Prevalence of a Non-Male-Killing Spiroplasma in Natural Populations of Drosophila hydei

Male-killing phenotypes are found in a variety of insects and are often associated with maternally inherited endosymbiotic bacteria. In several species of Drosophila, male-killing endosymbionts of the genus Spiroplasma have been found at low frequencies (0.1 to 3%). In this study, spiroplasma infection without causing male-killing was shown to be prevalent (23 to 66%) in Japanese populations of Drosophila hydei. Molecular phylogenetic analyses showed that D. hydei was infected with a single strain of spiroplasma, which was closely related to male-killing spiroplasmas from other Drosophila species. Artificial-transfer experiments suggested that the spiroplasma genotype rather than the host genotype was responsible for the absence of the male-killing phenotype. Infection densities of the spiroplasma in the natural host, D. hydei, and in the artificial host, Drosophila melanogaster, were significantly lower than those of the male-killing spiroplasma NSRO, which was in accordance with the hypothesis that a threshold infection density is needed for the spiroplasma-induced male-killing expression.     

Low Temperature Reveals Genetic Variability Against Male-Killing Spiroplasma in Drosophila melanogaster Natural Populations

Spiroplasma endosymbionts are maternally inherited microorganisms which infect many arthropod species. In some Drosophila species, it acts as a reproductive manipulator, spreading in populations by killing the sons of infected mothers. Distinct Drosophila melanogaster populations from Brazil exhibit variable male-killing Spiroplasma prevalences. In this study, we investigated the presence of variability for the male-killing phenotype among Drosophila and/or Spiroplasma strains and verified if it correlates with the endosymbiont prevalence in natural populations. For that, we analyzed the male-killing expression when Spiroplasma strains from different populations were transferred to a standard D. melanogaster line (Canton-S) and when a common Spiroplasma strain was transferred to different wild-caught D. melanogaster lines, both at optimal and challenging temperatures for the bacteria. No variation was observed in the male-killing phenotype induced by different Spiroplasma strains. No phenotypic variability among fly lines was detected at optimal temperature (23 °C), as well. Conversely, significant variation in the male-killing expression was revealed among D. melanogaster lines at 18.5 °C, probably caused by imperfect transmission of the endosymbiont. Distinct lines differed in their average sex ratios as well as in the pattern of male-killing expression as the infected females aged. Greater variation occurred among lines from one locality, although there was no clear correlation between the male-killing intensity and the endosymbiont prevalence in each population. Imperfect transmission or male killing may also occur in the field, thus helping to explain the low or intermediate prevalences reported in nature. We discuss the implications of our results for the dynamics of male-killing Spiroplasma in natural populations.     

Male killing Spiroplasma protects Drosophila melanogaster against two parasitoid wasps

Maternally transmitted associations between endosymbiotic bacteria and insects are diverse and widespread in nature. Owing to imperfect vertical transmission, many heritable microbes have evolved compensational mechanisms to enhance their persistence in host lineages, such as manipulating host reproduction and conferring fitness benefits to host. Symbiont-mediated defense against natural enemies of hosts is increasingly recognized as an important mechanism by which endosymbionts enhance host fitness. Members of the genus Spiroplasma associated with distantly related Drosophila hosts are known to engage in either reproductive parasitism (i.e., male killing) or defense against natural enemies (the parasitic wasp Leptopilina heterotoma and a nematode). A male-killing strain of Spiroplasma (strain Melanogaster Sex Ratio Organism (MSRO)) co-occurs with Wolbachia (strain wMel) in certain wild populations of the model organism Drosophila melanogaster. We examined the effects of Spiroplasma MSRO and Wolbachia wMel on Drosophila survival against parasitism by two common wasps, Leptopilina heterotoma and Leptopilina boulardi, that differ in their host ranges and host evasion strategies. The results indicate that Spiroplasma MSRO prevents successful development of both wasps, and confers a small, albeit significant, increase in larva-to-adult survival of flies subjected to wasp attacks. We modeled the conditions under which defense can contribute to Spiroplasma persistence. Wolbachia also confers a weak, but significant, survival advantage to flies attacked by L. heterotoma. The host protective effects exhibited by Spiroplasma and Wolbachia are additive and may provide the conditions for such cotransmitted symbionts to become mutualists. Occurrence of Spiroplasma-mediated protection against distinct parasitoids in divergent Drosophila hosts suggests a general protection mechanism.     

Asymmetrical Interactions between Wolbachia and Spiroplasma Endosymbionts Coexisting in the Same Insect Host

We investigated the interactions between the endosymbionts Wolbachia pipientis strain wMel and Spiroplasma sp. strain NSRO coinfecting the host insect Drosophila melanogaster. By making use of antibiotic therapy, temperature stress, and hemolymph microinjection, we established the following strains in the same host genetic background: the SW strain, infected with both Spiroplasma and Wolbachia; the S strain, infected with Spiroplasma only; and the W strain, infected with Wolbachia only. The infection dynamics of the symbionts in these strains were monitored by quantitative PCR during host development. The infection densities of Spiroplasma exhibited no significant differences between the SW and S strains throughout the developmental course. In contrast, the infection densities of Wolbachia were significantly lower in the SW strain than in the W strain at the pupal and young adult stages. These results indicated that the interactions between the coinfecting symbionts were asymmetrical, i.e., Spiroplasma organisms negatively affected the population of Wolbachia organisms, while Wolbachia organisms did not influence the population of Spiroplasma organisms. In the host body, the symbionts exhibited their own tissue tropisms: among the tissues examined, Spiroplasma was the most abundant in the ovaries, while Wolbachia showed the highest density in Malpighian tubules. Strikingly, basically no Wolbachia organisms were detected in hemolymph, the principal location of Spiroplasma. These results suggest that different host tissues act as distinct microhabitats for the symbionts and that the lytic process in host metamorphosis might be involved in the asymmetrical interactions between the coinfecting symbionts.     

Density dynamics of diverse Spiroplasma strains naturally infecting different species of Drosophila

Facultative heritable bacterial endosymbionts can have dramatic effects on their hosts, ranging from mutualistic to parasitic. Within-host bacterial endosymbiont density plays a critical role in maintenance of a symbiotic relationship, as it can affect levels of vertical transmission and expression of phenotypic effects, both of which influence the infection prevalence in host populations. Species of genus Drosophila are infected with Spiroplasma, whose characterized phenotypic effects range from that of a male-killing reproductive parasite to beneficial defensive endosymbiont. For many strains of Spiroplasma infecting at least 17 species of Drosophila, however, the phenotypic effects are obscure. The infection prevalence of these Spiroplasma vary within and among Drosophila species, and little is known about the within-host density dynamics of these diverse strains. To characterize the patterns of Spiroplasma density variation among Drosophila we used quantitative PCR to assess bacterial titer at various life stages of three species of Drosophila naturally-infected with two different types of Spiroplasma. For naturally infected Drosophila species we found that non-male-killing infections had consistently lower densities than the male-killing infection. The patterns of Spiroplasma titer change during aging varied among Drosophila species infected with different Spiroplasma strains. Bacterial density varied within and among populations of Drosophila, with individuals from the population with the highest prevalence of infection having the highest density. This density variation underscores the complex interaction of Spiroplasma strain and host genetic background in determining endosymbiont density.     

Male-Killing Spiroplasma Induces Sex-Specific Cell Death via Host Apoptotic Pathway

Some symbiotic bacteria cause remarkable reproductive phenotypes like cytoplasmic incompatibility and male-killing in their host insects. Molecular and cellular mechanisms underlying these symbiont-induced reproductive pathologies are of great interest but poorly understood. In this study, Drosophila melanogaster and its native Spiroplasma symbiont strain MSRO were investigated as to how the host''s molecular, cellular and morphogenetic pathways are involved in the symbiont-induced male-killing during embryogenesis. TUNEL (terminal deoxynucleotidyl transferase dUTP nick end labeling) staining, anti-cleaved-Caspase-3 antibody staining, and apoptosis-deficient mutant analysis unequivocally demonstrated that the host''s apoptotic pathway is involved in Spiroplasma-induced male-specific embryonic cell death. Double-staining with TUNEL and an antibody recognizing epidermal marker showed that embryonic epithelium is the main target of Spiroplasma-induced male-specific apoptosis. Immunostaining with antibodies against markers of differentiated and precursor neural cells visualized severe neural defects specifically in Spiroplasma-infected male embryos as reported in previous studies. However, few TUNEL signals were detected in the degenerate nervous tissues of male embryos, and the Spiroplasma-induced neural defects in male embryos were not suppressed in an apoptosis-deficient host mutant. These results suggest the possibility that the apoptosis-dependent epidermal cell death and the apoptosis-independent neural malformation may represent different mechanisms underlying the Spiroplasma-induced male-killing. Despite the male-specific progressive embryonic abnormality, Spiroplasma titers remained almost constant throughout the observed stages of embryonic development and across male and female embryos. Strikingly, a few Spiroplasma-infected embryos exhibited gynandromorphism, wherein apoptotic cell death was restricted to male cells. These observations suggest that neither quantity nor proliferation of Spiroplasma cells but some Spiroplasma-derived factor(s) may be responsible for the expression of the male-killing phenotype.     

Loss of reproductive parasitism following transfer of male-killing Wolbachia to Drosophila melanogaster and Drosophila simulans

Wolbachia manipulate insect host biology through a variety of means that result in increased production of infected females, enhancing its own transmission. A Wolbachia strain (wInn) naturally infecting Drosophila innubila induces male killing, while native strains of D. melanogaster and D. simulans usually induce cytoplasmic incompatibility (CI). In this study, we transferred wInn to D. melanogaster and D. simulans by embryonic microinjection, expecting conservation of the male-killing phenotype to the novel hosts, which are more suitable for genetic analysis. In contrast to our expectations, there was no effect on offspring sex ratio. Furthermore, no CI was observed in the transinfected flies. Overall, transinfected D. melanogaster lines displayed lower transmission rate and lower densities of Wolbachia than transinfected D. simulans lines, in which established infections were transmitted with near-perfect fidelity. In D. simulans, strain wInn had no effect on fecundity and egg-to-adult development. Surprisingly, one of the two transinfected lines tested showed increased longevity. We discuss our results in the context of host-symbiont co-evolution and the potential of symbionts to invade novel host species.     

Silence of the killers: discovery of male-killing suppression in a rearing strain of the small brown planthopper,Laodelphax striatellus

According to evolutionary theory, sex ratio distortions caused by reproductive parasites such as Wolbachia and Spiroplasma are predicted to be rapidly normalized by the emergence of host nuclear suppressors. However, such processes in the evolutionary arms race are difficult to observe because sex ratio biases will be promptly hidden and become superficially unrecognizable. The evolution of genetic suppressors has been reported in just two insect species so far. In the small brown planthopper, Laodelphax striatellus, female-biases caused by Spiroplasma, which is a ‘late’ male-killer, have been found in some populations. During the continuous rearing of L. striatellus, we noted that a rearing strain had a 1 : 1 sex ratio even though it harboured Spiroplasma. Through introgression crossing experiments with a strain lacking suppressors, we revealed that the L. striatellus strain had the zygotic male-killing suppressor acting as a dominant trait. The male-killing phenotype was hidden by the suppressor even though Spiroplasma retained its male-killing ability. This is the first study to demonstrate the existence of a late male-killing suppressor and its mode of inheritance. Our results, together with those of previous studies, suggest that the inheritance modes of male-killing suppressors are similar regardless of insect order or early or late male killing.     

Population Dynamics of Male-Killing and Non-Male-Killing Spiroplasmas in Drosophila melanogaster

The endosymbiotic bacteria Spiroplasma spp. are vertically transmitted through female hosts and are known to cause selective death of male offspring in insects. One strain of spiroplasma, NSRO, causes male killing in Drosophila species, and a non-male-killing variant of NSRO, designated NSRO-A, has been isolated. It is not known why NSRO-A does not kill males. In an attempt to understand the mechanism of male killing, we investigated the population dynamics of NSRO and NSRO-A throughout the developmental course of the laboratory host Drosophila melanogaster by using a quantitative PCR technique. In the early development of the host insect, the titers of NSRO were significantly higher than those of NSRO-A at the first- and second-instar stages, whereas at the egg, third-instar, and pupal stages, the titers of the two spiroplasmas were almost the same. Upon adult emergence, the titers of the two spiroplasmas were similar, around 2 × 10⁸ dnaA copy equivalents. However, throughout host aging, the two spiroplasmas showed strikingly different population growth patterns. The titers of NSRO increased exponentially for 3 weeks, attained a peak value of around 4 × 10⁹ dnaA copy equivalents per insect, and then decreased. In contrast, the titers of NSRO-A were almost constant throughout the adult portion of the life cycle. In adult females, consequently, the titer of NSRO was significantly higher than the titer of NSRO-A except for a short period just after emergence. Although infection of adult females with NSRO resulted in almost 100% male killing, production of some male offspring was observed within 4 days after emergence when the titers of NSRO were as low as those of NSRO-A. Based on these results, we proposed a threshold density hypothesis for the expression of male killing caused by the spiroplasma. The extents of the bottleneck in the vertical transmission through host generations were estimated to be 5 × 10⁻⁵ for NSRO and 3 × 10⁻⁴ for NSRO-A.     

Phenotype and transmission efficiency of artificial and natural male-killing Spiroplasma infections in Drosophila melanogaster

Many insect species carry inherited Spiroplasma bacteria which act as important partners and antagonists. The nature of symbioses between Spiroplasma and insects has been most extensively studied in the interaction between male-killing Spiroplasma infection and Drosophila melanogaster. For historical reasons, these studies have largely focussed on the Spiroplasma strain known as NSRO, derived from Drosophila nebulosa and transinfected into D. melanogaster. More recently, D. melanogaster naturally infected with Spiroplasma were discovered. Whilst the well studied strain NSRO is closely related to that found natively in D. melanogaster, it is unclear whether strains from D. nebulosa reflect a natural interaction when placed in D. melanogaster. In this paper, we determine if NSRO has similar or different properties from strains of Spiroplasma naturally infecting D. melanogaster in terms of transmission efficiency and the strength and timing of male-killing. Native infections were observed to have higher transmission efficiency than introduced NSRO infections during the early phases of host reproduction, but not during late reproduction. The timing and intensity of male-killing did not differ between infection classes. As a precautionary measure, it is proposed that future work seeking to reveal the nature of coevolved Spiroplasma-Drosophila interactions use the native strain.     

Fitness effects of Wolbachia and Spiroplasma in Drosophila melanogaster

Maternally inherited endosymbionts that manipulate the reproduction of their insect host are very common. Aside from the reproductive manipulation they produce, the fitness of these symbionts depends in part on the direct impact they have on the female host. Although this parameter has commonly been investigated for single infections, it has much more rarely been established in dual infections. We here establish the direct effect of infection with two different symbionts exhibiting different reproductive manipulation phenotypes, both alone and in combination, in the fruit fly Drosophila melanogaster. This species carries a cytoplasmic incompatibility inducing Wolbachia and a male-killing Spiroplasma, occurring as single or double (co-) infections in natural populations. We assessed direct fitness effects of these bacteria on their host, by comparing larval competitiveness and adult fecundity of uninfected, Wolbachia, Spiroplasma and Wolbachia–Spiroplasma co-infected females. We found no effect of infection status on the fitness of females for both estimates, that is, no evidence of any benefits or costs to either single or co-infection. This leads to the conclusion that both bacteria probably have other sources of benefits to persist in D. melanogaster populations, either by means of their reproductive manipulations (fitness compensation from male death in Spiroplasma infection and cytoplasmic incompatibility in Wolbachia infection) or by positive fitness interactions on other fitness components.     

Variation in Antiviral Protection Mediated by Different Wolbachia Strains in Drosophila simulans

Drosophila C virus (DCV) is a natural pathogen of Drosophila and a useful model for studying antiviral defences. The Drosophila host is also commonly infected with the widespread endosymbiotic bacteria Wolbachia pipientis. When DCV coinfects Wolbachia-infected D. melanogaster, virus particles accumulate more slowly and virus induced mortality is substantially delayed. Considering that Wolbachia is estimated to infect up to two-thirds of all insect species, the observed protective effects of Wolbachia may extend to a range of both beneficial and pest insects, including insects that vector important viral diseases of humans, animals and plants. Currently, Wolbachia-mediated antiviral protection has only been described from a limited number of very closely related strains that infect D. melanogaster. We used D. simulans and its naturally occurring Wolbachia infections to test the generality of the Wolbachia-mediated antiviral protection. We generated paired D. simulans lines either uninfected or infected with five different Wolbachia strains. Each paired fly line was challenged with DCV and Flock House virus. Significant antiviral protection was seen for some but not all of the Wolbachia strain-fly line combinations tested. In some cases, protection from virus-induced mortality was associated with a delay in virus accumulation, but some Wolbachia-infected flies were tolerant to high titres of DCV. The Wolbachia strains that did protect occurred at comparatively high density within the flies and were most closely related to the D. melanogaster Wolbachia strain wMel. These results indicate that Wolbachia-mediated antiviral protection is not ubiquitous, a finding that is important for understanding the distribution of Wolbachia and virus in natural insect populations.     

Interspecific transmission of endosymbiotic Spiroplasma by mites

The occurrence of closely related strains of maternally transmitted endosymbionts in distantly related insect species indicates that these infections can colonize new host species by lateral transfer, although the mechanisms by which this occurs are unknown. We investigated whether ectoparasitic mites, which feed on insect haemolymph, can serve as interspecific vectors of Spiroplasma poulsonii, a male-killing endosymbiont of Drosophila. Using Spiroplasma-specific primers for PCR, we found that mites can pick up Spiroplasma from infected Drosophila nebulosa females and subsequently transfer the infection to Drosophila willistoni. Some of the progeny of the recipient D. willistoni were infected, indicating successful maternal transmission of the Spiroplasma within the new host species. However, the transmission rate of the infection from recipient flies to their offspring was low, perhaps due to low Spiroplasma density in the recipient flies.     

Effects of crowding and temperature on Wolbachia infection density among life cycle stages of Aedes albopictus

Species of the genus Wolbachia are a group of Rickettsia-like, maternally-inherited bacteria (gram negative), which cause various reproductive alterations in their arthropod and nematode hosts including cytoplasmic incompatibility (CI), male-killing, parthenogenesis and feminization. They can be divided into supergroups such as A and B based on phylogenetic analysis of 16S rDNA sequences. In this study, we examined the relative infection densities of Wolbachia strains among life cycle stages in the mosquito, Aedes albopictus in terms of crowding effect and temperature effect. A. albopictus is known to be superinfected with both A- and B-supergroup Wolbachia which cause CI. The relative Wolbachia densities within each individual mosquito were determined and quantified by using real-time quantitative PCR assay based on the wsp gene. We found that B-supergroup Wolbachia strain densities in this host species were consistently and significantly higher than in the A-supergroup. Larval crowding also reduced adult size of mosquitoes. Our results show clearly that the higher densities of mosquito larvae cause lower densities of Wolbachia strains. Examination of the effect of temperature on Wolbachia density in each stage of the mosquito clearly revealed a significant decrease in bacterial density following exposure to elevated temperature (37 °C) in both males and females.     

Evolutionary Dynamics of wAu-Like Wolbachia Variants in Neotropical Drosophila spp.

Wolbachia bacteria are common intracellular symbionts of arthropods and have been extensively studied in Drosophila. Most research focuses on two Old Word hosts, Drosophila melanogaster and Drosophila simulans, and does not take into account that some of the Wolbachia associations in these species may have evolved only after their fast global expansion and after the exposure to Wolbachia of previously isolated habitats. Here we looked at Wolbachia of Neotropical Drosophila species. Seventy-one lines of 16 Neotropical Drosophila species sampled in different regions and at different time points were analyzed. Wolbachia is absent in lines of Drosophila willistoni collected before the 1970s, but more recent samples are infected with a strain designated wWil. Wolbachia is absent in all other species of the willistoni group. Polymorphic wWil-related strains were detected in some saltans group species, with D. septentriosaltans being coinfected with at least four variants. Based on wsp and ftsZ sequence data, wWil of D. willistoni is identical to wAu, a strain isolated from D. simulans, but can be discriminated when using a polymorphic minisatellite marker. In contrast to wAu, which infects both germ line and somatic tissues of D. simulans, wWil is found exclusively in the primordial germ line cells of D. willistoni embryos. We report on a pool of closely related Wolbachia strains in Neotropical Drosophila species as a potential source for the wAu strain in D. simulans. Possible evolutionary scenarios reconstructing the infection history of wAu-like Wolbachia in Neotropical Drosophila species and the Old World species D. simulans are discussed.     

Variable Incidence of Spiroplasma Infections in Natural Populations of Drosophila Species

Spiroplasma is widespread as a heritable bacterial symbiont in insects and some other invertebrates, in which it sometimes acts as a male-killer and causes female-biased sex ratios in hosts. Besides Wolbachia, it is the only heritable bacterium known from Drosophila, having been found in 16 of over 200 Drosophila species screened, based on samples of one or few individuals per species. To assess the extent to which Spiroplasma infection varies within and among species of Drosophila, intensive sampling consisting of 50–281 individuals per species was conducted for natural populations of 19 Drosophila species. Infection rates varied among species and among populations of the same species, and 12 of 19 species tested negative for all individuals. Spiroplasma infection never was fixed, and the highest infection rates were 60% in certain populations of D. hydei and 85% in certain populations of D. mojavensis. In infected species, infection rates were similar for males and females, indicating that these Spiroplasma infections do not confer a strong male-killing effect. These findings suggest that Spiroplasma has other effects on hosts that allow it to persist, and that environmental or host variation affects transmission or persistence leading to differences among populations in infection frequencies.     

The Bacterial Symbiont Wolbachia Induces Resistance to RNA Viral Infections in Drosophila melanogaster

Wolbachia are vertically transmitted, obligatory intracellular bacteria that infect a great number of species of arthropods and nematodes. In insects, they are mainly known for disrupting the reproductive biology of their hosts in order to increase their transmission through the female germline. In Drosophila melanogaster, however, a strong and consistent effect of Wolbachia infection has not been found. Here we report that a bacterial infection renders D. melanogaster more resistant to Drosophila C virus, reducing the load of viruses in infected flies. We identify these resistance-inducing bacteria as Wolbachia. Furthermore, we show that Wolbachia also increases resistance of Drosophila to two other RNA virus infections (Nora virus and Flock House virus) but not to a DNA virus infection (Insect Iridescent Virus 6). These results identify a new major factor regulating D. melanogaster resistance to infection by RNA viruses and contribute to the idea that the response of a host to a particular pathogen also depends on its interactions with other microorganisms. This is also, to our knowledge, the first report of a strong beneficial effect of Wolbachia infection in D. melanogaster. The induced resistance to natural viral pathogens may explain Wolbachia prevalence in natural populations and represents a novel Wolbachia–host interaction.     

The virulent Wolbachia strain wMelPop increases the frequency of apoptosis in the female germline cells of Drosophila melanogaster

Background

Wolbachia are bacterial endosymbionts of many arthropod species in which they manipulate reproductive functions. The distribution of these bacteria in the Drosophila ovarian cells at different stages of oogenesis has been amply described. The pathways along which Wolbachia influences Drosophila oogenesis have been, so far, little studied. It is known that Wolbachia are abundant in the somatic stem cell niche of the Drosophila germarium. A checkpoint, where programmed cell death, or apoptosis, can occur, is located in region 2a/2b of the germarium, which comprises niche cells. Here we address the question whether or not the presence of Wolbachia in germarium cells can affect the frequency of cyst apoptosis in the checkpoint.

Results

Our current fluorescent microscopic observations showed that the wMel and wMelPop strains had different effects on female germline cells of D. melanogaster. The Wolbachia strain wMel did not affect the frequency of apoptosis in cells of the germarium. The presence of the Wolbachia strain wMelPop in the D. melanogaster^w1118 ovaries increased the number of germaria where cells underwent apoptosis in the checkpoint. Based on the appearance in the electron microscope, there was no difference in morphological features of apoptotic cystocytes between Wolbachia-infected and uninfected flies. Bacteria with normal ultrastructure and large numbers of degenerating bacteria were found in the dying cyst cells.

Conclusions

Our current study demonstrated that the Wolbachia strain wMelPop affects the egg chamber formation in the D. melanogaster ovaries. This led to an increase in the number of germaria containing apoptotic cells. It is suggested that Wolbachia can adversely interfere either with the cystocyte differentiation into the oocyte or with the division of somatic stem cells giving rise to follicle cells and, as a consequence, to improper ratio of germline cells to follicle cells and, ultimately, to apoptosis of cysts. There was no similar adverse effect in D. melanogaster Canton S infected with the Wolbachia strain wMel. This was taken to mean that the observed increase in frequency of apoptosis was not the general effect of Wolbachia on germline cells of D. melanogaster, it was rather induced by the virulent Wolbachia strain wMelPop.

     

The pathology of embryo death caused by the male-killing Spiroplasma bacterium in Drosophila nebulosa

Background

Inherited bacteria that kill male offspring, male-killers, are known to be common in insects, but little is understood about the mechanisms used by male-killing bacteria to kill males. In this paper we describe the tempo and changes that occur during male-killing by Spiroplasma bacteria in the host Drosophila nebulosa.

Results

Spiroplasma infected D. nebulosa males were developmentally retarded from 6–8 h into embryonic development at 25°C, and arrested at between stages 12 and 13 of embryogenesis (10–12 h). Dying males were characterized by a failure to form segments, and ultimately disintegration of the normal oval embryonic shape. Prior to death, dying males exhibited widespread apoptosis, as testified by TUNEL staining.

Conclusion

The Spiroplasma kills male Drosophila in a narrow developmental period, shortly after the formation of the host dosage compensation complex that is required for male-killing. Male death is preceded by widespread apoptosis, but it is uncertain if this is primary or secondary apoptosis.