Wolbachia consequences in host life history traits

One of the expectations of deterministic models is the spread of infected females in the population and the correlative rarefaction of the males (Figure 12 .1) . Feminizing Wolbachia have the potential to reduce male proportion to values limiting mating possibilities and therefore limiting population size (Moreau and Rigaud, 2000) . Such a situation could promote the conditions of a sexual selection . It has been shown in A. vulgare that Wolbachia-infected females (feminized males) have lower mating success and receive less sperm than asymbiotic females (Moreau et al ., 2001) . Males exhibit an active mate choice by interacting more and making more mounting attempts with uninfected females . At the same time, feminized males exhibit a high proportion of nonsexual behavior that stop the mating sequence . Moreover, multiple male mating induces sperm depletion that affects fertility only in infected females (Rigaud and Moreau, 2004) Moreover, multiple male mating induces a competition for sperm precedence (Moreau et al ., 2002). These differences in mating rate and fertility of symbiotic females may limit the spread of Wolbachia and maintain infection polymorphism in populations Furthermore, male mating capacity (i e , the number of females that a single male can inseminate) seems correlated with the symbiotic phenotype: isopod hosts harboring feminizing Wolbachia have higher male mating capacity than species harboring CI Wolbachia (Moreau and Rigaud, 2003) . This pattern may be explained either by selection of high male mating capacity following male rareness or because the feminizing phenotype would lead to population extinction if male mating capacity is not sufficient.

As a continuous supply of migrants may counterbalance male rareness, the impact of host life history traits may nevertheless be more constrained by population structure To trace the coevolution of host and their sex-ratio distorters, host genetic differentiation was investigated in A. vulgare populations using mitochondrial DNA, which is maternally inherited as is Wolbachia. A. vulgare has an unusual ~42-kb-long mitochondrial genome consisting of three ~14-kb-long monomeric units (Raimond et al ., 1999). The nucleotide sequence of the genome was recently determined; it showed that the three units have virtually identical sequences, thereby confirming A. vulgare mitochondrial DNA suitability for population genetic analyses (Marcade et al ., 2007) .

Grandjean et al. (1993) and Rigaud et al . (1999a) analyzed mtDNA variation and the concomitant distribution of sex ratios distorters in French populations of A. vulgare. Due to its mode of inheritance, Wolbachia is expected to be associated to a given mitotype for a single infection event This hitchhiking process will lead to a decrease of within-population diversity On the other hand, association between a given mitotype and the f element could be broken by the possibility of paternal transmission of f Both studies revealed that A. vulgare populations are genetically structured in correlation with Wolbachia prevalence Moreover, introgression of the f feminizing factor into Wolbachia lineages was suggested Recently, population structure was investigated using both mitochondrial DNA and nuclear microsatellite markers (Verne et al ., 2006; Verne, 2007) . Significant host genetic structure consistent with isolation by distance was recorded on five microsatellite loci, whereas mitochondrial DNA genetic structuration is related to Wolbachia prevalence (Verne, 2007) . Deviations from neutrality suggest that both Wolbachia and other feminizing and masculinizing factors may be under frequency-dependant selection, inducing hitchhiking on host cytoplasm

Stable integration off into the host

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Selection of masculinizing genes

Selection of masculinizing genes

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Chromosomal Sex Determination

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Cytoplasmic Sex Determination

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(f) Wolbachia DNA fragment integrated into the host genome k.

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Figure 12.2 Schematic view of the evolution of sex determination and chromosomal sex determinants in A. vulgare (modified from Juchault and Mocquard, 1993) . The symbiotic association between feminizing Wolbachia leads to cytoplasmic sex determination where all individuals are homogametic ZZ . Originating from Wolbachia, a new Mendelian sex factor (f may be transferred into the host genome, leading to the selection of nuclear inhibitors (M genes) . When stably inserted into a Z chromosome, the f factor generates a W-like chromosome . Distribution of progenies (male ratio) and typical chromosomal sex determinants in a given population are represented at each step of this evolutionary course . Black bars: individuals harboring Wolbachia. White bars: asymbiotic individuals . Grey bars: individuals harboring f factor.

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