Isopods, along with their relatives the amphipods, are the only crustaceans of which strictly terrestrial species exist About half of the 8,000 isopod species described are terrestrial (Schmalfuss, 2003), and they can be used as potential guides to ecosystem activity in cultivated grasslands (Souty-Grosset et al ., 2005) . Terrestrial isopods (or woodlice) have been known for a long time to display aberrations in sex ratio both in the wild and in laboratory lineages (Vandel, 1941) . One of the most intensively studied examples, the pill bug, Armadillidium vulgare, has long been known to harbor intracellular microorganisms (Martin et al , 1973) responsible for inducing the development of genetic males into functional females (Legrand and Juchault, 1970) The phylogenetic status of these bacteria remained

* This chapter is dedicated to Pierre Juchault, for his pioneering and influential work on sex determination in crustaceans .

unknown for almost two decades following their discovery, until they were identified as Wolbachia pipientis, on the basis of morphological and life cycle features (Rigaud et al , 1991) This taxonomic assignment was formally confirmed one year later by sequencing of the 16S and 23S ribosomal RNA genes (Rousset et al , 1992) In this review, we highlight recent results that have offered new insight into our understanding of the Wolbachia/crus-tacean endosymbiosis, both at functional and at evolutionary levels

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