Misdirection of dosage compensation underlies bidirectional sex-specific death in Wolbachia-infected Ostrinia scapulalis

Zygosity-based sex determination in a butterfly drives hypervariability of Masculinizer

Nature has devised many ways of producing males and females. Here, we report on a previously undescribed mechanism for Lepidoptera that functions without a female-specific gene. The number of alleles or allele heterozygosity in a single Z-linked gene (BaMasc) is the primary sex-determining switch in Bicyclus anynana butterflies. Embryos carrying a single BaMasc allele develop into WZ (or Z0) females, those carrying two distinct alleles develop into ZZ males, while (ZZ) homozygotes initiate female development, have mismatched dosage compensation, and die as embryos. Consequently, selection against homozygotes has favored the evolution of spectacular allelic diversity: 205 different coding sequences of BaMasc were detected in a sample of 246 females. The structural similarity of a hypervariable region (HVR) in BaMasc to the HVR in Apis mellifera csd suggests molecular convergence between deeply diverged insect lineages. Our discovery of this primary switch highlights the fascinating diversity of sex-determining mechanisms and underlying evolutionary drivers.

Developmental Shifts in the Microbiome of a Cosmopolitan Pest: Unraveling the Role of Wolbachia and Dominant Bacteria

Wolbachia bacteria (phylum Proteobacteria) are ubiquitous intracellular parasites of diverse invertebrates. In insects, coevolution has forged mutualistic associations with Wolbachia species, influencing reproduction, immunity, development, pathogen resistance, and overall fitness. However, the impact of Wolbachia on other microbial associates within the insect microbiome, which are crucial for host fitness, remains less explored. The diamondback moth (Plutella xylostella), a major pest of cruciferous vegetables worldwide, harbors the dominant Wolbachia strain plutWB1, known to distort its sex ratio. This study investigated the bacterial community diversity and dynamics across different developmental life stages and Wolbachia infection states in P. xylostella using high-throughput 16S rDNA amplicon sequencing. Proteobacteria and Firmicutes dominated the P. xylostella microbiome regardless of life stage or Wolbachia infection. However, the relative abundance of dominant genera, including an unclassified genus of Enterobacteriaceae, Wolbachia, Carnobacterium, and Delftia tsuruhatensis, displayed significant stage-specific variations. While significant differences in bacterial diversity and composition were observed across life stages, Wolbachia infection had no substantial impact on overall diversity. Nonetheless, relative abundances of specific genera differed between infection states. Notably, Wolbachia exhibited a stable, high relative abundance across all stages and negatively correlated with an unclassified genus of Enterobacteriaceae, Delftia tsuruhatensis, and Carnobacterium. Our findings provide a foundational understanding of the complex interplay between the host, Wolbachia, and the associated microbiome in P. xylostella, paving the way for a deeper understanding of their complex interactions and potential implications for pest control strategies.

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.

Two Complete Genomes of Male-Killing Wolbachia Infecting Ostrinia Moth Species Illuminate Their Evolutionary Dynamics and Association with Hosts

Wolbachia is an extremely widespread intracellular symbiont which causes reproductive manipulation on various arthropod hosts. Male progenies are killed in Wolbachia-infected lineages of the Japanese Ostrinia moth population. While the mechanism of male killing and the evolutionary interaction between host and symbiont are significant concerns for this system, the absence of Wolbachia genomic information has limited approaches to these issues. We determined the complete genome sequences of wFur and wSca, the male-killing Wolbachia of Ostrinia furnacalis and Ostrinia scapulalis. The two genomes shared an extremely high degree of homology, with over 95% of the predicted protein sequences being identical. A comparison of these two genomes revealed nearly minimal genome evolution, with a strong emphasis on the frequent genome rearrangements and the rapid evolution of ankyrin repeat-containing proteins. Additionally, we determined the mitochondrial genomes of both species' infected lineages and performed phylogenetic analyses to deduce the evolutionary dynamics of Wolbachia infection in the Ostrinia clade. According to the inferred phylogenetic relationship, two possible scenarios were proposed: (1) Wolbachia infection was established in the Ostrinia clade prior to the speciation of related species such as O. furnacalis and O. scapulalis, or (2) Wolbachia infection in these species was introgressively transferred from a currently unidentified relative. Simultaneously, the relatively high homology of mitochondrial genomes suggested recent Wolbachia introgression between infected Ostrinia species. The findings of this study collectively shed light on the host-symbiont interaction from an evolutionary standpoint.

Cell-based analysis reveals that sex-determining gene signals in Ostrinia are pivotally changed by male-killing Wolbachia

Wolbachia, a maternally transmitted bacterium, shows male-killing, an adaptive phenotype for cytoplasmic elements, in various arthropod species during the early developmental stages. In lepidopteran insects, lethality of males is accounted for by improper dosage compensation in sex-linked genes owing to Wolbachia-induced feminization. Herein, we established Ostrinia scapulalis cell lines that retained sex specificity per the splicing pattern of the sex-determining gene doublesex (Osdsx). We found that Wolbachia transinfection in male cell lines enhanced the female-specific splice variant of Osdsx (Osdsx^F ) while suppressing the male-specific variant (Osdsx^M ), indicating that Wolbachia affects sex-determining gene signals even in vitro. Comparative transcriptome analysis isolated only two genes that behave differently upon Wolbachia infection. The two genes were respectively homologous to Masculinizer (BmMasc) and zinc finger-2 (Bmznf-2), male-specifically expressed sex-determining genes of the silkworm Bombyx mori that encode CCCH-type zinc finger motif proteins. By using cultured cells and organismal samples, OsMasc and Osznf-2 were found to be sex-determining genes of O. scapulalis that are subjected to sex-specific alternative splicing depending upon the chromosomal sex, developmental stage, and infection status. Overall, our findings expound the cellular autonomy in insect sex determination and the mechanism through which sex is manipulated by intracellular selfish microbes.

Natural and Engineered Sex Ratio Distortion in Insects

Insects have evolved highly diverse genetic sex-determination mechanisms and a relatively balanced male to female sex ratio is generally expected. However, selection may shift the optimal sex ratio while meiotic drive and endosymbiont manipulation can result in sex ratio distortion (SRD). Recent advances in sex chromosome genomics and CRISPR/Cas9-mediated genome editing brought significant insights into the molecular regulators of sex determination in an increasing number of insects and provided new ways to engineer SRD. We review these advances and discuss both naturally occurring and engineered SRD in the context of the Anthropocene. We emphasize SRD-mediated biological control of insects to help improve One Health, sustain agriculture, and conserve endangered species.

Sex determination systems as the interface between male-killing bacteria and their hosts

Arthropods host a range of sex-ratio-distorting selfish elements, including diverse maternally inherited endosymbionts that solely kill infected males. Male-killing heritable microbes are common, reach high frequency, but until recently have been poorly understood in terms of the host–microbe interaction. Additionally, while male killing should generate strong selection for host resistance, evidence of this has been scant. The interface of the microbe with host sex determination is integral to the understanding of how death is sex limited and how hosts can evolve evasion of male killing. We first review current knowledge of the mechanisms diverse endosymbionts use to induce male-specific death. We then examine recent evidence that these agents do produce intense selection for host nuclear suppressor elements. We argue, from our understanding of male-killing mechanisms, that suppression will commonly involve evolution of the host sex determination pathways and that the host's response to male-killing microbes thus represents an unrecognized driver of the diversity of arthropod sex determination. Further work is required to identify the genes and mechanisms responsible for male-killing suppression, which will both determine the components of sex determination (or other) systems associated with suppressor evolution, and allow insight into the mechanism of male killing itself.

Mutation of P-element somatic inhibitor induces male sterility in the diamondback moth, Plutella xylostella

BACKGROUND

Genetic manipulation of sex determination pathways in insects provides the basis for a broad range of strategies to benefit agricultural security and human health. The P-element somatic inhibitor (PSI) protein, an exon splicing silencer that promotes male-specific splicing of dsx, plays a critical role in male sexual differentiation and development. The functions of PSI have been characterized in the lepidopteran model species Bombyx mori. However, the molecular mechanism and functions of PSI in Plutella xylostella, a worldwide agricultural pest and taxonomically basal species, are still unknown.

RESULTS

Here we identified PxPSI transcripts and analyzed their spatiotemporal expression pattern in P. xylostella. Multiple sequence alignment revealed that PxPSI contains four KH domains and is highly conserved in lepidopterans. We used the CRISPR-Cas9 system to generate mutations of the PxPSI genomic locus. Disruptions of PxPSI caused male-specific defects in internal and external genitals. In addition, we detected female-specific Pxdsx transcripts in PxPSI male mutants. Mutations also caused changes in expression of several sex-biased genes and induced male sterility.

CONCLUSION

Our study demonstrates that PxPSI plays a key role in male sex determination in P. xylostella and suggests a potential molecular target for genetic-based pest management in lepidopteran pests. © 2021 Society of Chemical Industry.

Bacterial Symbionts of Tsetse Flies: Relationships and Functional Interactions Between Tsetse Flies and Their Symbionts

Tsetse flies (Glossina spp.) act as the sole vectors of the African trypanosome species that cause Human African Trypanosomiasis (HAT or African Sleeping Sickness) and Nagana in animals. These flies have undergone a variety of specializations during their evolution including an exclusive diet consisting solely of vertebrate blood for both sexes as well as an obligate viviparous reproductive biology. Alongside these adaptations, Glossina species have developed intricate relationships with specific microbes ranging from mutualistic to parasitic. These relationships provide fundamental support required to sustain the specializations associated with tsetse's biology. This chapter provides an overview on the knowledge to date regarding the biology behind these relationships and focuses primarily on four bacterial species that are consistently associated with Glossina species. Here their interactions with the host are reviewed at the morphological, biochemical and genetic levels. This includes: the obligate symbiont Wigglesworthia, which is found in all tsetse species and is essential for nutritional supplementation to the blood-specific diet, immune system maturation and facilitation of viviparous reproduction; the commensal symbiont Sodalis, which is a frequently associated symbiont optimized for survival within the fly via nutritional adaptation, vertical transmission through mating and may alter vectorial capacity of Glossina for trypanosomes; the parasitic symbiont Wolbachia, which can manipulate Glossina via cytoplasmic incompatibility and shows unique interactions at the genetic level via horizontal transmission of its genetic material into the genome in two Glossina species; finally, knowledge on recently observed relations between Spiroplasma and Glossina is explored and potential interactions are discussed based on knowledge of interactions between this bacterial Genera and other insect species. These flies have a simple microbiome relative to that of other insects. However, these relationships are deep, well-studied and provide a window into the complexity and function of host/symbiont interactions in an important disease vector.

Wolbachia increase germ cell mitosis to enhance the fecundity of Laodelphax striatellus

Wolbachia are intracellular bacteria that infect a wide range of invertebrates and have evolved various strategies to alter host reproduction for their own survival and dissemination. In small brown planthopper Laodelphax striatellus, Wolbachia-infected females lay more eggs than uninfected females. Our previous study has shown that Wolbachia are abundant in ovarian cells of L. striatellus and change the number of apoptotic nurse cells in a caspase-dependent manner to provide nutrition for oogenesis. The cellular and molecular bases of the Wolbachia-mediated alterations in L. striatellus oogenesis remain largely unknown. Here, we investigated whether germ cell mitosis, which has been implicated in determination of egg production rates, influences the interaction between fecundity and Wolbachia in L.striatellus. We used an anti-phospho-histone 3 (pH3) antibody to label and visualize mitotic cells. Microscopic observations indicated that the Wolbachia strain wStri increased the number of ovarioles that contained mitotic germ cells. The increased fecundity of Wolbachia-infected females was a result of mitosis of germ cells; the frequency of germ cell mitosis was much higher in infected females than in uninfected females. In addition, mitosis inhibition by Cdc20, CDK1, and CycB messenger RNA interference in Wolbachia-infected L. striatellus markedly decreased egg numbers. Live Wolbachia recolonization enhanced the egg production of uninfected L. striatellus by directly affecting mitosis regulators. Together, these data suggest that wStri might increase germ cell mitosis to enhance the fecundity of L. striatellus in a mitosis-regulating manner. Our findings establish a link between Wolbachia-induced mitosis and Wolbachia-mediated egg production effects.

Impact of Wolbachia infection on Drosophila female germline stem cells

Wolbachia pipientis, one of the most dominant insect-symbiotic bacteria, highjacks the female germline of insects for its own propagation across host generations. Such strict dependence on female gametes in trans-generational propagation has driven Wolbachia to devise ingenious strategies to enhance female fertility. In Drosophila melanogaster females with female-sterile mutant alleles of the master sex-determining gene Sex-lethal (Sxl), Wolbachia colonizing female germline stem cells (GSCs) support the maintenance of GSCs, thereby rescuing the defective ovarian development. In the germ cell cytoplasm, Wolbachia are often found in proximity to ribonucleoprotein-complex processing bodies (P bodies), where the Wolbachia-derived protein TomO interacts with RNAs encoding Nanos and Orb proteins, which support the GSC maintenance and oocyte polarization, respectively. Thus, manipulation of host RNA is the key to successful vertical transmission of Wolbachia.

Common and unique strategies of male killing evolved in two distinct Drosophila symbionts

Male killing is a selfish reproductive manipulation caused by symbiotic bacteria, where male offspring of infected hosts are selectively killed. The underlying mechanisms and the process of their evolution are of great interest not only in terms of fundamental biology, but also their potential applications. The two bacterial Drosophila symbionts, Wolbachia and Spiroplasma, have independently evolved male-killing ability. This raises the question whether the underlying mechanisms share some similarities or are specific to each bacterial species. Here, we analyse pathogenic phenotypes of D. bifasciata infected with its natural male-killing Wolbachia strain and compare them with those of D. melanogaster infected with male-killing Spiroplasma We show that male progeny infected with the Wolbachia strain die during embryogenesis with abnormal apoptosis. Interestingly, male-killing Wolbachia infection induces DNA damage and segregation defects in the dosage-compensated chromosome in male embryos, which are reminiscent of the phenotypes caused by male-killing Spiroplasma in D. melanogaster By contrast, host neural development seems to proceed normally unlike male-killing Spiroplasma infection. Our results demonstrate that the dosage-compensated chromosome is a common target of two distinct male killers, yet Spiroplasma uniquely evolved the ability to damage neural tissue of male embryos.

Evolution of Sex Chromosome Dosage Compensation in Animals: A Beautiful Theory, Undermined by Facts and Bedeviled by Details

Many animals with genetic sex determination harbor heteromorphic sex chromosomes, where the heterogametic sex has half the gene dose of the homogametic sex. This imbalance, if reflected in the abundance of transcripts or proteins, has the potential to deleteriously disrupt interactions between X-linked and autosomal loci in the heterogametic sex. Classical theory predicts that molecular mechanisms will evolve to provide dosage compensation that recovers expression levels comparable to ancestral expression prior to sex chromosome divergence. Such dosage compensating mechanisms may also, secondarily, result in balanced sex-linked gene expression between males and females. However, numerous recent studies addressing sex chromosome dosage compensation (SCDC) in a diversity of animals have yielded a surprising array of patterns concerning dosage compensation in the heterogametic sex, as well as dosage balance between sexes. These results substantially contradict longstanding theory, catalyzing both novel perspectives and new approaches in dosage compensation research. In this review, we summarize the theory, analytical approaches, and recent results concerning evolutionary patterns of SCDC in animals. We also discuss methodological challenges and discrepancies encountered in this research, which often underlie conflicting results. Finally, we discuss what outstanding questions and opportunities exist for future research on SCDC.

Uncovering the hidden players in Lepidoptera biology: the heritable microbial endosymbionts

The Lepidoptera is one of the most widespread and recognisable insect orders. Due to their remarkable diversity, economic and ecological importance, moths and butterflies have been studied extensively over the last 200 years. More recently, the relationship between Lepidoptera and their heritable microbial endosymbionts has received increasing attention. Heritable endosymbionts reside within the host’s body and are often, but not exclusively, inherited through the female line. Advancements in molecular genetics have revealed that host-associated microbes are both extremely prevalent among arthropods and highly diverse. Furthermore, heritable endosymbionts have been repeatedly demonstrated to play an integral role in many aspects of host biology, particularly host reproduction. Here, we review the major findings of research of heritable microbial endosymbionts of butterflies and moths. We promote the Lepidoptera as important models in the study of reproductive manipulations employed by heritable endosymbionts, with the mechanisms underlying male-killing and feminisation currently being elucidated in moths and butterflies. We also reveal that the vast majority of research undertaken of Lepidopteran endosymbionts concerns Wolbachia. While this highly prevalent bacterium is undoubtedly important, studies should move towards investigating the presence of other, and interacting endosymbionts, and we discuss the merits of examining the microbiome of Lepidoptera to this end. We finally consider the importance of understanding the influence of endosymbionts under global environmental change and when planning conservation management of endangered Lepidoptera species.

Feminizing Wolbachia endosymbiont disrupts maternal sex chromosome inheritance in a butterfly species

Wolbachia is a maternally inherited ubiquitous endosymbiotic bacterium of arthropods that displays a diverse repertoire of host reproductive manipulations. For the first time, we demonstrate that Wolbachia manipulates sex chromosome inheritance in a sexually reproducing insect. Eurema mandarina butterfly females on Tanegashima Island, Japan, are infected with the wFem Wolbachia strain and produce all‐female offspring, while antibiotic treatment results in male offspring. Fluorescence in situ hybridization (FISH) revealed that wFem‐positive and wFem‐negative females have Z0 and WZ sex chromosome sets, respectively, demonstrating the predicted absence of the W chromosome in wFem‐infected lineages. Genomic quantitative polymerase chain reaction (qPCR) analysis showed that wFem‐positive females lay only Z0 eggs that carry a paternal Z, whereas females from lineages that are naturally wFem‐negative lay both WZ and ZZ eggs. In contrast, antibiotic treatment of adult wFem females resulted in the production of Z0 and ZZ eggs, suggesting that this Wolbachia strain can disrupt the maternal inheritance of Z chromosomes. Moreover, most male offspring produced by antibiotic‐treated wFem females had a ZZ karyotype, implying reduced survival of Z0 individuals in the absence of feminizing effects of Wolbachia. Antibiotic treatment of wFem‐infected larvae induced male‐specific splicing of the doublesex (dsx) gene transcript, causing an intersex phenotype. Thus, the absence of the female‐determining W chromosome in Z0 individuals is functionally compensated by Wolbachia‐mediated conversion of sex determination. We discuss how Wolbachia may manipulate the host chromosome inheritance and that Wolbachia may have acquired this coordinated dual mode of reproductive manipulation first by the evolution of female‐determining function and then cytoplasmically induced disruption of sex chromosome inheritance.