An optimized method for mulberry silkworm, Bombyx mori (Bombycidae:Lepidoptera) sex classification using TLBPSGA-RFEXGBoost

Silkworm seed production is vital for silk farming, requiring precise breeding techniques to optimize yields. In silkworm seed production, precise sex classification is crucial for optimizing breeding and boosting silk yields. A non-destructive approach for sex classification addresses these challenges, offering an efficient alternative that enhances both yield and environmental responsibility. Southern India is a hub for mulberry silk and cocoon farming, with the high-yielding double-hybrid varieties FC1 (foundation cross 1) and FC2 (foundation cross 2) being popular. Traditional methods of silkworm pupae sex classification involve manual sorting by experts, necessitating the cutting of cocoons - a practice with a high risk of damaging the cocoon and affecting yield. To address this issue, this study introduces an accelerated histogram of oriented gradients (HOG) feature extraction technique that is enhanced by block-level dimensionality reduction. This non-destructive method allows for efficient and accurate silkworm pupae classification. The modified HOG features are then fused with weight features and processed through a machine learning classification model that incorporates recursive feature elimination (RFE). Performance evaluation shows that an RFE-hybridized XGBoost model attained the highest classification accuracy, achieving 97.2% for FC1 and 97.1% for FC2. The model further optimized with a novel teaching learning-based population selection genetic algorithm (TLBPSGA) achieved a remarkable accuracy of 98.5% for FC1 and 98.2% for FC2. These findings have far-reaching implications for improving both the ecological sustainability and economic efficiency of silkworm seed production.

Genomic evidence reveals three W-autosome fusions in Heliconius butterflies

Sex chromosomes are evolutionarily labile in many animals and sometimes fuse with autosomes, creating so-called neo-sex chromosomes. Fusions between sex chromosomes and autosomes have been proposed to reduce sexual conflict and to promote adaptation and reproductive isolation among species. Recently, advances in genomics have fuelled the discovery of such fusions across the tree of life. Here, we discovered multiple fusions leading to neo-sex chromosomes in the sapho subclade of the classical adaptive radiation of Heliconius butterflies. Heliconius butterflies generally have 21 chromosomes with very high synteny. However, the five Heliconius species in the sapho subclade show large variation in chromosome number ranging from 21 to 60. We find that the W chromosome is fused with chromosome 4 in all of them. Two sister species pairs show subsequent fusions between the W and chromosomes 9 or 14, respectively. These fusions between autosomes and sex chromosomes make Heliconius butterflies an ideal system for studying the role of neo-sex chromosomes in adaptive radiations and the degeneration of sex chromosomes over time. Our findings emphasize the capability of short-read resequencing to detect genomic signatures of fusion events between sex chromosomes and autosomes even when sex chromosomes are not explicitly assembled.

Multiple independent origins of the female W chromosome in moths and butterflies

Lepidoptera, the most diverse group of insects, exhibit female heterogamy (Z0 or ZW), which is different from most other insects (male heterogamy, XY). Previous studies suggest a single origin of the Z chromosome. However, the origin of the lepidopteran W chromosome remains poorly understood. Here, we assemble the genome from females down to the chromosome level of a model insect (Bombyx mori) and identify a W chromosome of approximately 10.1 megabase using a newly developed tool. In addition, we identify 3593 genes that were not previously annotated in the genomes of B. mori. Comparisons of 21 lepidopteran species (including 17 ZW and four Z0 systems) and three trichopteran species (Z0 system) reveal that the formation of Ditrysia W involves multiple mechanisms, including previously proposed canonical and noncanonical models, as well as a newly proposed mechanism called single-Z turnover. We conclude that there are multiple independent origins of the W chromosome in the Ditrysia (most moths and all butterflies) of Lepidoptera.

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.

Not all apparently gynandromorphic butterflies are gynandrous: The case of Polyommatus icarus and its relatives (Lepidoptera: Lycaenidae)

Beside the more than two thousand normal specimens of Polyommatus icarus (Rottemburg, 1775) yielded by rearing experiments, there was one perfectly bilateral dichromatic individual first considered to be gynandrous. On the basis of analysing genitalia traits, wing surface covering scale micromorphology, and the spectral characteristics of the blue colour generated by the cover scales, the gender of the specimen has been identified as female. This exemplar was investigated in comparison with gynandrous specimens from the collections of the Hungarian Natural History Museum exhibiting various degrees of intermixing of blue and brown coloration. Focus stacking microscopy for detailed scale morphology and UV-visible reflectance spectroscopy was used for the characterization of the optical properties. Inspecting literature references and the Lycaenidae collection of the museum, further examples have been found for female bilateral dichromatism in the closely related polyommatine lycaenid species Lysandra bellargus (Rottemburg, 1775) and Lysandra coridon (Poda, 1761) what suggests that polyommatine female dichromaticity may be displayed by the manner of bilaterality and mosaicism, phenomena hitherto solely connected to gynandromorphy.

Role of small nucleolar RNAs in alternative splicing of the doublesex gene in the silkworm, Bombyx mori

More and more evidence shows that small noncoding RNAs (ncRNAs) play diverse roles in development, stress response and other cellular processes, but functional study of intermediate-size ncRNAs is still rare. Here, the expression profile of 16 intermediate-size ncRNAs in ovary and testis of silkworm Bombyx mori were analyzed. Twelve ncRNAs, including 5 small nucleolar RNAs (snoRNAs) and 7 unclassified ncRNAs, accumulated more in the testis than in the ovary of silkworm, especially Bm-163, Bm-51 and Bm-68. Four ncRNAs (including three orphan snoRNAs and one unclassified ncRNA) had higher expression level in the ovary than in the testis, especially Bm-86. Overexpression of the testis-enriched snoRNA Bm-68 in the female led to the accumulation of male-specific isoform of doublesex (BmdsxM) and increased the expression ratio of BmdsxM: BmdsxF. While overexpression of ovary-enriched snoRNA Bm-86 in the male decreased the expression ratio of BmdsxM: BmdsxF, indicating the roles of the two snoRNAs played in the alternative splicing of Bmdsx of silkworm, which will provide new clues for the functional study of snoRNAs in insects.

Heatwaves are detrimental to fertility in the viviparous tsetse fly

Heatwaves are increasing in frequency and intensity due to climate change, pushing animals beyond physiological limits. While most studies focus on survival limits, sublethal effects on fertility tend to occur below lethal thresholds, and consequently can be as important for population viability. Typically, male fertility is more heat-sensitive than female fertility, yet direct comparisons are limited. Here, we measured the effect of experimental heatwaves on tsetse flies, Glossina pallidipes, disease vectors and unusual live-bearing insects of sub-Saharan Africa. We exposed males or females to a 3-day heatwave peaking at 36, 38 or 40°C for 2 h, and a 25°C control, monitoring mortality and reproduction over six weeks. For a heatwave peaking at 40°C, mortality was 100%, while a 38°C peak resulted in only 8% acute mortality. Females exposed to the 38°C heatwave experienced a one-week delay in producing offspring, whereas no such delay occurred in males. Over six weeks, heatwaves resulted in equivalent fertility loss in both sexes. Combined with mortality, this lead to a 10% population decline over six weeks compared to the control. Furthermore, parental heatwave exposure gave rise to a female-biased offspring sex ratio. Ultimately, thermal limits of both survival and fertility should be considered when assessing climate change vulnerability.

The Sex Ratio Indicates the Conclusion and Onset of Population Cycles in the Beet Webworm Loxostege sticticalis L. (Lepidoptera: Pyralidae)

Beet webworms, Loxostege sticticalis L. (Lepidoptera: Pyralidae), are one of the most destructive pest insects in northern China, and their populations outbreak periodically. Developing an indicator that defines the ending and beginning of the occurrence period cycle is urgent for the population forecast and theoretical study. The sex ratio can be a primary pathway through which species regulate population size. We measured the maximum mating potential of both females and males and the population net reproductive rate under different sex ratios (e.g., 3:1, 2:1, 1:1, 1:2, 1:3). The maximum mating frequency of males was 2.91 times that of females. The progeny contribution per mating decreased with increased mating times in males. The variation in population net reproductive rate affected by the sex ratio fits the parabolic curve analysis and peaked at 1.82 for females vs. males. Our results illustrate the quantitative connection phenomenon shown by the historical data: population outbreaks occur at a sex ratio of two or more and collapse at a sex rate lower than one. Simultaneously, the sex ratio may be utilized as a definite indicator for the beginning and end of the future occurrence cycle in the beet webworm.

Dichotomous sperm in Lepidopteran insects: a biorational target for pest management

Lepidoptera are unusual in possessing two distinct kinds of sperm, regular nucleated (eupyrene) sperm and anucleate (apyrene) sperm ('parasperm'). Sperm of both types are transferred to the female and are required for male fertility. Apyrene sperm play 'helper' roles, assisting eupyrene sperm to gain access to unfertilized eggs and influencing the reproductive behavior of mated female moths. Sperm development and behavior are promising targets for environmentally safer, target-specific biorational control strategies in lepidopteran pest insects. Sperm dimorphism provides a wide window in which to manipulate sperm functionality and dynamics, thereby impairing the reproductive fitness of pest species. Opportunities to interfere with spermatozoa are available not only while sperm are still in the male (before copulation), but also in the female (after copulation, when sperm are still in the male-provided spermatophore, or during storage in the female's spermatheca). Biomolecular technologies like RNAi, miRNAs and CRISPR-Cas9 are promising strategies to achieve lepidopteran pest control by targeting genes directly or indirectly involved in dichotomous sperm production, function, or persistence.

Sex separation by body color via a W-chromosome-linked transgene

Sex separation processes are important for commercial insect production and sterile insect techniques. Here, we describe the transgenic insertion of a DsRed expression cassette driven by the enhancer HR3 and strong promoter IE1 into the silkworm W chromosome as a dominant visible marker of sex separation. The obtained transgenic lines showed female-specific body color visible to the naked eye at the second- to fifth-instar larval, pupal and adult stages, and their performance traits were comparable to those of a nontransgenic practical silkworm variety. This strategy can greatly facilitate the sex separation of silkworms for male-only rearing and to obtain hybrids while avoiding sibling mating, and it can also be applied to the sex separation of other light-colored insects.

High-density linkage maps and chromosome level genome assemblies unveil direction and frequency of extensive structural rearrangements in wood white butterflies (Leptidea spp.)

Karyotypes are generally conserved between closely related species and large chromosome rearrangements typically have negative fitness consequences in heterozygotes, potentially driving speciation. In the order Lepidoptera, most investigated species have the ancestral karyotype and gene synteny is often conserved across deep divergence, although examples of extensive genome reshuffling have recently been demonstrated. The genus Leptidea has an unusual level of chromosome variation and rearranged sex chromosomes, but the extent of restructuring across the rest of the genome is so far unknown. To explore the genomes of the wood white (Leptidea) species complex, we generated eight genome assemblies using a combination of 10X linked reads and HiC data, and improved them using linkage maps for two populations of the common wood white (L. sinapis) with distinct karyotypes. Synteny analysis revealed an extensive amount of rearrangements, both compared to the ancestral karyotype and between the Leptidea species, where only one of the three Z chromosomes was conserved across all comparisons. Most restructuring was explained by fissions and fusions, while translocations appear relatively rare. We further detected several examples of segregating rearrangement polymorphisms supporting a highly dynamic genome evolution in this clade. Fusion breakpoints were enriched for LINEs and LTR elements, which suggests that ectopic recombination might be an important driver in the formation of new chromosomes. Our results show that chromosome count alone may conceal the extent of genome restructuring and we propose that the amount of genome evolution in Lepidoptera might still be underestimated due to lack of taxonomic sampling.

Accumulation of retrotransposons contributes to W chromosome differentiation in the willow beauty Peribatodes rhomboidaria (Lepidoptera: Geometridae)

The W chromosome of Lepidoptera is typically gene-poor, repeat-rich and composed of heterochromatin. Pioneering studies investigating this chromosome reported an abundance of mobile elements. However, the actual composition of the W chromosome varies greatly between species, as repeatedly demonstrated by comparative genomic hybridization (CGH) or fluorescence in situ hybridization (FISH). Here we present an analysis of repeats on the W chromosome in the willow beauty, Peribatodes rhomboidaria (Geometridae), a species in which CGH predicted an abundance of W-enriched or W-specific sequences. Indeed, comparative analysis of male and female genomes using RepeatExplorer identified ten putative W chromosome-enriched repeats, most of which are LTR or LINE mobile elements. We analysed the two most abundant: PRW LINE-like and PRW Bel-Pao. The results of FISH mapping and bioinformatic analysis confirmed their enrichment on the W chromosome, supporting the hypothesis that mobile elements are the driving force of W chromosome differentiation in Lepidoptera. As the W chromosome is highly underrepresented in chromosome-level genome assemblies of Lepidoptera, this recently introduced approach, combining bioinformatic comparative genome analysis with molecular cytogenetics, provides an elegant tool for studying this elusive and rapidly evolving part of the genome.

Descriptive versus causal morphology: gynandromorphism and intersexuality

In animal species with separate sexes, abnormal individuals with a mix of phenotypically male and phenotypically female body parts are generally indicated as gynandromorphs, whereas individuals with intermediate sexual phenotypic traits are generally indicated as intersexes. However, this distinction, clear as it may seem, is neither universally agreed upon, nor free of critical issues. In consideration of the role of sex anomalies in understanding normal development, we reassess these phenomena of abnormal sexual development, taking into consideration the more recent advances in the study of sex determination and sexual differentiation. We argue that a distinction between gynandromorphism and intersexuality, although useful for descriptive purposes, is not always possible or sensible. We discuss the conceptual and terminological intricacies of the literature on this subject and provide reasons for largely, although not strictly, preferring a terminology based on descriptive rather than causal morphology, that is, on the observed phenotypic patterns rather on the causal process behind them.

A history of the genetic and molecular identification of genes and their functions controlling insect sex determination

The genetics of the sex determination regulatory cascade in Drosophila melanogaster has a fascinating history, interlinked with the foundation of the Genetics discipline itself. The discovery that alternative splicing rather than differential transcription is the molecular mechanism underlying the upstream control of sex differences in the Drosophila model system was surprising. This notion is now fully integrated into the scientific canon, appearing in many genetics textbooks and online education resources. In the last three decades, it was a key reference point for starting evolutionary studies in other insect species by using homology-based approaches. This review will introduce a very brief history of Drosophila genetics. It will describe the genetic and molecular approaches applied for the identifying and cloning key genes involved in sex determination in Drosophila and in many other insect species. These comparative analyses led to supporting the idea that sex-determining pathways have evolved mainly by recruiting different upstream signals/genes while maintaining widely conserved intermediate and downstream regulatory genes. The review also provides examples of the link between technological advances and research achievements, to stimulate reflections on how science is produced. It aims to hopefully strengthen the related historical and conceptual knowledge of general readers of other disciplines and of younger geneticists, often focused on the latest technical-molecular approaches.

Identification of potential sex determination genes and functional analyses in Neoseiulus californicus under prey stress

BACKGROUND

Phytoseiid mites are important natural enemies of spider mites. Sex-determination mechanism are important basic scientific issues in the reproduction and evolution of predatory mites. Clarifying sex-determination mechanism may provide reference for exploring genetic approach to have the phytoseiid mites produce more female offspring, which could improve their effectiveness as a biological control agent.

RESULTS

We used transcriptome sequencing to identify and characterize 20 putative sex-determination genes in the phytoseiid mite Neoseiulus californicus, a species with uncommon pseudo-arrhenotoky, including doublesex-like (dsx1-like), transformer-2 (tra-2), intersex (ix), and fruitless-like (BTB2). A significant negative correlation was found between prey stress and offspring sex ratio. But the most genes identified showed no difference in expression between the groups with lowest and highest female offspring ratios. The hatching rate and sex ratio of female offspring were reduced when the ix gene was silenced, and the oviposition days and fecundity were reduced when the BTB2 gene was silenced. The fecundity was reduced when the tra2 gene was silenced and the snf gene is essential for oviposition in female. There was no effect on reproduction and female sex determination when silencing the dsx1-like and dsx2-like gene.

CONCLUSION

The genes BTB2, tra2 and snf are involved in oviposition, and ix may be involved in female sex determination and egg formation in Neoseiulus californicus. The results are conductive to further understanding molecular regulatory mechanism of sex determination in predatory mites and may provide a reference for better use of this predatory by producing more females. © 2022 Society of Chemical Industry.

A step forward in the genome characterization of the sugarcane borer, Diatraea saccharalis: karyotype analysis, sex chromosome system and repetitive DNAs through a cytogenomic approach

Moths of the family Crambidae include a number of pests that cause economic losses to agricultural crops. Despite their economic importance, little is known about their genome architecture and chromosome evolution. Here, we characterized the chromosomes and repetitive DNA of the sugarcane borer Diatraea saccharalis using a combination of low-pass genome sequencing, bioinformatics, and cytogenetic methods, focusing on the sex chromosomes. Diploid chromosome numbers differed between the sexes, i.e., 2n = 33 in females and 2n = 34 in males. This difference was caused by the occurrence of a WZ₁Z₂ trivalent in female meiosis, indicating a multiple sex-chromosome system WZ₁Z₂/Z₁Z₁Z₂Z₂. A strong interstitial telomeric signal was observed on the W chromosome, indicating a fusion of the ancestral W chromosome with an autosome. Among repetitive DNAs, transposable elements (TEs) accounted for 39.18% (males) to 41.35% (females), while satDNAs accounted for only 0.214% (males) and 0.215% (females) of the genome. FISH mapping revealed different chromosomal organization of satDNAs, such as single localized clusters, spread repeats, and non-clustered repeats. Two TEs mapped by FISH were scattered. Although we found a slight enrichment of some satDNAs in the female genome, they were not differentially enriched on the W chromosome. However, we found enriched FISH signals for TEs on the W chromosome, suggesting their involvement in W chromosome degeneration and differentiation. These data shed light on karyotype and repetitive DNA dynamics due to multiple chromosome fusions in D. saccharalis, contribute to the understanding of genome structure in Lepidoptera and are important for future genomic studies.

Hoisted with his own petard: How sex-ratio meiotic drive in Drosophila affinis creates resistance alleles that limit its spread

Meiotic drivers are selfish genetic elements that tinker with gametogenesis to bias their own transmission into the next generation of offspring. Such tinkering can have significant consequences on gametogenesis and end up hampering the spread of the driver. In Drosophila affinis, sex-ratio meiotic drive is caused by an X-linked complex that, when in males with a susceptible Y chromosome, results in broods that are typically more than 95% female. Interestingly, D. affinis males lacking a Y chromosome (XO) are fertile and males with the meiotic drive X and no Y produce only sons-effectively reversing the sex-ratio effect. Here, we show that meiotic drive dramatically increases the rate of nondisjunction of the Y chromosome (at least 750X), meaning that the driver is creating resistant alleles through the process of driving. We then model how the O might influence the spread, dynamics and equilibrium of the sex-ratio X chromosome. We find that the O can prevent the spread or reduce the equilibrium frequency of the sex-ratio X chromosome, and it can even lead to oscillations in frequency. Finally, with reasonable parameters, the O is unlikely to lead to the loss of the Y chromosome, but we discuss how it might lead to sex-chromosome turnover indirectly.

Masculinizer and Doublesex as Key Factors Regulate Sexual Dimorphism in Ostrinia furnacalis

Simple Summary

In animals, sexually dimorphic traits are ubiquitous and play vital roles in reproduction, courtship, and environmental adaptation, especially in insects. In this study, we used the CRISPR/Cas9 genome editing system to generate somatic mutations of the Masculinizer (Masc) and doublesex (dsx) genes in the sex determination pathway of Ostrinia furnacalis. The OfMasc and Ofdsx genes are structural orthologs of the key sex regulation factors in Bombyx mori. Mutation of the OfMasc and Ofdsx genes induced abnormal external genitalia, adult sterility, and sex reversal of sexually dimorphic traits including wing pigmentation, gene expression patterns, and dsx sex-specific splicing. These results demonstrate that the Masc and dsx genes are conserved factors in sexually dimorphic traits, and therefore represent potential target genes in the effort to control O. furnacalis and other lepidopteran pests.

Abstract

Sex determination is an important and traditional biological process. In Lepidoptera, Masculinizer (Masc) and doublesex (dsx) are the essential genes for sex determination and play critical roles in sexual differentiation and development. The functions of Masc and dsx have been characterized in several model insect species. However, the molecular mechanism and sex determination functions of Masc and dsx in Ostrinia furnacalis, an agricultural pest, are still unknown. Here, we successfully used the CRISPR/Cas9 genome editing system to knock out OfMasc and Ofdsx. Mutation of OfMasc induced male external genital defects and sterility. Disruptions of the Ofdsx common region caused sex-specific defects in the external genitals and adult sterility. In addition, we found that OfMasc and Ofdsx can regulate the pigmentation genes that control wing pigmentation patterns. These results demonstrate that OfMasc and Ofdsx play key roles in the sex determination of O. furnacalis, and suggest novel genetic control approaches for the management of pests, including O. furnacalis.

Mapping and CRISPR homology-directed repair of a recessive white eye mutation in Plodia moths

The pantry moth Plodia interpunctella is a worldwide pest of stored food products and a promising laboratory model system for lepidopteran functional genomics. Here we describe efficient methods for precise genome editing in this insect. A spontaneous recessive white-eyed phenotype maps to a frameshift deletion (c.737delC) in the white gene. CRISPR NHEJ mutagenesis of white replicates this phenotype with high rates of somatic biallelic knockout. G₀ individuals with mutant clones on both eyes produced 100% mutant progeny, making white an ideal marker for co-conversion when targeting other genes. CRISPR HDR experiments corrected c.737delC and reverted white eyes to a pigmented state in 37% of G₀ mosaic adults. These repaired alleles showed practical rates of germline transmission in backcrosses, demonstrating the potential of the technique for precise genome editing. Plodia offers a promising avenue for research in this taxon because of its lab-ready features, egg injectability, and editability.

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Plodia pantry moths are an emerging model organism for functional genomics in Lepidoptera

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Spontaneous and CRISPR-induced white mutations yield recessive-white eye phenotypes

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CRISPR HDR repair with ssODN donor result in practical rates of base editing

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We provide optimized protocols for Plodia handling and genome editing

Manipulating Insect Sex Determination Pathways for Genetic Pest Management: Opportunities and Challenges

Sex determination pathways in insects are generally characterised by an upstream primary signal, which is highly variable across species, and that regulates the splicing of a suite of downstream but highly-conserved genes (transformer, doublesex and fruitless). In turn, these downstream genes then regulate the expression of sex-specific characteristics in males and females. Identification of sex determination pathways has and continues to be, a critical component of insect population suppression technologies. For example, "first-generation" transgenic technologies such as fsRIDL (Female-Specific Release of Insects carrying Dominant Lethals) enabled efficient selective removal of females from a target population as a significant improvement on the sterile insect technique (SIT). Second-generation technologies such as CRISPR/Cas9 homing gene drives and precision-guided SIT (pgSIT) have used gene editing technologies to manipulate sex determination genes in vivo. The development of future, third-generation control technologies, such as Y-linked drives, (female to male) sex-reversal, or X-shredding, will require additional knowledge of aspects of sexual development, including a deeper understanding of the nature of primary signals and dosage compensation. This review shows how knowledge of sex determination in target pest species is fundamental to all phases of the development of control technologies.

A CCCH zinc finger gene regulates doublesex alternative splicing and male development in Bombyx mori

Recent identification of a Piwi-interacting RNA (piRNA)-initiated sex determination cascade in the silkworm, Bombyx mori, provides novel insights into high diversity of insect sex determination pathways. In this system, the W-chromosome-derived Fem piRNA is the primary sex determination signal. A CCCH-type zinc finger gene Masculinizer (Masc), which is targeted by Fem piRNA-PIWI complex in female animals, is indispensable for male-specific splicing of B. mori doublesex (Bmdsx). Although many genes involved in this cascade have been identified, the regulatory mechanisms of silkworm sex determination remain to be elucidated. Here we show that another CCCH-type zinc finger gene, Bmznf-2, is a masculinization factor in B. mori. Bmznf-2 shows testis-abundant expression and loss of Bmznf-2 function via clustered regularly interspaced short palindromic repeats / single-guide RNA-mediated mutagenesis results in feminized differentiation and appearance of the female-specific splicing variants of Bmdsx transcripts in males. In contrast, there is no phenotypic consequence in mutant females. In mutant males, relative messenger RNA expression levels of female-dominant genes such as vitellogenin and sex-specific storage protein 1 are significantly elevated while several male-dominant genes are significantly down-regulated. Furthermore, male mutants show delayed developmental timing, smaller body sizes of larvae and malformation of moth wings. Our data thus reveal that Bmznf-2 plays an indispensable role in silkworm male sexual differentiation.

Degenerated, Undifferentiated, Rearranged, Lost: High Variability of Sex Chromosomes in Geometridae (Lepidoptera) Identified by Sex Chromatin

Sex chromatin is a conspicuous body that occurs in polyploid nuclei of most lepidopteran females and consists of numerous copies of the W sex chromosome. It is also a cytogenetic tool used to rapidly assess the W chromosome presence in Lepidoptera. However, certain chromosomal features could disrupt the formation of sex chromatin and lead to the false conclusion that the W chromosome is absent in the respective species. Here we tested the sex chromatin presence in 50 species of Geometridae. In eight selected species with either missing, atypical, or normal sex chromatin patterns, we performed a detailed karyotype analysis by means of comparative genomic hybridization (CGH) and fluorescence in situ hybridization (FISH). The results showed a high diversity of W chromosomes and clarified the reasons for atypical sex chromatin, including the absence or poor differentiation of W, rearrangements leading to the neo-W emergence, possible association with the nucleolus, and the existence of multiple W chromosomes. In two species, we detected intraspecific variability in the sex chromatin status and sex chromosome constitution. We show that the sex chromatin is not a sufficient marker of the W chromosome presence, but it may be an excellent tool to pinpoint species with atypical sex chromosomes.

Evidence for multiple colonisations and Wolbachia infections shaping the genetic structure of the widespread butterfly Polyommatus icarus in the British Isles

The paradigm of isolation in southern refugia during glacial periods followed by expansions during interglacials, producing limited genetic differentiation in northern areas, dominates European phylogeography. However, the existence of complex structured populations in formerly glaciated areas, and islands connected to mainland areas during glacial maxima, call for alternative explanations. We reconstructed the mtDNA phylogeography of the widespread Polyommatus Icarus butterfly with an emphasis on the formerly glaciated and connected British Isles. We found distinct geographical structuring of CO1 haplogroups, with an ancient lineage restricted to the marginal European areas, including Northern Scotland and Outer Hebrides. Population genomic analyses, using ddRADSeq genomic markers, also reveal substantial genetic structuring within Britain. However, there is negligble mito-nuclear concordance consistent with independent demographic histories of mitochondrial versus nuclear DNA. While mtDNA-Wolbachia associations in northern Britain could account for the geographic structuring of mtDNA across most of the British Isles, for nuclear DNA markers (derived from ddRADseq data) butterflies from France cluster between northern and southern British populations - an observation consistent with a scenario of multiple recolonisation. Taken together our results suggest that contemporary mtDNA structuring in the British Isles (and potentially elsewhere in Europe) largely results from Wolbachia infections, however, nuclear genomic structuring suggests a history of at least two distinct colonisations. This two-stage colonisation scenario has previously been put forth to explain genetic diversity and structuring in other British flora and fauna. Additionally, we also present preliminary evidence for potential Wolbachia-induced feminization in the Outer Hebrides.

Identification and Characterization of the Masculinizing Function of the Helicoverpa armigera Masc Gene

The Masculinizer (Masc) gene has been known to control sex development and dosage compensation in lepidopterans. However, it remains unclear whether its ortholog exists and plays the same roles in distantly related lepidopterans such as Helicoverpa armigera. To address this question, we cloned Masc from H. armigera (HaMasc), which contains all essential functional domains of BmMasc, albeit with less than 30% amino acid sequence identity with BmMasc. Genomic PCR and qPCR analyses showed that HaMasc is a Z chromosome-linked gene since its genomic content in males (ZZ) was two times greater than that in females (ZW). RT-PCR and RT-qPCR analyses revealed that HaMasc expression was sex- and stage-biased, with significantly more transcripts in males and eggs than in females and other stages. Transfection of a mixture of three siRNAs of HaMasc into a male embryonic cell line of H. armigera led to the appearance of female-specific mRNA splicing isoforms of H. armigeradoublesex (Hadsx), a downstream target gene of HaMasc in the H. armigera sex determination pathway. The knockdown of HaMasc, starting from the third instar larvae resulted in a shift of Hadsx splicing from male to female isoforms, smaller male pupa and testes, fewer but larger/longer spermatocytes and sperm bundles, delayed pupation and internal fusion of the testes and follicles. These data demonstrate that HaMasc functions as a masculinizing gene in the H. armigera sex-determination cascade.

A conserved role of the duplicated Masculinizer gene in sex determination of the Mediterranean flour moth, Ephestia kuehniella

Sex determination in the silkworm, Bombyx mori, is based on Feminizer (Fem), a W-linked Fem piRNA that triggers female development in WZ individuals, and the Z-linked Masculinizer (Masc), which initiates male development and dosage compensation in ZZ individuals. While Fem piRNA is missing in a close relative of B. mori, Masc determines sex in several representatives of distant lepidopteran lineages. We studied the molecular mechanisms of sex determination in the Mediterranean flour moth, Ephestia kuehniella (Pyralidae). We identified an E. kuehniella Masc ortholog, EkMasc, and its paralog resulting from a recent duplication, EkMascB. Both genes are located on the Z chromosome and encode a similar Masc protein that contains two conserved domains but has lost the conserved double zinc finger domain. We developed PCR-based genetic sexing and demonstrated a peak in the expression of EkMasc and EkMascB genes only in early male embryos. Simultaneous knock-down experiments of both EkMasc and EkMascB using RNAi during early embryogenesis led to a shift from male- to female-specific splicing of the E. kuehniella doublesex gene (Ekdsx), their downstream effector, in ZZ embryos and resulted in a strong female-biased sex-ratio. Our results thus confirmed the conserved role of EkMasc and/or EkMascB in masculinization. We suggest that the C-terminal proline-rich domain, we have identified in all functionally confirmed Masc proteins, in conjunction with the masculinizing domain, is important for transcriptional regulation of sex determination in Lepidoptera. The function of the Masc double zinc finger domain is still unknown, but appears to have been lost in E. kuehniella.

The sex-determining cascade in the silkworm, Bombyx mori, differs greatly from those of other insects. In B. mori, female development is initiated by Fem piRNA expressed from the W chromosome during early embryogenesis. Fem piRNA silences Masculinizer (Masc) thereby blocking the male pathway resulting in female development. It is currently unknown whether this cascade is conserved across Lepidoptera. In the Mediterranean flour moth, Ephestia kuehniella, we identified an ortholog of Masc and discovered its functional duplication on the Z chromosome, which has not yet been found in any other lepidopteran species. We provide two lines of evidence that the EkMasc and/or EkMascB genes play an essential role in masculinization: (i) they show a peak of expression during early embryogenesis in ZZ but not in WZ embryos and (ii) their simultaneous silencing by RNAi results in female-specific splicing of the E. kuehniella doublesex gene (Ekdsx) in ZZ embryos and in a female-biased sex ratio. Our results suggest a conserved role of the duplicated Masc gene in sex determination of E. kuehniella.

Genome mapping coupled with CRISPR gene editing reveals a P450 gene confers avermectin resistance in the beet armyworm

The evolution of insecticide resistance represents a global constraint to agricultural production. Because of the extreme genetic diversity found in insects and the large numbers of genes involved in insecticide detoxification, better tools are needed to quickly identify and validate the involvement of putative resistance genes for improved monitoring, management, and countering of field-evolved insecticide resistance. The avermectins, emamectin benzoate (EB) and abamectin are relatively new pesticides with reduced environmental risk that target a wide number of insect pests, including the beet armyworm, Spodoptera exigua, an important global pest of many crops. Unfortunately, field resistance to avermectins recently evolved in the beet armyworm, threatening the sustainable use of this class of insecticides. Here, we report a high-quality chromosome-level assembly of the beet armyworm genome and use bulked segregant analysis (BSA) to identify the locus of avermectin resistance, which mapped on 15-16 Mbp of chromosome 17. Knockout of the CYP9A186 gene that maps within this region by CRISPR/Cas9 gene editing fully restored EB susceptibility, implicating this gene in avermectin resistance. Heterologous expression and in vitro functional assays further confirm that a natural substitution (F116V) found in the substrate recognition site 1 (SRS1) of the CYP9A186 protein results in enhanced metabolism of EB and abamectin. Hence, the combined approach of coupling gene editing with BSA allows for the rapid identification of metabolic resistance genes responsible for insecticide resistance, which is critical for effective monitoring and adaptive management of insecticide resistance.

The Dryas iulia Genome Supports Multiple Gains of a W Chromosome from a B Chromosome in Butterflies

In butterflies and moths, which exhibit highly variable sex determination mechanisms, the homogametic Z chromosome is deeply conserved and is featured in many genome assemblies. The evolution and origin of the female W sex chromosome, however, remains mostly unknown. Previous studies have proposed that a ZZ/Z0 sex determination system is ancestral to Lepidoptera, and that W chromosomes may originate from sex-linked B chromosomes. Here, we sequence and assemble the female Dryas iulia genome into 32 highly contiguous ordered and oriented chromosomes, including the Z and W sex chromosomes. We then use sex-specific Hi-C, ATAC-seq, PRO-seq, and whole-genome DNA sequence data sets to test if features of the D. iulia W chromosome are consistent with a hypothesized B chromosome origin. We show that the putative W chromosome displays female-associated DNA sequence, gene expression, and chromatin accessibility to confirm the sex-linked function of the W sequence. In contrast with expectations from studies of homologous sex chromosomes, highly repetitive DNA content on the W chromosome, the sole presence of domesticated repetitive elements in functional DNA, and lack of sequence homology with the Z chromosome or autosomes is most consistent with a B chromosome origin for the W, although it remains challenging to rule out extensive sequence divergence. Synteny analysis of the D. iulia W chromosome with other female lepidopteran genome assemblies shows no homology between W chromosomes and suggests multiple, independent origins of the W chromosome from a B chromosome likely occurred in butterflies.

Hybrid effects in field populations of the African monarch butterfly, Danaus chrysippus (L.) (Lepidoptera: Nymphalidae)

Heterosis, Haldane and Bateson-Dobzhansky-Muller effects have been widely documented amongst a range of plants and animals. However, typically these effects are shown by taking parents of known genotype into the laboratory and measuring components of the F1 progeny under laboratory conditions. This leaves in doubt the real significance of such effects in the field. Here we use the well-known colour pattern genotypes of the African monarch or queen (Danaus chrysippus), which also control wing length, to test these effects both in the laboratory and in a contact zone in the field. By measuring the wing lengths in animals of known colour pattern genotype we show clear evidence for all three hybrid effects at the A and BC colour patterning loci, and importantly, that these same effects persist in the same presumptive F1s when measured in hybrid populations in the field. This demonstrates the power of a system in which genotypes can be directly inferred in the field and highlights that all three hybrid effects can be seen in the East African contact zone of this fascinating butterfly.

A de novo transcriptional atlas in Danaus plexippus reveals variability in dosage compensation across tissues

A detailed knowledge of gene function in the monarch butterfly is still lacking. Here we generate a genome assembly from a Mexican nonmigratory population and used RNA-seq data from 14 biological samples for gene annotation and to construct an atlas portraying the breadth of gene expression during most of the monarch life cycle. Two thirds of the genes show expression changes, with long noncoding RNAs being particularly finely regulated during adulthood, and male-biased expression being four times more common than female-biased. The two portions of the monarch heterochromosome Z, one ancestral to the Lepidoptera and the other resulting from a chromosomal fusion, display distinct association with sex-biased expression, reflecting sample-dependent incompleteness or absence of dosage compensation in the ancestral but not the novel portion of the Z. This study presents extended genomic and transcriptomic resources that will facilitate a better understanding of the monarch's adaptation to a changing environment.

Draft nuclear genome and complete mitogenome of the Mediterranean corn borer, Sesamia nonagrioides, a major pest of maize

The Mediterranean corn borer (Sesamia nonagrioides, Noctuidae, Lepidoptera) is a major pest of maize in Europe and Africa. Here, we report an assembly of the nuclear and mitochondrial genome of a pool of inbred males and females third-instar larvae, based on short- and long-read sequencing. The complete mitochondrial genome is 15,330 bp and contains all expected 13 and 24 protein-coding and RNA genes, respectively. The nuclear assembly is 1021 Mb, composed of 2553 scaffolds and it has an N50 of 1105 kb. It is more than twice larger than that of all Noctuidae species sequenced to date, mainly due to a higher repeat content. A total of 17,230 protein-coding genes were predicted, including 15,776 with InterPro domains. We provide detailed annotation of genes involved in sex determination (doublesex, insulin-like growth factor 2 mRNA-binding protein, and P-element somatic inhibitor) and of alpha-amylase genes possibly involved in interaction with parasitoid wasps. We found no evidence of recent horizontal transfer of bracovirus genes from parasitoid wasps. These genome assemblies provide a solid molecular basis to study insect genome evolution and to further develop biocontrol strategies against S. nonagrioides.

Comparative studies on speciation: 30 years since Coyne and Orr

Understanding the processes of population divergence and speciation remains a core question in evolutionary biology. For nearly a hundred years evolutionary geneticists have characterized reproductive isolation (RI) mechanisms and specific barriers to gene flow required for species formation. The seminal work of Coyne and Orr provided the first comprehensive comparative analysis of speciation. By combining phylogenetic hypotheses and species range data with estimates of genetic divergence and multiple mechanisms of RI across Drosophila, Coyne and Orr's influential meta-analyses answered fundamental questions and motivated new analyses that continue to push the field forward today. Now 30 years later, we revisit the five questions addressed by Coyne and Orr, identifying results that remain well supported and others that seem less robust with new data. We then consider the future of speciation research, with emphasis on areas where novel methods and data motivate potential progress. While the literature remains biased towards Drosophila and other model systems, we are enthusiastic about the future of the field.

The Role of Satellite DNAs in Genome Architecture and Sex Chromosome Evolution in Crambidae Moths

Tandem repeats are important parts of eukaryotic genomes being crucial e.g., for centromere and telomere function and chromatin modulation. In Lepidoptera, knowledge of tandem repeats is very limited despite the growing number of sequenced genomes. Here we introduce seven new satellite DNAs (satDNAs), which more than doubles the number of currently known lepidopteran satDNAs. The satDNAs were identified in genomes of three species of Crambidae moths, namely Ostrinia nubilalis, Cydalima perspectalis, and Diatraea postlineella, using graph-based computational pipeline RepeatExplorer. These repeats varied in their abundance and showed high variability within and between species, although some degree of conservation was noted. The satDNAs showed a scattered distribution, often on both autosomes and sex chromosomes, with the exception of both satellites in D. postlineella, in which the satDNAs were located at a single autosomal locus. Three satDNAs were abundant on the W chromosomes of O. nubilalis and C. perspectalis, thus contributing to their differentiation from the Z chromosomes. To provide background for the in situ localization of the satDNAs, we performed a detailed cytogenetic analysis of the karyotypes of all three species. This comparative analysis revealed differences in chromosome number, number and location of rDNA clusters, and molecular differentiation of sex chromosomes.

The Sex Determination Cascade in the Silkworm

In insects, sex determination pathways involve three levels of master regulators: primary signals, which determine the sex; executors, which control sex-specific differentiation of tissues and organs; and transducers, which link the primary signals to the executors. The primary signals differ widely among insect species. In Diptera alone, several unrelated primary sex determiners have been identified. However, the doublesex (dsx) gene is highly conserved as the executor component across multiple insect orders. The transducer level shows an intermediate level of conservation. In many, but not all examined insects, a key transducer role is performed by transformer (tra), which controls sex-specific splicing of dsx. In Lepidoptera, studies of sex determination have focused on the lepidopteran model species Bombyx mori (the silkworm). In B. mori, the primary signal of sex determination cascade starts from Fem, a female-specific PIWI-interacting RNA, and its targeting gene Masc, which is apparently specific to and conserved among Lepidoptera. Tra has not been found in Lepidoptera. Instead, the B. mori PSI protein binds directly to dsx pre-mRNA and regulates its alternative splicing to produce male- and female-specific transcripts. Despite this basic understanding of the molecular mechanisms underlying sex determination, the links among the primary signals, transducers and executors remain largely unknown in Lepidoptera. In this review, we focus on the latest findings regarding the functions and working mechanisms of genes involved in feminization and masculinization in Lepidoptera and discuss directions for future research of sex determination in the silkworm.

Walter J, Liu Z, Xu J, Huang Y. Gtsf1 is essential for proper female sex determination and transposon silencing in the silkworm, Bombyx mori

Sex determination pathways are astoundingly diverse in insects. For instance, the silk moth Bombyx mori uniquely use various components of the piRNA pathway to produce the Fem signal for specification of the female fate. In this study, we identified BmGTSF1 as a novel piRNA factor which participates in B. mori sex determination. We found that BmGtsf1 has a distinct expression pattern compared to Drosophila and mouse. CRISPR/Cas9 induced mutation in BmGtsf1 resulted in partial sex reversal in genotypically female animals by shifting expression of the downstream targets BmMasc and Bmdsx to the male pattern. As levels of Fem piRNAs were substantially reduced in female mutants, we concluded that BmGtsf1 plays a critical role in the biogenesis of the feminizing signal. We also demonstrated that BmGTSF1 physically interacted with BmSIWI, a protein previously reported to be involved in female sex determination, indicating BmGTSF1 function as the cofactor of BmSIWI. BmGtsf1 mutation resulted in piRNA pathway dysregulation, including piRNA biogenesis defects and transposon derepression, suggesting BmGtsf1 is also a piRNA factor in the silkworm. Furthermore, we found that BmGtsf1 mutation leads to gametogenesis defects in both male and female. Our data suggested that BmGtsf1 is a new component involved in the sex determination pathway in B. mori.

Sex determination is a fundamentally important process in most sexually reproducing metazoan. Nevertheless, the underlying mechanisms of sex determination are highly diverse. In B. mori, piRNAs derived from the W-chromosome-linked Fem precursor serve as the primary female determining signal. However, we still know little about the initiation of B. mori sex determination and its relationship with piRNA pathway. Here, we provided evidence that BmGTSF1 is a novel piRNA factor which is indispensable for B.mori female sex determination. Mutations in BmGtsf1 resulted in dysregulation of the piRNA pathway and caused partial female-male sex reversal. We also detected dramatic diminution of Fem piRNA in female mutant, indicating BmGTSF1 regulates B. mori sex determination via piRNA pathway. More importantly, we showed that BmGTSF1 interacted with BmSIWI, which protein had been reported to be involved in piRNA pathway and sex determination in B. mori, supporting the conclusion that BmGTSF1 is a novel factor for piRNA pathway and sex determination.

The probability of fusions joining sex chromosomes and autosomes

Chromosome fusion and fission are primary mechanisms of karyotype evolution. In particular, the fusion of a sex chromosome and an autosome has been proposed as a mechanism to resolve intralocus sexual antagonism. If sexual antagonism is common throughout the genome, we should expect to see an excess of fusions that join sex chromosomes and autosomes. Here, we present a null model that provides the probability of a sex chromosome autosome fusion, assuming all chromosomes have an equal probability of being involved in a fusion. This closed-form expression is applicable to both male and female heterogametic sex chromosome systems and can accommodate unequal proportions of fusions originating in males and females. We find that over 25% of all chromosomal fusions are expected to join a sex chromosome and an autosome whenever the diploid autosome count is fewer than 16, regardless of the sex chromosome system. We also demonstrate the utility of our model by analysing two contrasting empirical datasets: one from Drosophila and one from the jumping spider genus Habronattus. We find that in the case of Habronattus, there is a significant excess of sex chromosome autosome fusions but that in Drosophila there are far fewer sex chromosome autosome fusions than would be expected under our null model.

An Efficient Duplex PCR Method for Sex Identification of the European Grapevine Moth Lobesia botrana (Lepidoptera: Tortricidae) at Any Developmental Stage

Many genetic studies in insects require sex identification of individuals in all developmental stages. The most common sex chromosome system in lepidopterans is WZ/ZZ; the W chromosome is present only in females. Based on two W chromosome-specific short sequences (CpW2 and CpW5) described in Cydia pomonella (L.) (Lepidoptera: Tortricidae), we identified homologous female-specific sequences in Lobesia botrana Den. & Schiff, a polyphagous and very harmful species present in Chile since 2008. From this starting point, we extended the sequence information using the inverse PCR method, identifying the first W-specific sequences described up to now for the moth. Finally, we developed a duplex PCR method for rapid and sensitive determination of sex in L. botrana from larva to adult. The method showed a detection limit of 1 pg of genomic DNA; a blind panel of samples exhibited exact correspondence with the morphological identification. These results will be very useful for studies requiring sex-specific analyses at any developmental stage, contributing also to the understanding of gene expression in the insect, as well as to the eventual development of control protocols against the moth, such as the development of genetic sexing strains for the implementation of the sterile insect technique.

The genome sequence of Samia ricini, a new model species of lepidopteran insect

Samia ricini, a gigantic saturniid moth, has the potential to be a novel lepidopteran model species. Samia ricini is far more resistant to diseases than the current model species Bombyx mori, and therefore can be more easily reared. In addition, genetic resources available for S. ricini rival those for B. mori: at least 26 ecoraces of S. ricini are reported and S. ricini can hybridize with wild Samia species, which are distributed throughout Asian countries, and produce fertile progenies. Physiological traits such as food preference, integument colour and larval spot pattern differ among S. ricini strains and wild Samia species so that those traits can be targeted in forward genetic analyses. To facilitate genetic research in S. ricini, we determined its whole genome sequence. The assembled genome of S. ricini was 458 Mb with 155 scaffolds, and the scaffold N50 length of the assembly was ~ 21 Mb. In total, 16,702 protein coding genes were predicted. While the S. ricini genome was mostly collinear with that of B. mori with some rearrangements and few S. ricini-specific genes were discovered, chorion genes and fibroin genes seemed to have expanded in the S. ricini lineage. As the first step of genetic analyses, causal genes for "Blue," "Yellow," "Spot," and "Red cocoon" phenotypes were mapped to chromosomes.

Intersexual differences in density-dependent dispersal and their evolutionary drivers

Dispersal is well recognized as a major driver of evolutionary processes in local populations. Nevertheless, dispersal abilities should also be perceived as a life-history trait, being subject to evolutionary changes in response to various drivers. Empirical studies investigating these drivers rarely consider that they may influence male and female dispersal differently. The purpose of our study was to document intersexual differences in density-dependent emigration from local habitat patches. As a model system, we used a metapopulation of Maculinea (Phengaris) teleius butterfly, in which densities of both sexes vary greatly throughout the flying season. Following intensive mark-release-recapture surveys, the parameters and predictors of dispersal were analysed with the Virtual Migration model and the multi-state recapture model. The emigration rate in males was substantially higher in the early season, especially at smaller habitat patches. With the proportion of females increasing with the season progression, males became reluctant to emigrate from their natal patches. In turn, higher female emigration in the later part of the season was most strongly associated with female tendency to reduce intraspecific competition experienced by their offspring. Our findings provide evidence for the impact of reproductive strategies on dispersal in both sexes. The difference in reproductive strategies of males and females explains sex-biased dispersal in different parts of the season, which carries important implications for metapopulation functioning.

High dynamism for neo-sex chromosomes: satellite DNAs reveal complex evolution in a grasshopper

A common characteristic of sex chromosomes is the accumulation of repetitive DNA, which accounts for their diversification and degeneration. In grasshoppers, the X0 sex-determining system in males is considered ancestral. However, in some species, derived variants like neo-XY in males evolved several times independently by Robertsonian translocation. This is the case of Ronderosia bergii, in which further large pericentromeric inversion in the neo-Y also took place, making this species particularly interesting for investigating sex chromosome evolution. Here, we characterized the satellite DNAs (satDNAs) and transposable elements (TEs) of the species to investigate the quantitative differences in repeat composition between male and female genomes putatively associated with sex chromosomes. We found a total of 53 satDNA families and 56 families of TEs. The satDNAs were 13.5% more abundant in males than in females, while TEs were just 1.02% more abundant in females. These results imply differential amplification of satDNAs on neo-Y chromosome and a minor role of TEs in sex chromosome differentiation. We showed highly differentiated neo-XY sex chromosomes owing to major amplification of satDNAs in neo-Y. Furthermore, chromosomal mapping of satDNAs suggests high turnover of neo-sex chromosomes in R. bergii at the intrapopulation level, caused by multiple paracentric inversions, amplifications, and transpositions. Finally, the species is an example of the action of repetitive DNAs in the generation of variability for sex chromosomes after the suppression of recombination, and helps understand sex chromosome evolution at the intrapopulation level.

Egg Sterilisation of Irradiated Nezara viridula (Hemiptera: Pentatomidae)

Simple Summary

Certain stink bugs are emerging as a serious threat to food production globally, as they invade new areas and feed on a wide range of crops. The control of these pests relies primarily on potentially harmful pesticides, but the increasing threat posed by stink bug pests has sparked investigation into environmentally-friendly methods that do not exert serious impacts on other species. One method receiving attention is the Sterile Insect Technique (SIT), which involves sterilising large numbers of a pest, through radiation exposure, and releasing them into the wild pest population where mating results in unfertilised eggs that do not hatch. In support of recent studies on SIT for the brown marmorated stink bug, we aimed to ascertain the feasibility of the method for another stink bug pest, the green vegetable bug. We exposed the insects to increasing levels of radiation and allowed mating to occur. Virtually all of the resulting eggs were sterile and did not hatch at the higher radiation doses tested. These results could be used to inform the potential development of SIT against stink bug pests, and in some circumstances, could form the basis of potential eradication programmes against new invasions.

Abstract

Nezara viridula Linnaeus (Hemiptera: Pentatomidae) is a polyphagous pest of a wide range of economically important crops. Because the control of this species and other pentatomids relies primarily on insecticide application, investigation into the Sterile Insect Technique (SIT) is warranted. We aimed to investigate the irradiation biology of N. viridula for the potential application of SIT against this pest. Male and female N. viridula were gamma-irradiated at doses between 4 and 28 Gy and mated with both irradiated and nonirradiated conspecifics. Sterility of the resulting eggs followed a dose-response in each case. Irradiated males crossed with untreated females showed higher F₁ egg sterility than crosses where the female was irradiated. The greatest F₁ egg sterility was observed when both parents were irradiated. There was no obvious dose-response for the longevity of irradiated males, and for the fecundity of nonirradiated females mated with irradiated males. The fecundity of irradiated females appeared to decrease with irradiation dose. These results can be applied to a potential future application of SIT against N. viridula, but predominantly supports the ongoing development of SIT for Halyomorpha halys Stål (Hemiptera: Pentatomidae) and hemipteran pests in general.

Advances and challenges in genetic technologies to produce single-sex litters

There is currently a requirement for single-sex litters for many applications, including agriculture, pest control, and reducing animal culling in line with the 3Rs principles: Reduction, Replacement, and Refinement. The advent of CRISPR/Cas9 genome editing presents a new opportunity with which to potentially generate all-female or all-male litters. We review some of the historical nongenetic strategies employed to generate single-sex litters and investigate how genetic and genome editing techniques are currently being used to produce all-male or all-female progeny. Lastly, we speculate on future technologies for generating single-sex litters and the possible associated challenges.

Characterization of the First W-Specific Protein-Coding Gene for Sex Identification in Helicoverpa armigera

Helicoverpa armigera is a globally-important crop pest with a WZ (female)/ZZ (male) sex chromosome system. The absence of discernible sexual dimorphism in its egg and larval stages makes it impossible to address any sex-related theoretical and applied questions before pupation unless a W-specific sequence marker is available for sex diagnosis. To this end, we used one pair of morphologically pre-sexed pupae to PCR-screen 17 non-transposon transcripts selected from 4855 W-linked candidate reads identified by mapping a publicly available egg transcriptome of both sexes to the male genome of this species and detected the read SRR1015458.67499 only in the female pupa. Subsequent PCR screenings of this read and the previously reported female-specific RAPD (random amplified polymorphic DNA) marker AF18 with ten more pairs of pre-sexed pupae and different annealing positions and/or temperatures as well as its co-occurrence with the female-specific transcript splicing isoforms of doublesex gene of H. armigera (Hadsx) and amplification and sequencing of their 5′ unknown flanking sequences in three additional pairs of pre-sexed pupae verified that SRR1015458.67499 is a single copy protein-coding gene unique to W chromosome (named GUW1) while AF18 is a multicopy MITE transposon located on various chromosomes. Test application of GUW1 as a marker to sex 30 neonates of H. armigera yielded a female/male ratio of 1.14: 1.00. Both GUW1 and Hadsx splicing isoforms assays revealed that the H. armigera embryo cell line QB-Ha-E-1 is a male cell line. Taken together, GUW1 is not only a reliable DNA marker for sexing all stages of H. armigera and its cell lines, but also represents the first W-specific protein-coding gene in lepidopterans.

Evolution of multiple sex-chromosomes associated with dynamic genome reshuffling in Leptidea wood-white butterflies

Sex-chromosome systems tend to be highly conserved and knowledge about their evolution typically comes from macroevolutionary inference. Rapidly evolving complex sex-chromosome systems represent a rare opportunity to study the mechanisms of sex-chromosome evolution at unprecedented resolution. Three cryptic species of wood-white butterflies—Leptidea juvernica, L. sinapis and L. reali—have each a unique set of multiple sex-chromosomes with 3–4 W and 3–4 Z chromosomes. Using a transcriptome-based microarray for comparative genomic hybridisation (CGH) and a library of bacterial artificial chromosome (BAC) clones, both developed in L. juvernica, we identified Z-linked Leptidea orthologs of Bombyx mori genes and mapped them by fluorescence in situ hybridisation (FISH) with BAC probes on multiple Z chromosomes. In all three species, we determined synteny blocks of autosomal origin and reconstructed the evolution of multiple sex-chromosomes. In addition, we identified W homologues of Z-linked orthologs and characterised their molecular differentiation. Our results suggest that the multiple sex-chromosome system evolved in a common ancestor as a result of dynamic genome reshuffling through repeated rearrangements between the sex chromosomes and autosomes, including translocations, fusions and fissions. Thus, the initial formation of neo-sex chromosomes could not have played a role in reproductive isolation between these Leptidea species. However, the subsequent species-specific fissions of several neo-sex chromosomes could have contributed to their reproductive isolation. Then, significantly increased numbers of Z-linked genes and independent neo-W chromosome degeneration could accelerate the accumulation of genetic incompatibilities between populations and promote their divergence resulting in speciation.

Shared and Independent Genetic Basis of Resistance to Bt Toxin Cry2Ab in Two Strains of Pink Bollworm

Evolution of pest resistance threatens the benefits of crops genetically engineered to produce insecticidal proteins from Bacillus thuringiensis (Bt). Field populations of the pink bollworm (Pectinophora gossypiella), a global pest of cotton, have evolved practical resistance to transgenic cotton producing Bt toxin Cry2Ab in India, but not in the United States. Previous results show that recessive mutations disrupting an autosomal ATP-binding cassette gene (PgABCA2) are associated with pink bollworm resistance to Cry2Ab in field-selected populations from India and in one lab-selected strain from the United States (Bt4-R2). Here we discovered that an independently derived, lab-selected Cry2Ab-resistant pink bollworm strain from the United States (BX-R) also harbors mutations that disrupt PgABCA2. Premature stop codons introduced by mis-splicing of PgABCA2 pre-mRNA were prevalent in field-selected larvae from India and in both lab-selected strains. The most common mutation in field-selected larvae from India was also detected in both lab-selected strains. Results from interstrain crosses indicate BX-R has at least one additional mechanism of resistance to Cry2Ab that does not involve PgABCA2 and is not completely recessive or autosomal. We conclude that recessive mutations disrupting PgABCA2 are the primary, but not the only, mechanism of resistance to Cry2Ab in pink bollworm.

Identification and characterization of a Masculinizer homologue in the diamondback moth, Plutella xylostella

Recently, a novel sex-determination system was identified in the silkworm (Bombyx mori) in which a piwi-interacting RNA (piRNA) encoded on the female-specific W chromosome silences a Z-linked gene (Masculinizer) that would otherwise initiate male sex-determination and dosage compensation. Masculinizer provides various opportunities for developing improved genetic pest management tools. A pest lepidopteran in which a genetic pest management system has been developed, but which would benefit greatly from such improved designs, is the diamondback moth, Plutella xylostella. However, Masculinizer has not yet been identified in this species. Here, focusing on the previously described 'masculinizing' domain of B. mori Masculinizer, we identify P. xylostella Masculinizer (PxyMasc). We show that PxyMasc is Z-linked, regulates sex-specific alternative splicing of doublesex and is necessary for male survival. Similar results in B. mori suggest this survival effect is possibly through failure to initiate male dosage compensation. The highly conserved function and location of this gene between these two distantly related lepidopterans suggests a deep role for Masculinizer in the sex-determination systems of the Lepidoptera.

Multilayered Tuning of Dosage Compensation and Z-Chromosome Masculinization in the Wood White (Leptidea sinapis) Butterfly

In species with genetic sex determination, dosage compensation can evolve to equal expression levels of sex-linked and autosomal genes. Current knowledge about dosage compensation has mainly been derived from male-heterogametic (XX/XY) model organisms, whereas less is understood about the process in female-heterogametic systems (ZZ/ZW). In moths and butterflies, downregulation of Z-linked expression in males (ZZ) to match the expression level in females (ZW) is often observed. However, little is known about the underlying regulatory mechanisms, or if dosage compensation patterns vary across ontogenetic stages. In this study, we assessed dynamics of Z-linked and autosomal expression levels across developmental stages in the wood white (Leptidea sinapis). We found that although expression of Z-linked genes in general was reduced compared with autosomal genes, dosage compensation was actually complete for some categories of genes, in particular sex-biased genes, but equalization in females was constrained to a narrower gene set. We also observed a noticeable convergence in Z-linked expression between males and females after correcting for sex-biased genes. Sex-biased expression increased successively across developmental stages, and male-biased genes were enriched on the Z-chromosome. Finally, all five core genes associated with the ribonucleoprotein dosage compensation complex male-specific lethal were detected in adult females, in correspondence with a reduction in the expression difference between autosomes and the single Z-chromosome. We show that tuning of gene dosage is multilayered in Lepidoptera and argue that expression balance across chromosomal classes may predominantly be driven by enrichment of male-biased genes on the Z-chromosome and cooption of available dosage regulators.

Cytogenetic markers applied to cytotaxonomy in two soybean pests: Anticarsia gemmatalis (Hübner, 1818) and Chrysodeixis includens (Walker, 1858)

Anticarsia gemmatalis (Hübner, 1818) and Chrysodeixis includens (Walker, 1858) are species of Lepidoptera that cause great damages in the soybean plantations of Brazil. Despite the importance they have in this regard, there are no studies on the chromosomal organization of these species and recently, A. gemmatalis, which belonged to the Noctuidae family, was allocated to the Erebidae family. Therefore, the objective of this paper was to analyze, through conventional and molecular cytogenetic markers, both species of Lepidoptera. A 2n = 62 was observed, with ZZ/ZW sex chromosome system and holokinetic chromosomes for both species. There was homogeneity in the number of 18S rDNA sites for both species. However, variations in heterochromatin distribution were observed between both species. The cytogenetic analyses enabled separation of the species, corroborating the transference of A. gemmatalis, from the family Noctuidae to the family Erebidae, suggesting new cytotaxonomic characteristics.

Absence of W Chromosome in Psychidae Moths and Implications for the Theory of Sex Chromosome Evolution in Lepidoptera

Moths and butterflies (Lepidoptera) are the largest group with heterogametic females. Although the ancestral sex chromosome system is probably Z0/ZZ, most lepidopteran species have the W chromosome. When and how the W chromosome arose remains elusive. Existing hypotheses place the W origin either at the common ancestor of Ditrysia and Tischeriidae, or prefer independent origins of W chromosomes in these two groups. Due to their phylogenetic position at the base of Ditrysia, bagworms (Psychidae) play an important role in investigating the W chromosome origin. Therefore, we examined the W chromosome status in three Psychidae species, namely Proutiabetulina, Taleporiatubulosa, and Diplodomalaichartingella, using both classical and molecular cytogenetic methods such as sex chromatin assay, comparative genomic hybridization (CGH), and male vs. female genome size comparison by flow cytometry. In females of all three species, no sex chromatin was found, no female-specific chromosome regions were revealed by CGH, and a Z-chromosome univalent was observed in pachytene oocytes. In addition, the genome size of females was significantly smaller than males. Overall, our study provides strong evidence for the absence of the W chromosome in Psychidae, thus supporting the hypothesis of two independent W chromosome origins in Tischeriidae and in advanced Ditrysia.