Ecology, Wolbachia infection frequency and mode of reproduction in the parasitoid wasp Tetrastichus coeruleus (Hymenoptera: Eulophidae) BARBARA M. REUMER,* JACQUES J. M. VAN ALPHEN*† and KEN KRAAIJEVELD* *Section Animal Ecology, Institute of Biology Leiden, Sylvius Laboratory, University of Leiden, P.O. Box 9505, 2300 RA Leiden, The Netherlands,†UMR 6553 ECOBIO, Universite ´ de Rennes I, Campus de Beaulieu, Avenue du Ge ´ne ´ral Leclerc, 35 042 Rennes cedex, France Abstract Whereas sexual reproduction may facilitate adaptation to complex environments with many biotic interactions, simplified environments are expected to favour asexual reproduction. In agreement with this, recent studies on invertebrates have shown a prevalence of asexual species in agricultural (simplified) but not in natural (complex) environments. We investigated whether the same correlation between reproductive mode and habitat can be found in different populations within one species. The parasitoid wasp Tetrastichus coeruleus forms an ideal model to test this question, since it occurs both in natural and agricultural environments. Further, we investigated whether Wolbachia infection caused parthenogenesis in female-biased populations. In contrast to the general pattern, in Dutch and French natural areas, we found Wolbachia-infected, highly female-biased populations that reproduce parthenogenetically. In contrast, populations on Dutch agricultural fields were not infected with Wolbachia, showed higher frequencies of males and reproduced sexually. However, we also found a female- only, Wolbachia-infected population on agricultural fields in north-eastern United States. All Wolbachia-infected populations were infected with the same Wolbachia strain. At this moment, we do not have a convincing explanation for this deviation from the general pattern of ecology and reproductive mode. It may be that asparagus agricultural fields differ from other crop fields in ways that favour sexual reproduction. Alternatively, Wolbachia may manipulate life history traits in its host, resulting in different fitness pay-offs in different habitats. The fixation of Wolbachia in the United States populations (where the species was introduced) may be due to founder effect and lack of uninfected, sexual source populations. Keywords: environmental specialization, MLST, parthenogenesis, sexual vs. asexual popula- tions, Tetrastichus coeruleus, Wolbachia Received 30 October 2009; revision received 10 February 2010; accepted 18 February 2010 Introduction Sexual reproduction is the predominant mode of repro- duction among eukaryotes. This is remarkable given the large fitness disadvantage of sexual reproduction, known as the twofold cost of sex (Maynard Smith 1978). Since an asexual female only produces daughters, all of her offspring will contribute to the next genera- tion, while only half of the offspring (with a 50 ⁄ 50 sex ratio) of a sexually reproducing female will contribute to the next generation. Therefore, an asexual population can grow faster than a sexual population and when they are in competition, the asexual population should outcompete the sexual one (Maynard Smith 1978). Also, asexual females transmit all of their genome to each Correspondence: Barbara M. Reumer, Fax: 0031 (0) 71 527 4900; E-mail: B.M.Reumer@biology.leidenuniv.nl Ó 2010 Blackwell Publishing Ltd Molecular Ecology (2010) 19, 1733–1744 doi: 10.1111/j.1365-294X.2010.04599.x