Evidence for sex ratio distortion by a new microsporidian parasite of a Corophiid amphipod S. I. MAUTNER 1 , K. A. COOK 1 , M. R. FORBES 1 *, D. G. MCCURDY 2 and A. M. DUNN 3 1 Department of Biology, 1125 Colonel By Drive, Carleton University, Ottawa, Ontario, K1S 5B6, Canada 2 Biology Department, Albion College, 611 East Porter Street, Albion, MI 49224, USA 3 Institute of Integrative and Comparative Biology, Faculty of Biological Sciences, University of Leeds, Leeds LS2 9JT, UK (Received 8 December 2006; revised 8 February 2007; accepted 25 April 2007; first published online 11 June 2007) SUMMARY In this paper, we describe the occurrence of a microsporidian parasite in female-biased populations of an intertidal amphipod, Corophium volutator (Pallas), at mudflat sites in the Bay of Fundy, Canada. Sequence data for the parasite’s 16S rDNA indicate that it is a novel microsporidian species. This parasite was found principally in female host gonads, indicating that it might be a vertically transmitted, sex-distorting microparasite. At 4 sites each sampled in early and mid- summer, parasite prevalence varied from 0 to 21 %. In the lab, infected mothers gave rise to more female-biased broods, than did uninfected mothers. Infection was not associated with size of females or with lowered survivorship of their young. Surprisingly, infected mothers actually had higher fertility controlling for body length than did uninfected mothers. Taken together, our results suggest that this novel microsporidian is likely a feminizing microparasite and is a contributing factor to local and widespread sex ratio distortion in C. volutator. Key words: amphipod, feminization, microsporidia, sex ratio distortion, vertical transmission. INTRODUCTION Many cytoplasmic parasites have evolved strategies of host sex ratio distortion that lead to increased production of female offspring to ensure their own transmission, because transmission to a male host is a dead end for the parasite. These strategies include induction of parthenogenesis, male killing and fem- inization (Bandi et al. 2001). Among crustacean hosts, cytoplasmic parasites frequently have been observed to induce feminization (converting males to females, which transmit the parasites). Besides the well-described Wolbachia bacteria commonly found in isopods (Bouchon et al. 1998), a variety of feminizing microsporidia have been recently ident- ified in amphipods (Terry et al. 2004). Feminization may be common in amphipods because of great plasticity in their sex determination system, which rarely has well-differentiated sex chromosomes (Le ´cher et al. 1995). Instead, sex is often influenced by environmental factors or sex-distorting elements (Bulnheim, 1978 ; Terry et al. 1998; Dunn et al. 2005 ; Rodgers-Gray et al. 2004). We studied sex ratio distortion in Corophium volutator, an intertidal amphipod (Crustacea : Am- phipoda) commonly found at coastal mudflats of North America and Europe (Meadows and Reid, 1966). For populations of this species in Great Britain and Canada, strongly female-biased sex ratios have been described in adult and juvenile cohorts (Watkin, 1941; Peer et al. 1986 ; Schneider et al. 1994). Factors that have an influence on population sex ratio in C. volutator include primary sex ratio and differential mortality of males and females. Primary sex ratios at the time of release from the mother’s brood pouch appear to indicate an already existing skew towards female offspring (Schneider et al. 1994). Higher mortality of mate-searching males compared to more sedentary adult females occurs, although it fails to explain strong sex ratio bias in juveniles, which are not yet subjected to differential predation (Forbes et al. 1996). Little is known about sex determination mechanisms in C. volutator, but so far no evidence for sex chromosomes has been re- ported. The strongly female-biased sex ratios in adult as well as juvenile cohorts (Schneider et al. 1994) suggest that a parasitic sex ratio distorter might be present and prevalent. In this study, we explored whether female-biased sex ratios of C. volutator amphipods at Bay of Fundy sites might be explained by parasitism by a micro- sporidian parasite. We first compared parasite preva- lence in male and female hosts, by screening ovarian and testicular tissue with microsporidian specific primers, after confirming successful DNA extrac- tion, as detailed below. Screening gonadal tissue also provided an indication of whether the parasite might * Corresponding author : Department of Biology, 1125 Colonel By Drive, Carleton University, Ottawa, Ontario, K1S 5B6, Canada. Tel: +613 520 2600 Ext.3873. Fax : +613 520 3539. E-mail : mforbes@connect.carleton.ca 1567 Parasitology (2007), 134, 1567–1573. f 2007 Cambridge University Press doi:10.1017/S0031182007003034 Printed in the United Kingdom