as compared to a genetically homogeneous population (Vrijenhoek, 1979; Tilman, 1999; Swanson et al., 2003; Hughes et al., 2008). Increased genetic diversity is promoted by sexual reproduction, which is hypothesized to have short-term benefits such as broader niche utilization, and hence a decreased competition (Maynard-Smith, 1978; Bell, 1985; Koella, 1988). Moreover, these advantages Handling editor: Diego Fontaneto N. Dimas-Flores  M. Serra  M. J. Carmona (&) Institut Cavanilles de Biodiversitat i Biologia Evolutiva, Universitat de Vale `ncia, A.O. 22085, 46071 Valencia, Spain e-mail: maria.j.carmona@uv.es 123 Hydrobiologia DOI 10.1007/s10750-012-1378-4