Sex chromosomes control vertical transmission of feminizing Wolbachia symbionts in an isopod.

Microbial endosymbiosis is widespread in animals, with major ecological and evolutionary implications. Successful symbiosis relies on efficient vertical transmission through host generations. However, when symbionts negatively affect host fitness, hosts are expected to evolve suppression of symbiont effects or transmission. Here, we show that sex chromosomes control vertical transmission of feminizing Wolbachia endosymbionts in the isopod Armadillidium nasatum. Theory predicts that the invasion of an XY/XX species by cytoplasmic sex ratio distorters is unlikely because it leads to fixation of the unusual (and often lethal or infertile) YY genotype. We demonstrate that A. nasatum X and Y sex chromosomes are genetically highly similar and that YY individuals are viable and fertile, thereby enabling Wolbachia spread in this XY-XX species. Nevertheless, we show that Wolbachia cannot drive fixation of YY individuals, because infected YY females do not transmit Wolbachia to their offspring, unlike XX and XY females. The genetic basis fits the model of a Y-linked recessive allele (associated with an X-linked dominant allele), in which the homozygous state suppresses Wolbachia transmission. Moreover, production of all-male progenies by infected YY females restores a balanced sex ratio at the host population level. This suggests that blocking of Wolbachia transmission by YY females may have evolved to suppress feminization, thereby offering a whole new perspective on the evolutionary interplay between microbial symbionts and host sex chromosomes.

Legionella pneumophila decreases velocity of Acanthamoeba castellanii.

Acanthamoeba castellanii is a free-living amoeba commonly found in aquatic environment. It feeds on bacteria even if some bacteria resist amoebal digestion. Thus, A. castellanii is described as a Trojan horse able to harbor pathogenic bacteria. L. pneumophila is one of the amoeba-resisting bacteria able to avoid host degradation by phagocytosis and to multiply inside the amoeba. When infecting its host, L. pneumophila injects hundreds of effectors via a type IV secretion system that change physiology of the amoeba to its profit. In this study, we assess mobility of A. castellanii upon infection with L. pneumophila. Electron-microscopy analysis of amoebae revealed a reduction of acanthopodia on cells infected with L. pneumophila. Analysis of velocity showed that migration of A. castellanii infected with L. pneumophila was significantly impaired compare to uninfected cells. Taken together, infection with L. pneumophila could prevent formation of cytoplasmic extensions such as acanthopodia with consequences on the shape, adherence and mobility of A. castellanii.

Bidirectional cytoplasmic incompatibility caused by Wolbachia in the terrestrial isopod Porcellio dilatatus.

In the terrestrial isopod species Porcellio dilatatus, unidirectional Cytoplasmic Incompatibility (CI) between two morphs (P. d. dilatatus and P. d. petiti) caused by a Wolbachia strain (wPet) infecting the morph P. d. petiti has been previously described by experiments initiated four decades ago. Here, we studied another Wolbachia that has been recently detected in a population of the morph P. d. dilatatus. The MLST markers reveal that this Wolbachia is a new strain called wDil distinct from wPet also belonging to the isopod clade of Wolbachia. Quantifications of both Wolbachia strains in the gonads of the two P. dilatatus morphs revealed that all males exhibit similar Wolbachia titers while the titers in females depend on the Wolbachia strain they host. Crossing experiments showed that both wDil and wPet induced partial unidirectional CI with different intensities. Moreover, these two strains induced bidirectional CI when individuals were both infected with one of the two different Wolbachia strains. This way, we demonstrated that P. dilatatus can be infected by two closely related Wolbachia strains (wDil and wPet), that seem to have different modification-rescue systems.

Experimental evidence of Wolbachia introgressive acquisition between terrestrial isopod subspecies.

Wolbachia are the most widespread endosymbiotic bacteria in animals. In many arthropod host species, they manipulate reproduction via several mechanisms that favor their maternal transmission to offspring. Among them, cytoplasmic incompatibility (CI) promotes the spread of the symbiont by specifically decreasing the fertility of crosses involving infected males and uninfected females, via embryo mortality. These differences in reproductive efficiency may select for the avoidance of incompatible mating, a process called reinforcement, and thus contribute to population divergence. In the terrestrial isopod Porcellio dilatatus, the Wolbachia wPet strain infecting the subspecies P. d. petiti induces unidirectional CI with uninfected individuals of the subspecies P. d. dilatatus. To study the consequences of CI on P. d. dilatatus and P. d. petiti hybridization, mitochondrial haplotypes and Wolbachia infection dynamics, we used population cages seeded with different proportions of the 2 subspecies in which we monitored these genetic parameters 5 and 7 years after the initial setup. Analysis of microsatellite markers allowed evaluating the degree of hybridization between individuals of the 2 subspecies. These markers revealed an increase in P. d. dilatatus nuclear genetic signature in all mixed cages, reflecting an asymmetry in hybridization. Hybridization led to the introgressive acquisition of Wolbachia and mitochondrial haplotype from P. d. petiti into nuclear genomes dominated by alleles of P. d. dilatatus. We discuss these results with regards to Wolbachia effects on their host (CI and putative fitness cost), and to a possible reinforcement that may have led to assortative mating, as possible factors contributing to the observed results.

Molecular phylogeny of Wolbachia strains in arthropod hosts based on groE-homologous gene sequences.

Wolbachia is a genus of maternally inherited, intracellular, alpha-Proteobacteria which are widespread among arthropods and cause various reproductive alterations such as cytoplasmic incompatibility and feminization. Wolbachia strains in arthropods have been phylogenetically divided into two supergroups, A and B. Here, we determined the phylogenetic relationships among Wolbachia strains harbored by insects and isopod crustaceans by using groE operon sequences, which encode highly conserved bacterial heat shock proteins. This study includes the first survey for Wolbachia in isopods from a Southeast Asian country, i.e., Thailand. Two isopod species from different parts of Thailand were found to be infected by Wolbachia. Their groE sequences were also included in the phylogenetic analysis. Our results showed clearly that 19 strains from tropical insects, 11 strains from French isopods, and two strains from Thai isopods were in the B supergroup, and 15 strains from tropical insects were in the A supergroup. This is the first report of phylogenetic analysis of a large data set comprising Wolbachia groE sequences from both insects and isopod crustaceans.

Mass drives mating success in Armadillidium vulgare (Crustacea, Oniscidea).

In the terrestrial crustacean Armadillidium vulgare, a large size range exists in natural populations within which males and females could potentially mate. Because of continuous growth far beyond sexual maturity, the largest individuals can be nearly ten times the live mass of the smallest sexually mature individuals. In this study, we explored the influence of male and female body mass on the mating behaviour and success. Starting with a representative panel of males and females in which females are significantly larger than males in average, we followed the sexual behaviour of 23 groups of 20 mixed-sex virgin animals under conditions comparable with natural field situation during the early breeding season. We found a correlation between paired individuals showing an assortative pairing. During pairing male stimulates female and duration of stimulation is determinant for pairing follow-up: efficient stimulation is correlated with female size and not with male size. In consequence, pairs in mating show a reversed size dimorphism between male and female where female are about 20% smaller. Largest females were not mated. During copulation behaviour, the quantity of sperm transferred is positively correlated with copulation duration. Stored sperm can be used for immediate breeding by the female and stored in the spermatheca for future breeding. The last option allows to largest females in the field to continue breeding without additional mating, avoiding the lack of availability of large males able to stimulate them efficiently.

NEIGHBOUR-IN: Image processing software for spatial analysis of animal grouping.

Animal grouping is a very complex process that occurs in many species, involving many individuals under the influence of different mechanisms. To investigate this process, we have created an image processing software, called NEIGHBOUR-IN, designed to analyse individuals' coordinates belonging to up to three different groups. The software also includes statistical analysis and indexes to discriminate aggregates based on spatial localisation of individuals and their neighbours. After the description of the software, the indexes computed by the software are illustrated using both artificial patterns and case studies using the spatial distribution of woodlice. The added strengths of this software and methods are also discussed.

Wolbachia mediate variation of host immunocompetence.

BACKGROUND: After decades during which endosymbionts were considered as silent in their hosts, in particular concerning the immune system, recent studies have revealed the contrary. In the present paper, we addressed the effect of Wolbachia, the most prevalent endosymbiont in arthropods, on host immunocompetence. To this end, we chose the A. vulgare-Wolbachia symbiosis as a model system because it leads to compare consequences of two Wolbachia strains (wVulC and wVulM) on hosts from the same population. Moreover, A. vulgare is the only host-species in which Wolbachia have been directly observed within haemocytes which are responsible for both humoral and cellular immune responses. METHODOLOGY/PRINCIPAL FINDINGS: We sampled gravid females from the same population that were either asymbiotic, infected with wVulC, or infected with wVulM. The offspring from these females were tested and it was revealed that individuals harbouring wVulC exhibited: (i) lower haemocyte densities, (ii) more intense septicaemia in their haemolymph and (iii) a reduced lifespan as compared to individuals habouring wVulM or asymbiotic ones. Therefore, individuals in this population of A. vulgare appeared to suffer more from wVulC than from wVulM. Symbiotic titer and location in the haemocytes did not differ for the two Wolbachia strains showing that these two parameters were not responsible for differences observed in their extended phenotypes in A. vulgare. CONCLUSION/SIGNIFICANCE: The two Wolbachia strains infecting A. vulgare in the same population induced variation in immunocompetence and survival of their hosts. Such variation should highly influence the dynamics of this host-symbiont system. We propose in accordance with previous population genetic works, that wVulM is a local strain that has attenuated its virulence through a long term adaptation process towards local A. vulgare genotypes whereas wVulC, which is a widespread and invasive strain, is not locally adapted.