Meiotic drivers are selfish genetic elements that manipulate the process of meiosis to increase their transmission, circumventing Mendel’s Law of equal segregation. Because meiosis fundamentally differs between the sexes, meiotic drivers use different mechanisms and strategies in male and female meiosis. Only one out of the four products of female meiosis can participate in fertilization, so female meiotic drivers attempt to alter chromosome segregation to ensure they end up in the fertilizing gamete. The mechanisms that female meiotic drivers use to alter segregation are not well understood. Here, we study the segregation distortion caused by a recently discovered selfish X chromosome that causes female meiotic drive in the woodland fly Drosophila testacea. We used controlled crosses to test whether one or two copies of this selfish X chromosome cause higher rates of nondisjunction, or chromosome segregation failure, in female flies. We found that females carrying a selfish X chromosome failed to transmit either of their X chromosomes at a significantly greater rate than wildtype females; this was even more pronounced in females with two selfish X chromosomes, creating a large number of sterile sons with an XO genotype. Interestingly and on the other hand, females carrying two selfish X chromosomes almost never transmitted both of their X chromosomes. These results suggest that in D. testacea females that are homozygous for the selfish X chromosome, X chromosomes interfere with each other over segregation. Our results show that female meiotic drive is associated with nondisjunction, resulting in negative fitness consequences, and provides insights into the transmission of this unusual selfish chromosome.

[This corrects the article DOI: 10.1371/journal.ppat.1006431.].

Some species of insects harbour strains of the endosymbiotic bacteria Wolbachia that do not cause obvious reproductive manipulations, and so it is unclear why they persist in host populations. There is some evidence that some of these endosymbionts may provide their hosts with protection against viruses, which would help to explain their persistence, but few studies have explored associations between Wolbachia and naturally occurring, common viruses in natural populations. Here, we asked whether individuals of the invasive vinegar fly Drosophila suzukii infected with the wSuz strain of Wolbachia were less likely to be infected by naturally occurring viruses in its invaded range, in western North America and in Europe. First, using next-generation sequencing, we conducted a virome survey of adult and larval D. suzukii in British Columbia, Canada, finding eight candidate RNA viruses and two candidate DNA viruses; all but one have not been reported previously. Only the previously described Teise virus, an RNA virus, was abundant in our virome survey. We then screened individual flies from British Columbia and Germany for Teise virus and Wolbachia. Wolbachia-infected D. suzukii from the field were not less likely to be infected by Teise virus. Overall, our results do not provide conclusive evidence that wSuz provides strong protection for D. suzukii against viruses that are common in natural populations. However, the other viruses that we discovered in this study deserve further characterization in terms of their pathogenicity to D. suzukii and the frequency and dynamics of infection in wild populations.

Comparative genomic analyses among closely related species provide an opportunity to assess their evolutionary history. The relatedness between species can depend on a variety of factors, including reproductive isolation, introgression, and incomplete lineage sorting, and this can impact divergence across the genome. Here, we use a combination of long- and short-read sequencing and HI-C scaffolding to assemble genomes for each of the four species in the testacea species group of Drosophila, including D. testacea, D. orientacea, D. neotestacea, and D. putrida, and its outgroup, D. bizonata. First, among species, we find many structural rearrangements across the genome as well as a large size difference in the dot chromosome that we infer is due to the expansion of repetitive elements. Second, we assess phylogenetic discordance and uncover a difference in the phylogeny inferred from genes on Muller E and the mitogenome relative to the rest of the genome, which may be due to recent hybridization. Lastly, we assess the rate of molecular evolution of genes shared across all species and identify genes evolving at different rates across the phylogeny. Our results present genomic resources for this species group and begin to probe into some of the evolutionary characteristics that contribute to variation in genome structure, while highlighting the need for high-quality genome resources to fully capture and understand the evolutionary history among closely related species.

Spalangia wasps have evolved a new association with Sodalis bacteria, transmitting them in an unusual way. They inject them into a host, along with their venom and an egg; the wasp larva then ingests them while feeding on host tissue.

Meiotic drivers are selfish genetic elements that bias their own transmission during meiosis or gamete formation. Due to the fundamental differences between male and female meiosis in animals and plants, meiotic drivers operate through distinct mechanisms in the two sexes: In females, they exploit the asymmetry of meiosis to ensure their inclusion in the egg, whereas in males, they eliminate competing gametes after symmetric meiosis. Meiotic drive is commonly reported in males, where it strongly influences the evolution of spermatogenesis, while the few known cases in females have highlighted its crucial role in centromere evolution. Despite a growing number of examples in a wide range of organisms, meiotic drive has so far only been observed in one sex or the other since its discovery nearly 100 y ago. Here, we show that a selfish X chromosome known to cause meiotic drive in male Drosophila testacea flies also causes meiotic drive in females. We find that this X chromosome has supergene architecture, harboring extensive structural rearrangements that suppress recombination between the two X chromosomes. This has contributed to a substantial expansion of its size compared to the wild-type chromosome, partly due to the accumulation of species-specific repetitive elements. Our findings suggest that female meiotic drive may play an important role in the evolutionary dynamics of polymorphic structural variants that suppress recombination, including inversions, translocations, and supergenes.

FLASH radiotherapy is an emerging technique in radiation oncology that may improve clinical outcomes by reducing normal tissue toxicities. The physical radiation characteristics needed to induce the radiobiological benefits of FLASH are still an active area of investigation. To determine the dose rate, range of doses and delivery time structure necessary to trigger the FLASH effect, Drosophila melanogaster were exposed to ultrahigh dose rate (UHDR) or conventional radiotherapy dose rate (CONV) 120-kVp X-rays. A conventional X-ray tube outfitted with a shutter system was used to deliver 17- to 44-Gy doses to third-instar D. melanogaster larvae at both UHDR (210 Gy/s) and CONV (0.2-0.4 Gy/s) dose rates. The larvae were then tracked through development to adulthood and scored for eclosion and lifespan. Larvae exposed to UHDR eclosed at higher rates and had longer median survival as adults compared to those treated with CONV at the same doses. Eclosion rates at 24 Gy were 68% higher for the UHDR group (P < 0.05). Median survival from 22 Gy was >22 days for UHDR and 17 days for CONV (P < 0.01). Two normal tissue-sparing effects were observed for D. melanogaster irradiated with UHDR 120-kVp X-rays. The effects appeared only at intermediate doses and may be useful in establishing the dose range over which the benefits of FLASH can be obtained. This work also demonstrates the usefulness of a high-throughput fruit fly model and a low-cost X-ray tube system for radiobiological FLASH research.

Wolbachia bacteria are maternally inherited symbionts that commonly infect terrestrial arthropods. Many Wolbachia reach high frequencies in their hosts by manipulating their reproduction, for example by causing reproductive incompatibilities between infected male and uninfected female hosts. However, not all strains manipulate reproduction, and a key unresolved question is how these non-manipulative Wolbachia persist in their hosts, often at intermediate to high frequencies. One such strain, wSuz, infects the invasive fruit pest Drosophila suzukii, spotted-wing drosophila. Here, we tested the hypothesis that wSuz infection provides a competitive benefit when resources are limited. Over the course of one season, we established population cages with varying amounts of food in a semi-field setting and seeded them with a 50:50 mixture of flies with and without Wolbachia. We predicted that Wolbachia-infected individuals should have higher survival and faster development than their uninfected counterparts when there was little available food. We found that while food availability strongly impacted fly fitness, there was no difference in development times or survival between Wolbachia-infected and uninfected flies. Interestingly, however, Wolbachia infection frequencies changed dramatically, with infections either increasing or decreasing by as much as 30% in a single generation, suggesting the possibility of unidentified factors shaping Wolbachia infection over the course of the season.

wHow genes attributed to KEGG genes.

Identified genes with secretion capabilities of wHow.

Wolbachia symbionts are the most successful host-associated microbes on the planet, infecting arthropods and nematodes. Their role in nematodes is particularly enigmatic, with filarial nematode species either 100% infected and dependent on symbionts for reproduction and development, or not at all infected. We have discovered a highly divergent strain of Wolbachia in an insect-parasitic tylenchid nematode, Howardula sp., in a nematode clade that has not previously been known to harbour Wolbachia. While this nematode is 100% infected with Wolbachia, we did not detect it in related species. We sequenced the Howardula symbiont (wHow) genome and found that it is highly reduced, comprising only 550 kilobase pairs of DNA, approximately 35% smaller than the smallest Wolbachia nematode symbiont genomes. The wHow genome is a subset of all other Wolbachia genomes and has not acquired any new genetic information. While it has lost many genes, including genes involved in cell wall synthesis and cell division, it has retained the entire haem biosynthesis pathway, suggesting that haem supplementation is critical. wHow provides key insights into our understanding of what are the lower limits of Wolbachia cells, as well as the role of Wolbachia symbionts in the biology and convergent evolution of diverse parasitic nematodes.

Accession numbers of novel sequences and previously published sequences used for analysis.

Cytoplasmic incompatibility (CI) is a form of reproductive manipulation caused by maternally inherited endosymbionts infecting arthropods, like Wolbachia, whereby matings between infected males and uninfected females produce few or no offspring. We report the discovery of a new CI symbiont, a strain of Spiroplasma causing CI in the parasitoid wasp Lariophagus distinguendus. Its extracellular occurrence enabled us to establish CI in uninfected adult insects by transferring Spiroplasma-infected hemolymph. We sequenced the CI-Spiroplasma genome and did not find any homologues of any of the cif genes discovered to cause CI in Wolbachia, suggesting independent evolution of CI. Instead, the genome contains other potential CI-causing candidate genes, such as homologues of high-mobility group (HMG) box proteins that are crucial in eukaryotic development but rare in bacterial genomes. Spiroplasma's extracellular nature and broad host range encompassing medically and agriculturally important arthropods make it a promising tool to study CI and its applications.

Obligate symbioses involving intracellular bacteria have transformed eukaryotic life, from providing aerobic respiration and photosynthesis to enabling colonization of previously inaccessible niches, such as feeding on xylem and phloem, and surviving in deep-sea hydrothermal vents. A major challenge in the study of obligate symbioses is to understand how they arise. Because the best studied obligate symbioses are ancient, it is especially challenging to identify early or intermediate stages. Here we report the discovery of a nascent obligate symbiosis in Howardula aoronymphium, a well-studied nematode parasite of Drosophila flies. We have found that Haoronymphium and its sister species harbor a maternally inherited intracellular bacterial symbiont. We never find the symbiont in nematode-free flies, and virtually all nematodes in the field and the laboratory are infected. Treating nematodes with antibiotics causes a severe reduction in fly infection success. The association is recent, as more distantly related insect-parasitic tylenchid nematodes do not host these endosymbionts. We also report that the Howardula nematode symbiont is a member of a widespread monophyletic group of invertebrate host-associated microbes that has independently given rise to at least four obligate symbioses, one in nematodes and three in insects, and that is sister to Pectobacterium, a lineage of plant pathogenic bacteria. Comparative genomic analysis of this group, which we name Candidatus Symbiopectobacterium, shows signatures of genome erosion characteristic of early stages of symbiosis, with the Howardula symbiont's genome containing over a thousand predicted pseudogenes, comprising a third of its genome.

We report the presence of two Asian species of larval parasitoids of spotted wing Drosophila, Drosophila suzukii (Matsumura) (Diptera: Drosophilidae), in northwestern North America. Leptopilina japonica Novkovic & Kimura and Ganaspis brasiliensis (Ihering) (Hymenoptera: Figitidae) were found foraging near and emerging from fruits infested by D. suzukii at several locations across coastal British Columbia, Canada in the summer and fall of 2019. While G. brasiliensis was found in British Columbia for the first time in 2019, re-inspection of previously collected specimens suggests that L. japonica has been present since at least 2016. Additionally, we found a species of Asobara associated with D. suzukii in British Columbia that is possibly Asobara rufescens (Förster) (known only from the Palearctic Region) based on COI DNA barcode data. These findings add to the list of cases documenting adventive establishment of candidate classical biological control agents outside of their native ranges. The findings also illustrate the need for revisiting species concepts within Asobara , as well as host and geographic distribution data due to cryptic and/or misidentified species.

All organisms have evolved a wide range of strategies to protect against natural enemies, from complex avoidance behaviours to sophisticated immune systems, and insects are no exception. A more recently recognized, and likely widespread, mode of protection for insects is through the use of microbial symbionts. Many of the best studied examples of defensive symbiosis in insects involve infections with maternally inherited symbionts. While inherited symbionts can protect their insect hosts by priming innate immune systems, or competing with enemies for scarce recourses, they have also been found to synthesize a wide diversity of toxins that effectively kill, weaken, or deter parasitic wasps, parasitic nematodes, pathogenic fungi, and predatory spiders. In this chapter, we provide an overview of defensive symbioses in insects, focusing in particular on toxin-based protection.

Driving X chromosomes (X D s) bias their own transmission through males by killing Y‐bearing gametes. These chromosomes can in theory spread rapidly in populations and cause extinction, but many are found as balanced polymorphisms or as “cryptic” X D s shut down by drive suppressors. The relative likelihood of these outcomes and the evolutionary pathways through which they come about are not well understood. An X D was recently discovered in the mycophagous fly, Drosophila testacea , presenting the opportunity to compare this X D with the well‐studied X D of its sister species, Drosophila neotestacea . Comparing features of independently evolved X D s in young sister species is a promising avenue towards understanding how X D s and their counteracting forces change over time. In contrast to the X D of D. neotestacea , we find that the X D of D. testacea is old, with its origin predating the radiation of three species: D. testacea , D. neotestacea and their shared sister species, Drosophila orientacea . Motivated by the suggestion that older X D s should be more deleterious to carriers, we assessed the effect of the X D on both male and female fertility. Unlike what is known from D. neotestacea , we found a strong fitness cost in females homozygous for the X D in D. testacea : a large proportion of homozygous females failed to produce offspring after being housed with males for several days. Our male fertility experiments show that although X D male fertility is lower under sperm‐depleting conditions, X D males have comparable fertility to males carrying a standard X chromosome under a free‐mating regime, which may better approximate conditions in wild populations of D. testacea . Lastly, we demonstrate the presence of autosomal suppression of X chromosome drive. Our results provide support for a model of X D evolution where the dynamics of young X D s are governed by fitness consequences in males, whereas in older X D systems, both suppression and fitness consequences in females likely supersede male fitness costs.