Role of parasite load and differential habitat preferences in maintaining the coexistence of sexual and asexual competitors in fish of the Cobitis taenia hybrid complex JAN KOTUSZ 1 *, MARCIN POPIOLEK 2 , PAVEL DROZD 3 , KOEN DE GELAS 4,5 , VERA ŠLECHTOVÁ 6 and KAREL JANKO 6,7,8 1 Museum of Natural History, Faculty of Biological Sciences, University of Wroclaw, Wroclaw, Poland 2 Institute of Biology, Department of Invertebrate Systematics and Ecology, Wroclaw University of Environmental and Life Sciences, Wroclaw, Poland 3 Faculty of Sciences, University of Ostrava, Ostrava, Czech Republic 4 Research Institute for Nature and Forest (INBO), Brussels, Belgium 5 Biogenomics, K.U. Leuven Research and Development, Leuven, Belgium 6 Laboratory of Fish Genetics, Department of Vertebrate Evolutionary Biology and Genetics, Institute of Animal Physiology and Genetics, AS CR, Libechov, Czech Republic 7 Institute of Vertebrate Biology, AS CR, Brno, Czech Republic 8 Life Science Research Centre, Department of Biology and Ecology, Faculty of Natural Sciences, University of Ostrava, Ostrava, Czech Republic Received 5 February 2014; revised 9 April 2014; accepted for publication 10 April 2014 In the context of the paradoxical ubiquity of sex, we tested whether stable coexistence of sexual and asexual fish of the genus Cobitis is mediated by parasites, as asexual fish suffer more from parasitic infections because of their lower genetic variability [the Red Queen hypothesis (RQH)], or by partial niche shift of the two strains differing in mode of reproduction. We did not find a clear correlation between infection risk with a helminth parasite and the proportion of sexuals, and we found similar infection rates among sexual females and co-occurring asexuals in general, including the most frequent clone in particular. These results suggest that the mechanisms of the RQH are not directly engaged in stabilizing this asexual complex. On the other hand, the temporally stable gradient in sexual/asexual proportions along the river correlated with gradients in environmental parameters (physicochemical water parameters, velocity, and shading of the habitat) and turnover in the fish assemblage structure. Sexual and asexual forms thus appear to prefer different habitats. The Cobitis teania asexual complex thus contributes to the view that persistence of sex may, as in many taxa, be driven by case-specific processes. © 2014 The Linnean Society of London, Biological Journal of the Linnean Society, 2014, 113, 220–235. ADDITIONAL KEYWORDS: diploid–polyploid complexes – European distribution – habitat partitioning – niche shift – parasite-mediated coexistence – Red Queen hypothesis – spined loach – unisexuality. INTRODUCTION Following the studies by George C. Williams (1975) and John Maynard Smith (1978), much research effort has been invested in understanding the ubiq- uity of sexual reproduction among multicellular organisms, which became poetically called ‘the queen of problems in evolutionary biology’. The importance of this question becomes obvious when considering the various costs of sex, especially the fact that an asexual female’s genes will, on average, be transmit- ted to twice as many offspring compared with the genes of a sexual female. This implies that, all else being equal, asexually reproducing organisms should quickly outcompete their sexual counterparts. However, the ubiquity of sex evidences that the *Corresponding author. E-mail: kotusz@biol.uni.wroc.pl Biological Journal of the Linnean Society, 2014, 113, 220–235. With 5 figures © 2014 The Linnean Society of London, Biological Journal of the Linnean Society, 2014, 113, 220–235 220