Hervé Merçot

Evolutionary consequences of Wolbachia infections

The past decade has revealed the bacterium Wolbachia as the most widespread symbiont of arthropods and nematodes. Behind this evolutionary success is an remarkable variety of effects on host biology, ranging from manipulation of reproduction in favor of females to more classical mutualistic interactions. Here we discuss the potential of Wolbachia for promoting evolutionary changes in its hosts.

Multiple Rescue Factors Within a Wolbachia Strain

Wolbachia-induced cytoplasmic incompatibility (CI) is expressed when infected males are crossed with either uninfected females or females infected with Wolbachia of different CI specificity. In diploid insects, CI results in embryonic mortality, apparently due to the the loss of the paternal set of chromosomes, usually during the first mitotic division. The molecular basis of CI has not been determined yet; however, several lines of evidence suggest that Wolbachia exhibits two distinct sex-dependent functions: in males, Wolbachia somehow “imprints” the paternal chromosomes during spermatogenesis (mod function), whereas in females, the presence of the same Wolbachia strain(s) is able to restore embryonic viability (resc function). On the basis of the ability of Wolbachia to induce the modification and/or rescue functions in a given host, each bacterial strain can be classified as belonging in one of the four following categories: mod+ resc+, mod− resc+, mod− resc−, and mod+ resc−. A so-called “suicide” mod+ resc− strain has not been found in nature yet. Here, a combination of embryonic cytoplasmic injections and introgression experiments was used to transfer nine evolutionary, distantly related Wolbachia strains (wYak, wTei, wSan, wRi, wMel, wHa, wAu, wNo, and wMa) into the same host background, that of Drosophila simulans (STCP strain), a highly permissive host for CI expression. We initially characterized the modification and rescue properties of the Wolbachia strains wYak, wTei, and wSan, naturally present in the yakuba complex, upon their transfer into D. simulans. Confocal microscopy and multilocus sequencing typing (MLST) analysis were also employed for the evaluation of the CI properties. We also tested the compatibility relationships of wYak, wTei, and wSan with all other Wolbachia infections. So far, the cytoplasmic incompatibility properties of different Wolbachia variants are explained assuming a single pair of modification and rescue factors specific to each variant. This study shows that a given Wolbachia variant can possess multiple rescue determinants corresponding to different CI systems. In addition, our results: (a) suggest that wTei appears to behave in D. simulans as a suicide mod+ resc− strain, (b) unravel unique CI properties, and (c) provide a framework to understand the diversity and the evolution of new CI-compatibility types.

Wolbachia infections in Drosophila melanogaster and D. simulans: polymorphism and levels of cytoplasmic incompatibility.

Wolbachia are endosymbiotic bacteria, widespread in terrestrial Arthropods. They are mainly transmitted vertically, from mothers to offspring and induce various alterations of their hosts’ sexuality and reproduction, the most commonly reported phenomenon being Cytoplasmic Incompatibility (CI), observed in Drosophila melanogaster and D. simulans. Basically, CI results in a more or less intense embryonic mortality, occurring in crosses between males infected by Wolbachia and uninfected females. In D. simulans, Wolbachia and CI were observed in 1986. Since then, this host species has become a model system for investigating the polymorphism of Wolbachia infections and CI. In this review we describe the different Wolbachia infections currently known to occur in D. melanogaster and D. simulans. The two species are highly contrasting with regard to symbiotic diversity: while five Wolbachia variants have been described in D. simulans natural populations, D. melanogaster seems to harbor one Wolbachia variant only. Another marked difference between these two Drosophila species is their permissiveness with regard to CI, which seems to be fully expressed in D. simulans but partially or totally repressed in D. melanogaster, demonstrating the involvement of host factors in the control of CI levels. The potential of the two host species regarding the understanding of CI and its evolution is also discussed.

Wolbachia Transfer from Rhagoletis cerasi to Drosophila simulans: Investigating the Outcomes of Host-Symbiont Coevolution

ABSTRACT Wolbachia is an endosymbiont of diverse arthropod lineages that can induce various alterations of host reproduction for its own benefice. Cytoplasmic incompatibility (CI) is the most common phenomenon, which results in embryonic lethality when males that bear Wolbachia are mated with females that do not. In the cherry fruit fly, Rhagoletis cerasi, Wolbachia seems to be responsible for previously reported patterns of incompatibility between populations. Here we report on the artificial transfer of two Wolbachia variants (wCer1 and wCer2) from R. cerasi into Drosophila simulans, which was performed with two major goals in mind: first, to isolate wCer1 from wCer2 in order to individually test their respective abilities to induce CI in the new host; and, second, to test the theoretical prediction that recent Wolbachia-host associations should be characterized by high levels of CI, fitness costs to the new host, and inefficient transmission from mothers to offspring. wCer1 was unable to develop in the new host, resulting in its rapid loss after successful injection, while wCer2 was established in the new host. Transmission rates of wCer2 were low, and the infection showed negative fitness effects, consistent with our prediction, but CI levels were unexpectedly lower in the new host. Based on these parameter estimates, neither wCer1 nor wCer2 could be naturally maintained in D. simulans. The experiment thus suggests that natural Wolbachia transfer between species might be restricted by many factors, should the ecological barriers be bypassed.

THE SEX-RATIO TRAIT IN DROSOPHILA SIMULANS : GEOGRAPHICAL DISTRIBUTION OF DISTORTION AND RESISTANCE

The sex‐ratio trait we describe here in Drosophila simulans results from X‐linked meiotic drive. Males bearing a driving X chromosome can produce a large excess of females (about 90%) in their progeny. This is, however, rarely the case in the wild, where resistance factors, including autosomal suppressors and insensitive Y chromosomes, prevent the expression of the driver. In this study, we searched for drive and resistance factors in strains of Drosophila simulans collected all over the world. Driving X chromosomes were found in all populations whenever a good sample size was available. Their frequency may reach up to 60%. However, the presence of driving X chromosomes never results in an excess of females, due to the systematic co‐occurrence of resistance factors. The highest frequencies of driving X chromosomes were observed in islands, while populations from East and Central Africa (the supposed center of origin of the species) showed the highest level of resistance. The geographical pattern of drive and resistance factors, as well as the results of crosses between strains from different geographical areas, suggest that the sex‐ratio system described here has a unique and ancient origin in the species.

Alcohol tolerance, ADH activity, and ecological niche of Drosophila species

In vitro alcohol dehydrogenase (ADH) activity was measured in adults of species belonging to Drosophila and to the related genus Zaprionus. Data were analyzed according to the known breeding sites and the level of ethanol tolerance of these species. Alcohol dehydrogenase activity was assayed with both ethanol (E) and isopropanol (I). Our results show a very broad range of activities among the 71 species investigated, the ratio of the highest value observed (D. melanogaster) to the lowest (D. pruinosa) being 65:1. A general positive correlation was found between the level of ADH activity and the capacity to detoxify ethanol. Nevertheless, many species show exceptions to this rule. Contrary to a logical expectation, adaptation to high alcoholic resources, which has been a recurrent evolutionary event, was not mediated by a more efficient use of ethanol, that is, an increase of the E/I ratio. This ratio seems to be quite variable according to the phylogeny and is especially low in the subgenus Sophophora as well as in Zaprionus. Alcohol tolerance clearly is related to the larval habitat of the species and shows that adaptation to alcoholic resources has been a major evolutionary challenge in drosophilids. This adaptation is not related to phylogeny, having occurred independently several times during the evolution of the group. Finally, it should be borne in mind that, besides metabolization and detoxification, other physiological processes such as nervous‐system tolerance or ethanol excretion may be involved in ethanol tolerance, and such functions also should be investigated. Environmental ethanol, which is certainly a major ecological parameter for many drosophilids, has selected a diversity of physiological adaptations, all related to the Adh locus, but presumably much more complicated than was previously believed.

Wolbachia segregation rate in Drosophila simulans naturally bi-infected cytoplasmic lineages

Wolbachia are maternally transmitted endocellular bacteria infecting several arthropod species. In order to study Wolbachia segregation rate, Drosophila simulans females from an Indo-Pacific population (Seychelles) bi-infected by the two Wolbachia variants wHa and wNo were backcrossed to uninfected males in two conditions. In the first case, Seychelles males from a stock cured from its Wolbachia by tetracycline treatment were used. In the second case, the males came from a naturally uninfected Tunisian population. It was found that (i) the two Wolbachia variants can segregate, so that bi-infected females can produce a few offspring infected only by wHa or wNo. This occurs in both backcross conditions. (ii) Segregation leads more frequently to wHa than to wNo mono-infection. (iii) Wolbachia transmission is lower when the Seychelles genome is introgressed by the Tunisian genome, suggesting that host genomic factors might influence infection fate.

Characterization of non-cytoplasmic incompatibility inducing Wolbachia in two continental African populations of Drosophila simulans

Wolbachia is an endocellular bacterium infecting arthropods and nematodes. In arthropods, it invades host populations through various mechanisms, affecting host reproduction, the most common of which being cytoplasmic incompatibility (CI). CI is an embryonic mortality occurring when infected males mate with uninfected females or females infected by a different Wolbachia strain. This phenomenon is observed in Drosophila simulans, an intensively studied Wolbachia host, harbouring at least five distinct bacterial strains. In this study, we investigate various aspects of the Wolbachia infections occurring in two continental African populations of D. simulans: CI phenotype, phylogenetic position based on the wsp gene and associated mitochondrial haplotype. From the East African population (Tanzania), we show that (i) the siIII mitochondrial haplotype occurs in continental populations, which was unexpected based on the current views of D. simulans biogeography, (ii) the wKi strain (that rescues from CI while being unable to induce it) is very closely related to the CI-inducing strain wNo, (iii) wKi and wNo might not derive from a unique infection event, and (iv) wKi is likely to represent the same entity as the previously described wMa variant. In the West African population (Cameroon), the Wolbachia infection was found identical to the previously described wAu, which does not induce CI. This finding supports the view that wAu might be an ancient infection in D. simulans.

EVOLUTION OF WOLBACHIA-INDUCED CYTOPLASMIC INCOMPATIBILITY IN DROSOPHILA SIMULANS AND D. SECHELLIA

Abstract.— The intracellular bacterium Wolbachia invades arthropod host populations through various mechanisms, the most common of which being cytoplasmic incompatibility (CI). CI involves elevated embryo mortality when infected males mate with uninfected females or females infected with different, incompatible Wolbachia strains. The present study focuses on this phenomenon in two Drosophila species: D. simulans and D. sechellia. Drosophila simulans populations are infected by several Wolbachia strains, including w Ha and w No. Drosophila sechellia is infected by only two Wolbachia: w Sh and w Sn. In both Drosophila species, double infections with Wolbachia are found. As indicated by several molecular markers, w Ha is closely related to w Sh, and w No to w Sn. Furthermore, the double infections in the two host species are associated with closely related mitochondrial haplotypes, namely si I (associated with w Ha and w No in D. simulans) and se (associated with w Sh and w Sn in D. sechellia). To test the theoretical prediction that Wolbachia compatibility types can diverge rapidly, we injected w Sh and w Sn into D. simulans, to compare their CI properties to those of their sister strains w Ha and w No, respectively, in the same host genetic background. We found that within each pair of sister strains CI levels were similar and that sister strains were fully compatible. We conclude that the short period for which the Wolbachia sister strains have been evolving separated from each other was not sufficient for their CI properties to diverge significantly.

Wolbachia transmission in a naturally bi‐infected Drosophila simulans strain from New‐Caledonia

Wolbachia are maternally‐transmitted endocellular bacteria infecting several arthropod species. In order to study the possibility of Wolbachia segregation in a naturally bi‐infected host, isofemale lines from a bi‐infected Drosophila simulans (Sturtevant) strain from Nouméa (New Caledonia) were backcrossed using uninfected males carrying the same nuclear background. Uninfected males were used to avoid the cytoplasmic incompatibility syndrome (CI) associated with the presence of Wolbachia in males. Each line was established using a female infected simultaneously by the two different Wolbachia variants wHa and wNo. The backcross led to some individuals carrying only one type of infection being recovered among the progeny of the bi‐infected foundress females. Rarely, uninfected individuals were also recovered. Isolated for the first time in its natural host, wNo exhibited a significantly weaker CI phenotype than the isolated wHa variant. Infection fate when backcross conditions were relaxed varied depending on rearing conditions of the host. Under favourable conditions, the infection was generally maintained, while it was frequently lost under unfavourable conditions. This result probably reflects the direct fitness dependence of the symbiont on its host.