1999) 469-475 Received 29 January 1999, accepted 25 June 1999 Wolbachia infectio

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1999) 469-475 Received 29 January 1999, accepted 25 June 1999 Wolbachia infection in the terrestrial isopod Oniscus asellus: sex ratio distortion and effect on fecundity THIERRY RIGAUD, JÉRÓME MOREAU& PIERRE JUCHAULT Universiné de Poitiers, Laboratoire de Genetique et Biologie des Populations de Crustaces, UMR CNRS 8556 40 avenue du Recteur Pineau, F-86022 Poltiers Cedex, France Maternally inherited Wolbachia bacteria are widespread in arthropods where they are responsible for various reproductive alterations. In terrestrial isopods (woodlice), Wolbachia may induce feminiza tion or cytoplasmic incompatibility (CI), but their effect remains unknown in most host species. To our understanding of host/symbiont interactions in terrestrial isopods, the effect in the oniscidean Oniscus asellus, mainly to discriminate between females feminization and CI. The Wolbachia infection was not linked with a Ci phenomenon, but infected with Wolbachia produced female-biased broods compared with uninfected females. The fecundity of infected females was slightly lower than that of uninfected, but the number of young at the adult stage was similar between the two female categories. The experimental transfer of the symbiont into uninfected strains showed that Wolbachia was responsible for the feminization of a number of genetic males. In female-biased broods, Wolbachia were vertically transmitted to around 88% of the offspring, but the transmission rate was lower in the few male-biased progenies. The feminizing activity of these symbionts was not systematic, as many phenotypic males were infected These results contrasted with what is known in another woodlouse species, and indicated that feminization has evolved in different ways in terrestrial isopods. Keywords: Crustacea, cytoplasmic bacteria, feminization, Oniscidea, sex determination. microorganism responsible for feminization is a bacteri- um of the genus Wolbachia (Martin et al, 1973, Rousset Because of anisogamy, vertically transmitted cytoplas- e al, 1992). All infected individuals develop a female e zero and transmit the symbiont to most of their offspring Introduction pheno type whatever their sex chromosome mie genes (organelles, endosymbionts, etc.) tend to be herited exclusively from the mothers and ha fitness in males. A Fisherian sex ratio (stable l l sex (around 90% on average. ratio) is therefore not in the evolutionary interest of ratios. In the wild, all infected females found so far have cytoplasmic elements, because their spread can increase been of a ZZ (male) chromosomal composition, and only if they favour the female sex in their host (Cosmides &the; uninfected offspring develop a male phenotype. In Tooby, 1981). Among other strategies (reviewed by such infected strains, the sex ratio therefore depends only Werren &O;'Neill, 1997), inherited cytoplasmic elements on the bacterial transmission rate. To put it in another might induce the production of female-biased sex ratios. way, the cytoplasmic Wolbachia represent the only female A variety of such cytoplasmic sex ratio distorters is determinant in infected strains, whereas the male deter- known in animals (Hurst, 1993). These include some minants are the Z chromosomes and autosomal genes endosymbionts that produce feminization (Rigaud, repressing the transmission of the symbiont (Rigaud& 1997; Kageyama et al., 1998). This phenomenon has Juchault, 1992). been discovered in the terrestrial isopod (woodlouse) Armadillidium vulgare, a species where sex is normally terrestrial isopods (Juchault et al, 1994; Bouchon et al.. determined by heterochromosomes (female heteroga- 1998), most of the time inducing feminization, with the mety: females WZ and males ZZ). In this species, the exception of Porcellio dilatatus, where Wolbachia- Wolbachia have recently been found in the majority of induced cytoplasmie incompatibility (CI), a Wolbachia reproductive manipulation common in insects, is seen (Werren & O'Neill, 1997). However, the effects Correspondence. E-mail: thierry rigaud@campus.univ-poitiers.fr 469 1999 The Genetical Society of Great Britain.

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Objective of the study: The objective of the study is to determine the effect of Wolbachia in host species (Oniscus asellus) to differentiate the feminization and cytoplasmic compatibility.

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