What maintains noncytoplasmic incompatibility inducing Wolbachia in their hosts: a case study from a natural Drosophila yakuba population.

Cytoplasmic incompatibility (CI) allows Wolbachia to invade hosts populations by specifically inducing sterility in crosses between infected males and uninfected females. In some species, non-CI inducing Wolbachia, that are thought to derive from CI-inducing ancestors, are common. In theory, the maintenance of such infections is not possible unless the bacterium is perfectly transmitted to offspring--and/or provides a fitness benefit to infected females. The present study aims to test this view by investigating a population of Drosophila yakuba from Gabon, West Africa. We did not find any evidence for CI using wild caught females. Infected females from the field transmitted the infection to 100% of their offspring. A positive effect on female fecundity was observed one generation after collecting, but this was not retrieved five generations later, using additional lines. Similarly, the presence of Wolbachia was found to affect mating behaviour, but the results of two experiments realized five generations apart were not consistent. Finally, Wolbachia was not found to affect sex ratio. Overall, our results would suggest that Wolbachia behaves like a neutral or nearly neutral trait in this species, and is maintained in the host by perfect maternal transmission.

Wolbachia segregation dynamics and levels of cytoplasmic incompatibility in Drosophila sechellia.

In Drosophila sechellia, the endocellular bacterium Wolbachia induces cytoplasmic incompatibility (CI): in crosses involving infected males, a partial or complete embryonic mortality occurs unless the female bears the same Wolbachia. D. sechellia is known to harbour two Wolbachia variants, namely wSh and wSn, closely related to wHa and wNo, respectively, two strains infecting the populations of D. simulans from the Seychelles archipelago and New Caledonia. Strikingly, the two species show similar infection patterns: in D. sechellia, wSh can be present on its own or in double infection with wSn, but individuals carrying wSn only do not occur; in D. simulans, wHa can be present on its own or in double infection with wNo, but individuals carrying wNo only do not occur, or occur at very low frequency. Previous experiments on D. simulans showed that lines singly infected by wNo can be obtained by segregation, and stably maintained. Here we investigate this issue in D. sechellia through an 18 generation experiment, and show that wSn and wSh singly infected lines can arise by segregation. Using singly infected lines obtained in this experiment, we estimate the CI intensities of wSh and wSn in D. sechellia, and compare these to the CI intensities of the same Wolbachia injected into D. simulans. Our results do not suggest any consistent effect of the host species on the CI induced by wSh. On the contrary, it seems that wSn expression is repressed by host factors in D. sechellia.

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.

On the mod resc model and the evolution of Wolbachia compatibility types.

Cytoplasmic incompatibility (CI) is induced by the endocellular bacterium Wolbachia. It results in an embryonic mortality occurring when infected males mate with uninfected females. The mechanism involved is currently unknown, but the mod resc model allows interpretation of all observations made so far. It postulates the existence of two bacterial functions: modification (mod) and rescue (resc). The mod function acts in the males' germline, before Wolbachia are shed from maturing sperm. If sperm is affected by mod, zygote development will fail unless resc is expressed in the egg. Interestingly, CI is also observed in crosses between infected males and infected females when the two partners bear different Wolbachia strains, demonstrating that mod and resc interact in a specific manner: Two Wolbachia strains are compatible with each other only if they harbor the same compatibility type. Here we focus on the evolutionary process involved in the emergence of new compatibility types from ancestral ones. We argue that new compatibility types are likely to evolve under a wider range of conditions than previously thought, through a two-step process. First, new mod variants can arise by mutation and spread by drift. This is possible because mod is expressed in males and Wolbachia is transmitted by females. Second, once such a mod variant achieves a certain frequency, it can create the conditions for the deterministic invasion of a new resc variant, allowing the invasion of a new mod resc pair. Furthermore, we show that a stable polymorphism might be maintained in natural populations, allowing the long-term existence of "suicidal" Wolbachia strains.

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.

Wolbachia transfer from Drosophila melanogaster into D. simulans: Host effect and cytoplasmic incompatibility relationships.

Wolbachia are maternally transmitted endocellular bacteria causing a reproductive incompatibility called cytoplasmic incompatibility (CI) in several arthropod species, including Drosophila. CI results in embryonic mortality in incompatible crosses. The only bacterial strain known to infect Drosophila melanogaster (wDm) was transferred from a D. melanogaster isofemale line into uninfected D. simulans isofemale lines by embryo microinjections. Males from the resulting transinfected lines induce >98% embryonic mortality when crossed with uninfected D. simulans females. In contrast, males from the donor D. melanogaster line induce only 18-32% CI on average when crossed with uninfected D. melanogaster females. Transinfected D. simulans lines do not differ from the D. melanogaster donor line in the Wolbachia load found in the embryo or in the total bacterial load of young males. However, >80% of cysts are infected by Wolbachia in the testes of young transinfected males, whereas only 8% of cysts are infected in young males from the D. melanogaster donor isofemale line. This difference might be caused by physiological differences between hosts, but it might also involve tissue-specific control of Wolbachia density by D. melanogaster. The wDm-transinfected D. simulans lines are unidirectionally incompatible with strains infected by the non-CI expressor Wolbachia strains wKi, wMau, or wAu, and they are bidirectionally incompatible with strains infected by the CI-expressor Wolbachia strains wHa or wNo. However, wDm-infected males do not induce CI toward females infected by the CI-expressor strain wRi, which is found in D. simulans continental populations, while wRi-infected males induce partial CI toward wDm-infected females. This peculiar asymmetrical pattern could reflect an ongoing divergence between the CI mechanisms of wRi and wDm. It would also confirm other results indicating that the factor responsible for CI induction in males is distinct from the factor responsible for CI rescue in females.

The relationship between structural variation and dysgenic properties of P elements in long-established P-transformed lines of Drosophila simulans.

Ten lines of Drosophila simulans were investigated with respect to P activity, P susceptibility and the number and structure of their P copies, eight years after transformation with the P element. All 10 were found to have reached a steady state. They exhibited varying levels of P activity (from 0 per cent to 96 per cent GD sterility) and, with the exception of one line, were not P-susceptible. In contrast with P element behaviour in D. melanogaster: (i) no relationship was found between the molecular pattern of P copies in a line and its ability to induce or to repress P expression in D. simulans; (ii) peculiar P element derivatives were observed in this species; (iii) the average number of P copies per genome was only half of that in D. melanogaster. This may result from transposon-host genome interactions, which lead to a low invading power of the P element in D. simulans.

Variability within the Seychelles cytoplasmic incompatibility system in Drosophila simulans.

In Drosophila simulans, we described a cytoplasmic incompatibility (CI) system (Seychelles) restricted to insular populations that harbor the mitochondrial type SiI. Since then, these populations have been shown to be heterogeneous, some being infected by one Wolbachia genetic variant only (wHa), while others are infected simultaneously by wHa and by another variant (wNo) always found in association with wHa. We have experimentally obtained two D. simulans strains only infected by the wNo variant. This variant determines its own cytoplasmic incompatibility type. In particular, the cross between wNo-bearing flies and wHa-bearing ones is bidirectionally incompatible. The Seychelles CI type, stricto sensu, is distinguished by being determined by the simultaneous presence of two Wolbachia variants that we found to be mutually incompatible. In addition, we observed incomplete maternal transmission of the Wolbachia.

Phenotypic expression of ADH regulatory genes in Drosophila melanogaster: a comparative study between a paleartic and a tropical population.

In vitro ADH activity was studied in D. melanogaster males from two sets of third chromosome substitution lines, one from a paleartic population (Gigean, France), the other from a tropical population (Brazzaville, Congo). As a linear model with raw ADH activity dependent on fresh weight was significant in both sets of lines, the raw activity was adjusted by regression on weight. Two main results were found: (a) the well-known substantial intrapopulation variability; and (b) third chromosome geographical origin did not affect the mean ADH activity. Unlike the structural Adh gene polymorphism which allows the two populations to be distinguished, the polymorphism of the third chromosome ADH regulatory genes (or more exactly their phenotypic expression) does not allow to discriminate between them. These results are discussed in the context of the adaptation of D. melanogaster to the alcoholic substrates in light of a model proposed by Hedrick and McDonald (1980) in order to interpret variations in both structural and regulatory gene polymorphisms.

Latitudinal variation of Adh gene frequencies in Drosophila melanogaster: a Mediterranean instability.

The relationship between allelic frequencies at the Adh locus and latitude of origin was studied using selected published data from various parts of the world and original observations. An overall increase of Adh-F with increasing latitude was observed but the relationship is not linear. Tropical populations are generally similar, having a low frequency of the F allele (average 15 per cent) and a smooth increase with latitude (one per cent for one degree). Between 30 and 42 degrees latitude, populations living in a Mediterranean climate in various parts of the world (Mediterranean countries, Australia's east coast and North America's west coast) are also similar, with a much higher average frequency of F (70 per cent), a steeper slope (two per cent) and a broader range of variability for a given latitude. In a restricted area (near Cordoba in southern Spain) numerous wild collected samples also showed a large variability, sometimes over a very short distance. Allelic frequencies in Mediterranean countries are thus quite unstable and it is proposed that this phenomenon be called a "Mediterranean instability". Further north, numerous samples from France were characterized by an even higher frequency of F (95 per cent) and a greater homogeneity over a broad geographic area. These observations are discussed and the need for more field studies is emphasized.

ADH activity and ethanol tolerance in third chromosome substitution lines in Drosophila melanogaster.

In vitro ADH activity and ethanol tolerance were studied in males of a series of third chromosome substitution lines in Drosophila melanogaster. The lines were divided into those with a random third chromosome from a vineyard population (VO lines) and those with a selected third chromosome from males obtained after an egg-to-adult ethanol survival test on the F4 of the previous population (VE lines). Both ADH activity and ethanol tolerance varied significantly among the lines, but the characters showed no significant correlation. Ethanol tolerance (at the higher ethanol concentrations) was higher in the selected lines (VE lines) but ADH activity was not. In our lines, the in vitro ADH activity variability, linked to the regulatory genes (located on the third chromosome) and unrelated to the polymorphism of the Adh locus (located on the second chromosome), is not involved in the ethanol tolerance variability. The data suggest that in this population ethanol tolerance was acquired in nature, at least partially, by means other than increasing ADH activity.

A MOLECULAR APPROACH TO THE ROLE OF HISTORICITY IN EVOLUTION. I. EXPERIMENTAL DESIGN WITH ENZYME POLYMORPHISM IN DROSOPHILA MELANOGASTER.

I investigate the role of the past history (characterized by two inverse sequences of three environments) experienced by Drosophila melanogaster populations, on the allozyme frequencies at four loci (α-Gpdh, Adh, Est-6, Pgm) in a common and constant final environment. The only locus for which an effect of past history was detected is Adh. Although not unambiguous, this result is discussed in terms of interactions between this locus and other polymorphic loci. For Est-6 and Pgm, no influence of past history was seen. As for α-Gpdh, the substantial heterogeneity between replicate populations belonging to the same historical series is probably due to a hitch-hiking effect of genes surrounding this locus. More generally, the role of historicity (i.e., the factors originating from the past history of a population and being accountable for its contemporary evolution) in creating genetic diversity between populations and among species is discussed.

Multiple matings, effective population size and sexual selection in Drosophila melanogaster.

The distribution of sexually efficient males has been analysed in two populations made up of 5 times more females than males. When observation stops as soon as the number of inseminated females equals the number of males, 25 per cent of males in both strains had not mated, 50 per cent of the matings were performed by 50 per cent of the males who mated only once and the other 50 per cent of matings were performed by 25 per cent of the males who mated 2 or 3 times. Productivity of the successive matings falls drastically after the third mating. The relative role of sexual selection and multiple matings as components of fitness are discussed, as well as their implications for the effective population size.