Endosymbiotic bacteria affecting sex determination or reproduction of arthropod hosts

The reproductive systems of arthropod hosts are often manipulated by endosymbiotic bacteria such as Spiroplasma, Rickettsia, Wolbachia, Arsenophonus, and Cardinium (O'Neill et al ., 1997; Bourtzis and Miller, 2003, 2006) . Among these, Wolbachia are particularly focused upon due to their high prevalence (approximately 30% of insect species are infected) and the various types of reproductive manipulations they induce

The most common type of Wolbachia-induced reproductive manipulation is cytoplasmic incompatibility. Cytoplasmic incompatibility results in embryonic mortality after matings between insects with differing Wolbachia infection statuses (Bourtzis et al , 2003), and can be either unidirectional or bidirectional. Unidirectional cytoplasmic incompatibility is typically expressed when an infected male mates with an uninfected female The reciprocal mating is fully compatible, as are matings between infected individuals Bidirectional cytoplasmic incompatibility usually occurs in matings between infected individuals harboring different strains of Wolbachia (Bourtzis and Miller, 2003) The underlying mechanism of cytoplasmic incompatibility is basically considered to be a modification-rescue system In other words, a Wolbachia strain in males modifies the sperm in order to kill the offspring during embryogenesis . If the same Wolbachia strain is also possessed by females, the offspring will be rescued by removal of the modification (Poinsot et al ., 2003; Bourtzis and Miller, 2003)

Wolbachia also induces various types of sex-ratio distortion, such as male killing, whereby male individuals (i e , the nontransmitting sex) are selectively killed (Bourtzis and Miller, 2003), thelytokous parthenogenesis, whereby females reproduce without fertilization (O'Neill et al , 1997), and feminization, whereby genetic males are transformed into functional females (O'Neill et al , 1997; Hiroki et al , 2002; Negri et al , 2006) Feminization is likely to occur in a relatively small number of species At present, naturally occurring feminization has only been reported in the butterfly E. hecabe (Hiroki et al , 2002, 2004; Narita et al ., 2007a) and a leafhopper, Zyginidia pullula (Negri et al ., 2006) . In E. hecabe, genetic males are completely transformed into functional females, whereas in Z. pullula, genetic males are incompletely feminized and exhibit deformed morphologies Outside insects, Wolbachia-induced feminization is known to occur in crustacean species, such as woodlice, and has been extensively examined in Armadillidium vulgare (for a review, see Rigaud, 1997)

CHwHecFem

in most populations all-9

CHwHecFem in most populations in Okinawajima and Tanegashima in Okinawajima and Tanegashima

Cytoplasmic incompatibility 1:1 sex ratio

Feminization all-9

Cytoplasmic incompatibility 1:1 sex ratio

Feminization

Figure 13.2 (Color figure follows p. 238.) Different Wolbachia infection types and their pheno-types in E. hecabe. (a): In most populations, butterflies singly infected with wHecCI exhibit cytoplasmic incompatibility (b): In populations from Okinawajima and Tanegashima, butterflies doubly infected with wHecCI and wHecFem exhibit feminization . (c): Butterflies singly infected with wHec-Fem have never been found in natural populations or in the laboratory. Right: A female adult of E. hecabe in the natural condition . (Photo provided by Dr. Masashi Nomura, Chiba University.)

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