Maternally transmitted non-bacterial male killer in Drosophila biauraria

Cell-based assays and comparative genomics revealed the conserved and hidden effects of Wolbachia on insect sex determination

It is advantageous for maternally transmitted endosymbionts to skew the sex ratio of their hosts toward females. Some endosymbiotic bacteria, such as Wolbachia, cause their insect hosts to exclusively produce female offspring through male killing (MK) or feminization. In some lepidopteran insects, MK is achieved by affecting the sex-determining process in males, and a unique mechanism of MK and its functional link with feminization have been implicated. However, comparative analysis of these phenotypes is often difficult because they have been analyzed in different host-symbiont systems, and transinfection of Wolbachia across different hosts is often challenging. In this study, we demonstrated the effects of nine Wolbachia strains on the splicing of sex-determining genes in Lepidoptera by fixing the host genetic background using a cell culture system. Cell transinfection assays confirmed that three MK-inducing Wolbachia strains and one feminization-inducing Wolbachia strain increased the female-type splicing products of the core sex-determining genes doublesex, masculinizer, and zinc finger protein 2. Regarding Wolbachia strains that do not induce MK/feminization, three had no effect on these sex-determining genes, whereas two strains induced female-type splicing of masculinizer and doublesex but not zinc finger protein 2. Comparative genomics confirmed that homologs of oscar, the Wolbachia gene responsible for MK in Ostrinia, were encoded by four MK/feminizing Wolbachia strains, but not by five non-MK/nonfeminizing strains. These results support the conserved effects underlying MK and feminization induced by oscar-bearing Wolbachia and suggested other potential mechanisms that Wolbachia might employ to manipulate host sex.

Ecological determinants of prevalence of the male-killing bacterium Arsenophonus nasoniae

Male-killing bacteria are found in a broad range of arthropods. Arsenophonus nasoniae is a male-killing bacterium, causing a 80% reduction of the male progeny in infected Nasonia vitripennis wasps. Although the discovery of A. nasoniae dates from the early 80's, knowledge about the biology and ecology of this endosymbiont is still scarce. One of these poorly studied features is the ecological factors underlying A. nasoniae incidence on its Nasonia spp. hosts in different geographical locations. Here, we studied the prevalence of A. nasoniae in Iberian wild populations of its host N. vitripennis. This wasp species is a common parasitoid of the blowfly Protocalliphora azurea pupae, which in turn is a parasite of hole-nesting birds, such as the blue tit (Cyanistes caeruleus). We also examined the effects of bird rearing conditions on the prevalence of A. nasoniae through a brood size manipulation experiment (creating enlarged, control and reduced broods). Both the wasp and bacterium presence were tested through PCR assays in blowfly pupae. We found A. nasoniae in almost half (47%) of nests containing blowflies parasitized by N. vitripennis. The prevalence of A. nasoniae was similar in the two geographical areas examined (central Portugal and southeastern Spain) and the probability of infection by A. nasoniae was independent of the number of blowfly pupae in the nest. Experimental manipulation of brood size did not affect the prevalence of A. nasoniae nor the prevalence of its host, N. vitripennis. These results suggest that the incidence of A. nasoniae in natural populations of N. vitripennis is high in the Iberian Peninsula, and the infestation frequency of nests by N. vitripennis carrying A. nasoniae is spatially stable in this geographical region independently of bird rearing conditions.

Two male-killing Wolbachia from Drosophila birauraia that are closely related but distinct in genome structure

Insects harbour diverse maternally inherited bacteria and viruses, some of which have evolved to kill the male progeny of their hosts (male killing: MK). The fly species Drosophila biauraria carries a maternally transmitted MK-inducing partiti-like virus, but it was unknown if it carries other MK-inducing endosymbionts. Here, we identified two male-killing Wolbachia strains (wBiau1 and wBiau2) from D. biauraria and compared their genomes to elucidate their evolutionary processes. The two strains were genetically closely related but had exceptionally different genome structures with considerable rearrangements compared with combinations of other Wolbachia strains. Despite substantial changes in the genome structure, the two Wolbachia strains did not experience gene losses that would disrupt the male-killing expression or persistence in the host population. The two Wolbachia-infected matrilines carried distinct mitochondrial haplotypes, suggesting that wBiau1 and wBiau2 have invaded D. biauraria independently and undergone considerable genome changes owing to unknown selective pressures in evolutionary history. This study demonstrated the presence of three male-killers from two distinct origins in one fly species and highlighted the diverse and rapid genome evolution of MK Wolbachia in the host.

Male-killing virus in a noctuid moth Spodoptera litura

A female-biased sex ratio is considered advantageous for the cytoplasmic elements that inhabit sexually reproducing organisms. There are numerous examples of bacterial symbionts in the arthropod cytoplasm that bias the host sex ratio toward females through various means, including feminization and male killing. Recently, maternally inherited RNA viruses belonging to the family Partitiviridae were found to cause male killing in moths and flies, but it was unknown whether male-killing viruses were restricted to Partitiviridae or could be found in other taxa. Here, we provide compelling evidence that a maternally inherited RNA virus, Spodoptera litura male-killing virus (SlMKV), selectively kills male embryos of the tobacco caterpillar Spodoptera litura, resulting in all-female broods. SlMKV injected into uninfected S. litura can also be inherited maternally and causes male killing. SlMKV has five genomic segments encoding seven open reading frames, has no homolog of known male-killing genes, and belongs to an unclassified group of arthropod-specific viruses closely related to Tolivirales. When transinfected into larvae, both male and female recipients allow SlMKV to proliferate, but only males die at the pupal stage. The viral RNA levels in embryonic and pupal male killing suggest that the mechanism of male killing involves the constitutive expression of viral products that are specifically lethal to males, rather than the male-specific expression of viral products. Our results, together with recent findings on male-killing partiti-like viruses, suggest that diverse viruses in arthropods tend to acquire male killing independently and that such viruses may be important components of intragenomic conflict in arthropods.

The son-killer microbe Arsenophonus nasoniae is a widespread associate of the parasitic wasp Nasonia vitripennis in Europe

Heritable microbes that exhibit reproductive parasitism are common in insects. One class of these are the male-killing bacteria, which are found in a broad range of insect hosts. Commonly, our knowledge of the incidence of these microbes is based on one or a few sampling sites, and the degree and causes of spatial variation are unclear. In this paper, we examine the incidence of the son-killer microbe Arsenophonus nasoniae across European populations of its wasp host, Nasonia vitripennis. In preliminary work, we noticed two female N. vitripennis producing highly female biased sex ratios in a field study from the Netherlands and Germany. When tested, the brood from Germany was revealed to be infected with A. nasoniae. We then completed a broad survey in 2012, in which fly pupal hosts of N. vitripennis were collected from vacated birds' nests from four European populations, N. vitripennis wasps allowed to emerge and then tested for A. nasoniae presence through PCR assay. We then developed a new screening methodology based on direct PCR assays of fly pupae and applied this to ethanol-preserved material collected from great tit (Parus major) nests in Portugal. These data show A. nasoniae is found widely in European N. vitripennis, being present in Germany, the UK, Finland, Switzerland and Portugal. Samples varied in the frequency with which they carry A. nasoniae, from being rare to being present in 50% of the pupae parasitised by N. vitripennis. Direct screening of ethanol-preserved fly pupae was an effective method for revealing both wasp and A. nasoniae infection, and will facilitate sample transport across national boundaries. Future research should examine the causes of variation in frequency, in particular testing the hypothesis that N. vitripennis superparasitism rates drive the variation in A. nasoniae frequency through providing opportunities for infectious transmission.

A male-killing gene encoded by a symbiotic virus of Drosophila

In most eukaryotes, biparentally inherited nuclear genomes and maternally inherited cytoplasmic genomes have different evolutionary interests. Strongly female-biased sex ratios that are repeatedly observed in various arthropods often result from the male-specific lethality (male-killing) induced by maternally inherited symbiotic bacteria such as Spiroplasma and Wolbachia. However, despite some plausible case reports wherein viruses are raised as male-killers, it is not well understood how viruses, having much smaller genomes than bacteria, are capable of inducing male-killing. Here we show that a maternally inherited double-stranded RNA (dsRNA) virus belonging to the family Partitiviridae (designated DbMKPV1) induces male-killing in Drosophila. DbMKPV1 localizes in the cytoplasm and possesses only four genes, i.e., one gene in each of the four genomic segments (dsRNA1-dsRNA4), in contrast to ca. 1000 or more genes possessed by Spiroplasma or Wolbachia. We also show that a protein (designated PVMKp1; 330 amino acids in size), encoded by a gene on the dsRNA4 segment, is necessary and sufficient for inducing male-killing. Our results imply that male-killing genes can be easily acquired by symbiotic viruses through reassortment and that symbiotic viruses are hidden players in arthropod evolution. We anticipate that host-manipulating genes possessed by symbiotic viruses can be utilized for controlling arthropods.

Coexistence of Two Male-Killers and Their Impact on the Development of Oriental Tea Tortrix Homona magnanima

Male-killing, the death of male offspring induced by maternally transmitted microbes, is classified as early, or late, male-killing. The primary advantage afforded by early male-killing, which typically occurs during embryogenesis, is the reallocation of resources to females, that would have otherwise been consumed by males. Meanwhile, the key advantage of late male-killing, which typically occurs during late larval development, is the maximized potential for horizontal transmission. To date, no studies have reported on the associated developmental and physiological effects of host coinfection with early and late male-killers, which may have a significant impact on the population dynamics of the male-killers. Here we used a lepidopteran tea pest Homona magnanima as a model, which is a unique system wherein an early male-killer (a Spiroplasma bacterium) and a late male-killer (an RNA virus) can coexist in nature. An artificially established matriline, coinfected with both Spiroplasma and RNA virus, exhibited embryonic death (early male-killing) as seen in the host line singly infected with Spiroplasma. Moreover, the coinfected line also exhibited developmental retardation and low pupal weight similar to the host line singly infected with the RNA virus. A series of field surveys revealed that Spiroplasma-RNA virus coinfection occurs in nature at a low frequency. Hence, although the two male-killers are capable of coexisting within the H. magnanima population independently, high associated fitness cost appears to limit the prevalence of male-killer coinfection in the field host population.

[Osugoroshi virus, a male-killer virus]

In insects, sex ratio bias is sometimes introduced by feminization, parthenogenesis, cytoplasmic incompatibility, or male-killing. Some intracellular bacteria such as Wolbachia or Spiroplasma has been known as male-killing agents. Here I introduce an example of non-bacterial male-killing agent, Osugoroshi virus found in oriental tea tortrix.

Microorganisms in the reproductive tissues of arthropods

Microorganisms that reside within or transmit through arthropod reproductive tissues have profound impacts on host reproduction, health and evolution. In this Review, we discuss select principles of the biology of microorganisms in arthropod reproductive tissues, including bacteria, viruses, protists and fungi. We review models of specific symbionts, routes of transmission, and the physiological and evolutionary outcomes for both hosts and microorganisms. We also identify areas in need of continuing research, to answer the fundamental questions that remain in fields within and beyond arthropod-microorganism associations. New opportunities for research in this area will drive a broader understanding of major concepts as well as the biodiversity, mechanisms and translational applications of microorganisms that interact with host reproductive tissues.

Wolbachia infection in Argentinean populations of Anastrepha fraterculus sp1: preliminary evidence of sex ratio distortion by one of two strains

Background

Wolbachia, one of the most abundant taxa of intracellular Alphaproteobacteria, is widespread among arthropods and filarial nematodes. The presence of these maternally inherited bacteria is associated with modifications of host fitness, including a variety of reproductive abnormalities, such as cytoplasmic incompatibility, thelytokous parthenogenesis, host feminization and male-killing. Wolbachia has attracted much interest for its role in biological, ecological and evolutionary processes as well as for its potential use in novel and environmentally-friendly strategies for the control of insect pests and disease vectors including a major agricultural pest, the South American fruit fly, Anastrepha fraterculus Wiedemann (Diptera: Tephritidae).

Results

We used wsp, 16S rRNA and a multilocus sequence typing (MLST) scheme including gatB, coxA, hcpA, fbpA, and ftsZ genes to detect and characterize the Wolbachia infection in laboratory strains and wild populations of A. fraterculus from Argentina. Wolbachia was found in all A. fraterculus individuals studied. Nucleotide sequences analysis of wsp gene allowed the identification of two Wolbachia nucleotide variants (named wAfraCast1_A and wAfraCast2_A). After the analysis of 76 individuals, a high prevalence of the wAfraCast2_A variant was found both, in laboratory (82%) and wild populations (95%). MLST analysis identified both Wolbachia genetic variants as sequence type 13. Phylogenetic analysis of concatenated MLST datasets clustered wAfraCast1/2_A in the supergroup A. Paired-crossing experiments among single infected laboratory strains showed a phenotype specifically associated to wAfraCast1_A that includes slight detrimental effects on larval survival, a female-biased sex ratio; suggesting the induction of male-killing phenomena, and a decreased proportion of females producing descendants that appears attributable to the lack of sperm in their spermathecae.

Conclusions

We detected and characterized at the molecular level two wsp gene sequence variants of Wolbachia both in laboratory and wild populations of A. fraterculus sp.1 from Argentina. Crossing experiments on singly-infected A. fraterculus strains showed evidence of a male killing-like mechanism potentially associated to the wAfraCast1_A - A. fraterculus interactions. Further mating experiments including antibiotic treatments and the analysis of early and late immature stages of descendants will contribute to our understanding of the phenotypes elicited by the Wolbachia variant wAfraCast1_A in A. fraterculus sp.1.

Life-History Traits of a Subtropical Cerambycid Beetle, Ropica honesta (Coleoptera: Cerambycidae)

Ropica honesta Pascoe is a small-sized cerambycid that has been recorded in tropical to subtropical Asia. In this study, life-history traits were examined for a local population collected from Iriomote Island (24.3°N, 123.8°E), Okinawa, Japan, by rearing insects on artificial diet as larval food. The egg period was 5.9 ± 0.3 d at 25°C. There was no significant difference in the duration of the larval, pupal, and adult preoviposition periods between long-day (14:10 [L:D]) and short-day (12:12 [L:D]) photoperiods at both 20 and 25°C. These periods at 25°C (14:10 [L:D]) were 28.5 ± 1.4, 8.4 ± 0.5, and 9.6 ± 1.9 d (mean ± SD), respectively. The relationship between the developmental rate and temperature followed the law of total effective temperature; thus, the developmental threshold temperature and thermal constant were estimated based on these data. Together with the finding that R. honesta may not have diapause in the egg stage, it is suggested that this beetle does not have diapause in the life cycle. Furthermore, when larvae were reared on natural food (dead twigs of hardwoods) adults emerged from the twigs 47.6 ± 2.9 d after oviposition, and this value was close to the total duration of the egg to pupal periods. Together with the data for annual temperature of the habitat and the fact that food resources for the species (dead twigs and leaves of hardwoods) are available throughout the year, we conclude that R. honesta develops and reproduces all year round, with five generations at maximum.

Genotype to phenotype: Diet-by-mitochondrial DNA haplotype interactions drive metabolic flexibility and organismal fitness

Diet may be modified seasonally or by biogeographic, demographic or cultural shifts. It can differentially influence mitochondrial bioenergetics, retrograde signalling to the nuclear genome, and anterograde signalling to mitochondria. All these interactions have the potential to alter the frequencies of mtDNA haplotypes (mitotypes) in nature and may impact human health. In a model laboratory system, we fed four diets varying in Protein: Carbohydrate (P:C) ratio (1:2, 1:4, 1:8 and 1:16 P:C) to four homoplasmic Drosophila melanogaster mitotypes (nuclear genome standardised) and assayed their frequency in population cages. When fed a high protein 1:2 P:C diet, the frequency of flies harbouring Alstonville mtDNA increased. In contrast, when fed the high carbohydrate 1:16 P:C food the incidence of flies harbouring Dahomey mtDNA increased. This result, driven by differences in larval development, was generalisable to the replacement of the laboratory diet with fruits having high and low P:C ratios, perturbation of the nuclear genome and changes to the microbiome. Structural modelling and cellular assays suggested a V161L mutation in the ND4 subunit of complex I of Dahomey mtDNA was mildly deleterious, reduced mitochondrial functions, increased oxidative stress and resulted in an increase in larval development time on the 1:2 P:C diet. The 1:16 P:C diet triggered a cascade of changes in both mitotypes. In Dahomey larvae, increased feeding fuelled increased β-oxidation and the partial bypass of the complex I mutation. Conversely, Alstonville larvae upregulated genes involved with oxidative phosphorylation, increased glycogen metabolism and they were more physically active. We hypothesise that the increased physical activity diverted energy from growth and cell division and thereby slowed development. These data further question the use of mtDNA as an assumed neutral marker in evolutionary and population genetic studies. Moreover, if humans respond similarly, we posit that individuals with specific mtDNA variations may differentially metabolise carbohydrates, which has implications for a variety of diseases including cardiovascular disease, obesity, and perhaps Parkinson's Disease.

A Tangled Web: Origins of Reproductive Parasitism

While typically a flea parasite and opportunistic human pathogen, the presence of Rickettsia felis (strain LSU-Lb) in the non-blood-feeding, parthenogenetically reproducing booklouse, Liposcelis bostrychophila, provides a system to ascertain factors governing not only host transitions but also obligate reproductive parasitism (RP). Analysis of plasmid pLbAR, unique to R. felis str. LSU-Lb, revealed a toxin–antitoxin module with similar features to prophage-encoded toxin–antitoxin modules utilized by parasitic Wolbachia strains to induce another form of RP, cytoplasmic incompatibility, in their arthropod hosts. Curiously, multiple deubiquitinase and nuclease domains of the large (3,841 aa) pLbAR toxin, as well the entire antitoxin, facilitated the detection of an assortment of related proteins from diverse intracellular bacteria, including other reproductive parasites. Our description of these remarkable components of the intracellular mobilome, including their presence in certain arthropod genomes, lends insight on the evolution of RP, while invigorating research on parasite-mediated biocontrol of arthropod-borne viral and bacterial pathogens.

Maternally transmitted non-bacterial male killer in Drosophila biauraria

A maternally inherited, all-female trait is widely found among arthropods, which is caused by bacterial endosymbionts such as Wolbachia, Rickettsia, Spiroplasma and Cardinium We discovered a single female of Drosophila biauraria, collected from Tomakomai, Hokkaido, Japan, that produced all-female offspring. This all-female trait was maternally inherited in the iso-female line (SP12F) by backcrossing with males of a normal line (SP11-20) with a 1 : 1 sex ratio derived from the same population. The all-female trait was not affected by tetracycline treatment performed for two consecutive generations. However, the microinjection of filter-sterilized homogenate of SP12F females into SP11-20 females established all-female matrilines. Our data suggest the role of transmissible agents, most likely viruses, but not bacteria or protists, as the possible cause of the all-female phenotype, which is likely to be achieved by killing of male embryos because egg hatch rates of SP12F were nearly half those of SP11-20. This is the first report in Diptera to demonstrate a maternally inherited virus-like element as the cause of the male-killing phenotype in D. biauraria.