A chimeric gene paternally instructs female sex determination in the haplodiploid wasp Nasonia

Understanding the genetics of sex determination in insects and its relevance to genetic pest management

Sex determination pathways regulate male and female-specific development and differentiation and offer potential targets for genetic pest management methods. Insect sex determination pathways are comprised of primary signals, relay genes and terminal genes. Primary signals of coleopteran, dipteran, hymenopteran and lepidopteran species are highly diverse and regulate the sex-specific splicing of relay genes based on the primary signal dosage, amino acid composition or the interaction with paternally inherited genes. In coleopterans, hymenopterans and some dipterans, relay genes are Transformer orthologs from the serine-arginine protein family that regulate sex-specific splicing of the terminal genes. Alternative genes regulate the splicing of the terminal genes in dipterans that lack Transformer orthologs and lepidopterans. Doublesex and Fruitless orthologs are the terminal genes. Doublesex and Fruitless orthologs are highly conserved zinc-finger proteins that regulate the expression of downstream proteins influencing physical traits and courtship behaviours in a sex-specific manner. Genetic pest management methods can use different mechanisms to exploit or disrupt female-specific regions of different sex determination genes. Female-specific regions of sex determination genes can be exploited to produce a lethal gene only in females or disrupted to impede female development or fertility. Reducing the number of fertile females in pest populations creates a male-biased sex ratio and eventually leads to the local elimination of the pest population. Knowledge on the genetic basis of sex determination is important to enable these sex determination pathways to be exploited for genetic pest management.

Function of doublesex and transformer-2 genes and its respond to environment factors in Dendroctonus armandi Tsai et Li (Coleoptera: Curculionidae: Scolytinae)

The doublesex (dsx) and transformer-2 (tra-2) genes play important roles in insect sex determination. However, the sex determination of Dendroctonus armandi in the native and coniferous forests of the Qinling Mountains is still unclear. In this study, we cloned and identified the full-length sequences of two Dadsx isoform (Dadsx1 and Dadsx2) and two isoform Datra-2 (Datra-2A and Datra-2B) in D. armandi. These four isoforms are expressed in larvae, pupae, as well as in males and females, and their expression levels are highest during the pupae stage. The relative expression levels of Dadsx and Datra-2 genes showed significant differences under different environment treatments (including temperature, nutrient, feeding duration, and terpenoid), with male Dadsx1 relative expression levels significantly higher than females and female Dadsx2 relative expression levels significantly higher than males. The silencing of Datra-2A and Datra-2B isoforms both leads to the downregulation of Dadsx1 isoform. Compared to the control group (average mortality rate: 36.7 %; average deformity rate: 0.2 %; emergence rate: 60.9 %; sex ratio: 1.11), when Dadsx1 (average mortality rate: 69.0 %; average deformity rate: 16.5 %; emergence rate: 29.6 %; sex ratio: 0.28), Dadsx2 (average mortality rate: 64.4 %; average deformity rate: 17.4 %; emergence rate: 25.4 %; sex ratio: 2.41), Datra-2A (average mortality rate: 65.1 %; average deformity rate: 16.1 %; emergence rate: 15.4 %; sex ratio: 0.31) and Datra-2B (average mortality rate: 65.4 %; average deformity rate: 17.2 %; emergence rate: 13.8 %; sex ratio: 0.33) isoform s were silenced, it showed a significant increase in mortality and deformity rates, a significant decrease in emergence rate, and a severe sex imbalance. The results indicate that the relative expression levels of Dadsx and Datra-2 genes are influenced by external factors and play a crucial role in maintaining the sex ratio of D. armandi and ensuring its lifecycle development.

Genome editing in hymenoptera

The application of genome editing tools in Hymenoptera has transformative potential for functional genetics and understanding their unique biology. Hymenoptera comprise one of the most diverse Orders of animals, and the development of methods for efficiently creating precise genome modifications could have applications in conservation, pest management and agriculture. To date, sex determination, DNA methylation, taste and smell sensory systems as well as phenotypic markers have been selected for gene editing investigations. From these data, insights into eusociality, the nature of haplodiploidy and the complex communication systems that Hymenoptera possess have provided an understanding of their evolutionary history that has led them to become so diverse and successful. Insights from these functional genetics analyses have been supported by the ever-improving suite of CRIPSR tools and further expansion will allow more specific biological hypotheses to be tested and applications beyond the lab. Looking ahead, genome editing tools have potential for Hymenopteran applications in modifying biocontrol agents of agricultural pests and for use in managing invasive species through the development of technologies such as gene drives. This review provides accessibility to information regarding the status of Hymenopteran genome editing, intending to support the considered development of CRISPR tools in novel species as well as innovation and refinement of methods in species in which it has already been achieved.

Heterozygosity at a conserved candidate sex determination locus is associated with female development in the clonal raider ant (Ooceraea biroi)

Sex determination is a developmental switch that triggers sex-specific developmental programs. This switch is "flipped" by the expression of genes that promote male- or female-specific development. Many lineages have evolved sex chromosomes that act as primary signals for sex determination. However, haplodiploidy (males are haploid and females are diploid), which occurs in ca. 12% of animal species, is incompatible with sex chromosomes. Haplodiploid taxa must, therefore, rely on other strategies for sex determination. One mechanism, "complementary sex determination" (CSD), uses heterozygosity as a proxy for diploidy. In CSD, heterozygosity at a sex determination locus triggers female development, while hemizygosity or homozygosity permits male development. CSD loci have been mapped in honeybees and two ant species, but we know little about their evolutionary history. Here, we investigate sex determination in the clonal raider ant, Ooceraea biroi. We identified a 46kb candidate CSD locus at which all females are heterozygous, but most diploid males are homozygous for either allele. As expected for CSD loci, the candidate locus has more alleles than most other loci, resulting in a peak of nucleotide diversity. This peak negligibly affects the amino acid sequences of protein-coding genes, suggesting that heterozygosity of a non-coding genomic sequence triggers female development. This locus is distinct from the CSD locus in honeybees but homologous to a CSD locus mapped in two distantly related ant species, implying that this molecular mechanism has been conserved since a common ancestor that lived approximately 112 million years ago.

A case of polyploid utility in biocontrol: reproductively-impaired triploid Nasonia vitripennis have high host-killing ability

BACKGROUND

Intentionally impairing the fecundity of mass-reared insects has important utility in controlling pest species. Typically, sterilized individuals are competed against wild counterparts, reducing pest population size. A novel consideration is creating biocontrol agents with lower reproductive capacity that are less likely to establish permanently or admix with wild populations, which are both emerging as legal barriers. Hymenopterans have diploid females, but archetypically infertile polyploid triploid females occur for various parasitoid species. As a first test of polyploid utility for these biocontrol concerns, we assessed the species with the best characterized polyploid biology, the gregarious idiobiont Nasonia vitripennis, for triploid female host-killing ability on pupal blowfly hosts (Calliphora vomitoria).

RESULTS

We examined four polyploid lines: the old Whiting polyploid line (WPL) derived from a spontaneous mutation, and new polyploid lines made through RNAi knockdown of sex determination genes transformer, transformer-2 and wasp-overruler-of-masculinization. For diploid and triploid females of each polyploid line, and control diploids of the STDR and oyster lines used to maintain them, we measured lifetime number of hosts killed; lifetime number of hosts that produced at least one offspring; the percentage of the hosts killed and the percentage of hosts that produced offspring out of those offered; and lifespan. For all lines, triploids produced viable offspring in far fewer hosts than their diploid counterparts (≤70% less). Surprisingly though, they killed as many or more hosts than diploids over similar lifespans. The offspring production ability of the WPL triploid was half that of the other lines, but lines varied only slightly in the number of hosts killed (±10) among the polyploids.

CONCLUSION

The ability of reproductively impaired triploids to kill as many hosts as fertile diploids demonstrate high biocontrol utility for polyploidized females, and downstream potential for reducing ecological risk. © 2024 The Author(s). Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Identification and functional analysis of sex-determining genes in the spongy moth, Lymantria dispar (lepidoptera: Erebidae)

The spongy moth (Lymantria dispar) employs a female heterogametic sex-determination system, where the female sex-determining factor (F factor) is located on the W chromosome, and the male sex-determining factor (M factor) is located on the Z chromosome. The sex-determining capabilities of the F factor and M factor vary among subspecies. Consequently, L. dispar serves as an excellent model for studying the mechanisms underlying the evolution and diversity of sex-determining genes. However, the genes encoding the F and M factors, as well as the molecular functions of their translation products, remain unidentified. In this study, we identified a L. dispar Masculinizer ortholog (LdMasc) and found that this gene is highly expressed in male embryos during the sex-determination stage. When LdMasc expression was silenced using embryonic RNA interference (RNAi), the expression pattern of L. dispar doublesex (Lddsx), the master regulatory gene for sex differentiation, shifted from the male-specific form to the female-specific form in male embryos. To identify potential F factors, we screened for genes that were exclusively expressed in females across multiple tissues and located only within the female genome. This screening yielded four unigenes with sequences displaying high homology to each other. These unigenes formed a tandem repeat, comprising approximately 100 copies within a 200 kbp region of the W chromosome-derived contig. We designated these unigenes as Fet-W (female-specifically expressed transcript from the W chromosome). RT-PCR analysis revealed that Fet-W was expressed in a female-specific manner during the sex-determination stage. Suppression of Fet-W expression by embryonic RNAi led to an increase in LdMasc expression in females and a corresponding shift in dsx expression patterns from the female-specific to the male-specific form. These findings strongly suggest that the F factor in L. dispar is Fet-W, whereas the M factor is LdMasc.

In vivo mechanism of the interaction between trimethylamine lyase expression and glycolytic pathways

Recent studies confirmed that host-gut microbiota interactions modulate disease-linked metabolite TMA production via TMA lyase. However, microbial enzyme production mechanisms remain unclear. In the present study, we investigated the impact of dietary and intervention factors on gut microbiota, microbial gene expression, and the interplay between TMA lyase and glycolytic pathways in mice. Using 16S rRNA gene sequencing, metagenomics, and metabolomics, the gut microbiota composition and microbial functional gene expression profiles related to TMA lyase and glycolytic enzymes were determined. The results revealed that distinct diets and intervention factors altered gut microbiota, gene expression, and metabolites linked to glycine metabolism and glycolysis. Notably, an arabinoxylan-rich diet suppressed genes linked to choline, glycine, glycolysis, and TMA lyase, favoring glycine utilization via pyruvate pathways. Glycolytic inhibitors amplified these effects, mainly inhibiting pyruvate kinase. Our findings underscored the crosstalk between TMA lyase and glycolytic pathways, regulating glycine levels, and suggested avenues for targeted interventions and personalized diets to curb choline TMA lyase production.

Honeybees' novel complementary sex-determining system: function and origin

Complementary sex determination regulates female and male development in honeybees (Apis mellifera) via heterozygous versus homo-/hemizygous genotypes of the csd (complementary sex determiner) gene involving numerous naturally occurring alleles. This lineage-specific function offers a rare opportunity to understand an undescribed regulatory mechanism and the molecular evolutionary path leading to this mechanism. We reviewed recent advances in understanding how Csd recognizes different versus identical protein variants, how these variants regulate downstream pathways and sexual differentiation, and how this mechanism has evolved and been shaped by evolutionary forces. Finally, we highlighted the shared regulatory principles of sex determination despite the diversity of primary signals and demonstrated that lineage-specific mutations are very informative for characterizing newly evolved functions.

The state of parasitoid wasp genomics

Parasitoid wasps represent a group of parasitic insects with high species diversity that have played a pivotal role in biological control and evolutionary studies. Over the past 20 years, developments in genomics have greatly enhanced our understanding of the biology of these species. Technological leaps in sequencing have facilitated the improvement of genome quality and quantity, leading to the availability of hundreds of parasitoid wasp genomes. Here, we summarize recent progress in parasitoid wasp genomics, focusing on the evolution of genome size (GS) and the genomic basis of several key traits. We also discuss the contributions of genomics in studying venom evolution and endogenization of viruses. Finally, we advocate for increased sequencing and functional research to better understand parasitoid biology and enhance biological control.

Creating insect neopolyploid lines to study animal polyploid evolution

Whole-genome duplication (polyploidy) poses many complications but is an important driver for eukaryotic evolution. To experimentally study how many challenges from the cellular (including gene expression) to the life history levels are overcome in polyploid evolution, a system in which polyploidy can be reliably induced and sustained over generations is crucial. Until now, this has not been possible with animals, as polyploidy notoriously causes first-generation lethality. The parasitoid wasp Nasonia vitripennis emerges as a stunningly well-suited model. Polyploidy can be induced in this haplodiploid system through (1) silencing genes in the sex determination cascade and (2) by colchicine injection to induce meiotic segregation failure. Nasonia polyploids produce many generations in a short time, making them a powerful tool for experimental evolution studies. The strong variation observed in Nasonia polyploid phenotypes aids the identification of polyploid mechanisms that are the difference between evolutionary dead ends and successes. Polyploid evolution research benefits from decades of Nasonia research that produced extensive reference-omics data sets, facilitating the advanced studies of polyploid effects on the genome and transcriptome. It is also possible to create both inbred lines (to control for genetic background effects) and outbred lines (to conduct polyploid selection regimes). The option of interspecific crossing further allows to directly contrast autopolyploidy (intraspecific polyploidy) to allopolyploidy (hybrid polyploidy). Nasonia can also be used to investigate the nascent field of using polyploidy in biological control to improve field performance and lower ecological risk. In short, Nasonia polyploids are an exceptional tool for researching various biological paradigms.

Direct Parental (DIPA) CRISPR in the jewel wasp, Nasonia vitripennis

While clustered regularly interspaced short palindromic repeats (CRISPR)-Cas9 technology has demonstrated remarkable promise as a gene-editing tool, its application in certain insects, such as the jewel wasp, Nasonia vitripennis, has been hindered by a lack of a tractable method for reagent delivery. Direct Parental (DIPA-) CRISPR recently emerged as a facile way to induce gene lesions because it involves adult injection with commercially available Cas9-sgRNA with no helper reagent. However, DIPA-CRISPR has so far been tested in only a few insects. Here, we have assessed the amenability of DIPA-CRISPR in N. vitripennis by targeting two eye pigmentation genes, cinnabar and vermilion, which function in the ommochrome pathway. Successful generation of lesions in both genes demonstrated the functionality of DIPA-CRISPR in N. vitripennis and its potential application to other genes, thereby expanding the range of insects suitable for this method. We varied two parameters, Cas9-sgRNA concentration and injection volume, to determine optimal injection conditions. We found that the larger injection volume coupled with either higher or lower reagent concentration was needed for consistent mutation production. However, DIPA-CRISPR yields an overall low mutation rate in N. vitripennis when compared to other tested insects, a characteristic that may be attributed to a proportionally low vitellogenic import efficiency in the jewel wasp. We discuss different factors that may be considered in determining when DIPA-CRISPR may be preferable over other reagent delivery methods.

PSRs: Selfish chromosomes that manipulate reproductive development

B chromosomes are intriguing "selfish" genetic elements, many of which exhibit higher-than-Mendelian transmission. This perspective highlights a group of B chromosomes known as Paternal Sex Ratio chromosomes (PSRs), which are found in several insects with haplo-diploid reproduction. PSRs harshly alter the organism's reproduction to facilitate their own inheritance. A manifestation of this effect is the conversion of female destined individuals into males. Key to this conversion is the mysterious ability of PSRs to cause elimination of the sperm-inherited half of the genome during zygote formation. Here we discuss how PSRs were discovered, what is known about how they alter paternal chromatin dynamics to cause sex conversion, and how PSR-induced genome elimination is different from other forms of programmed genome elimination in different insects. PSRs also stand out because their DNA sequence compositions differ in remarkable ways from their insect's essential chromosomes, a characteristic suggestive of interspecies origins. Broadly, we also highlight poorly understood aspects of PSR dynamics that need to be investigated.

LncRNA gene ANTSR coordinates complementary sex determination in the Argentine ant

Animals have evolved various sex determination systems. Here, we describe a newly found mechanism. A long noncoding RNA (lncRNA) transduces complementary sex determination (CSD) signal in the invasive Argentine ant. In this haplodiploid species, we identified a 5-kilobase hyper-polymorphic region underlying CSD: Heterozygous embryos become females, while homozygous and hemizygous embryos become males. Heterozygosity at the CSD locus correlates with higher expression of ANTSR, a gene that overlaps with the CSD locus and specifies an lncRNA transcript. ANTSR knockdown in CSD heterozygotes leads to male development. Comparative analyses indicated that, in Hymenoptera, ANTSR is an ancient yet rapidly evolving gene. This study reveals an lncRNA involved in genetic sex determination, alongside a previously unknown regulatory mechanism underlying sex determination based on complementarity among noncoding alleles.

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.

Identification of Parthenogenesis-Inducing Effector Proteins in Wolbachia

Bacteria in the genus Wolbachia have evolved numerous strategies to manipulate arthropod sex, including the conversion of would-be male offspring to asexually reproducing females. This so-called "parthenogenesis induction" phenotype can be found in a number of Wolbachia strains that infect arthropods with haplodiploid sex determination systems, including parasitoid wasps. Despite the discovery of microbe-mediated parthenogenesis more than 30 yr ago, the underlying genetic mechanisms have remained elusive. We used a suite of genomic, computational, and molecular tools to identify and characterize two proteins that are uniquely found in parthenogenesis-inducing Wolbachia and have strong signatures of host-associated bacterial effector proteins. These putative parthenogenesis-inducing proteins have structural homology to eukaryotic protein domains including nucleoporins, the key insect sex determining factor Transformer, and a eukaryotic-like serine-threonine kinase with leucine-rich repeats. Furthermore, these proteins significantly impact eukaryotic cell biology in the model Saccharomyces cerevisiae. We suggest that these proteins are parthenogenesis-inducing factors and our results indicate that this would be made possible by a novel mechanism of bacterial-host interaction.

Effects of polyploidization and their evolutionary implications are revealed by heritable polyploidy in the haplodiploid wasp Nasonia vitripennis

Recurrent polyploidization occurred in the evolutionary history of most Eukaryota. However, how neopolyploid detriment (sterility, gigantism, gene dosage imbalances) has been overcome and even been bridged to evolutionary advantage (gene network diversification, mass radiation, range expansion) is largely unknown, particularly for animals. We used the parasitoid wasp Nasonia vitripennis, a rare insect system with heritable polyploidy, to begin addressing this knowledge gap. In Hymenoptera the sexes have different ploidies (haploid males, diploid females) and neopolyploids (diploid males, triploid females) occur for various species. Although such polyploids are usually sterile, those of N. vitripennis are reproductively capable and can even establish stable polyploid lines. To assess the effects of polyploidization, we compared a long-established polyploid line, the Whiting polyploid line (WPL) and a newly generated transformer knockdown line (tKDL) for fitness traits, absolute gene expression, and cell size and number. WPL polyploids have high male fitness and low female fecundity, while tKDL polyploids have poor male mate competition ability and high fertility. WPL has larger cells and cell number reduction, but the tKDL does not differ in this respect. Expression analyses of two housekeeping genes indicated that gene dosage is linked to sex irrespective of ploidy. Our study suggests that polyploid phenotypic variation may explain why some polyploid lineages thrive and others die out; a commonly proposed but difficult-to-test hypothesis. This documentation of diploid males (tKDL) with impaired competitive mating ability; triploid females with high fitness variation; and hymenopteran sexual dosage compensation (despite the lack of sex chromosomes) all challenges general assumptions on hymenopteran biology. We conclude that polyploidization is dependent on the duplicated genome characteristics and that genomes of different lines are unequally suited to survive diploidization. These results demonstrate the utility of N. vitripennis for delineating mechanisms of animal polyploid evolution, analogous to more advanced polyploid plant models.

Recognition of polymorphic Csd proteins determines sex in the honeybee

Sex in honeybees, Apis mellifera, is genetically determined by heterozygous versus homo/hemizygous genotypes involving numerous alleles at the single complementary sex determination locus. The molecular mechanism of sex determination is however unknown because there are more than 4950 known possible allele combinations, but only two sexes in the species. We show how protein variants expressed from complementary sex determiner (csd) gene determine sex. In females, the amino acid differences between Csd variants at the potential-specifying domain (PSD) direct the selection of a conserved coiled-coil domain for binding and protein complexation. This recognition mechanism activates Csd proteins and, thus, the female pathway. In males, the absence of polymorphisms establishes other binding elements at PSD for binding and complexation of identical Csd proteins. This second recognition mechanism inactivates Csd proteins and commits male development via default pathway. Our results demonstrate that the recognition of different versus identical variants of a single protein is a mechanism to determine sex.

Masculinizer gene controls male sex determination in the codling moth, Cydia pomonella

The molecular mechanisms of sex determination in moths and butterflies (Lepidoptera) with female heterogamety (WZ/ZZ) are poorly understood, except in the silkworm Bombyx mori. However, the Masculinizer (Masc) gene that controls male development and dosage compensation in B. mori, appears to be conserved in Lepidoptera, as its masculinizing function was recently confirmed in several moth species. In this work, we investigated the role of the Masc gene in sex determination of the codling moth Cydia pomonella (Tortricidae), a globally important pest of pome fruits and walnuts. The gene structure of the C. pomonella Masc ortholog, CpMasc, is similar to B. mori Masc. However, unlike B. mori, we identified 14 splice variants of CpMasc in the available transcriptomes. Subsequent screening for sex specificity and genetic variation using publicly available data and RT-PCR revealed three male-specific splice variants. Then qPCR analysis of these variants revealed sex-biased expression showing a peak only in early male embryos. Knockdown of CpMasc by RNAi during early embryogenesis resulted in a shift from male-to female-specific splicing of the C. pomonella doublesex (Cpdsx) gene, its downstream effector, in ZZ embryos, leading to a strongly female-biased sex ratio. These data clearly demonstrate that CpMasc functions as a masculinizing gene in the sex-determining cascade of C. pomonella. Our study also showed that CpMasc transcripts are provided maternally, as they were detected in unfertilized eggs after oviposition and in mature eggs dissected from virgin females. This finding is unique, as maternal provision of mRNA has rarely been studied in Lepidoptera.

The identification and expression pattern of the sex determination genes and their sex-specific variants in the egg parasitoid Trichogramma dendrolimi Matsumura (Hymenoptera: Trichogrammatidae)

Introduction: Trichogramma wasps are egg parasitoids of agricultural lepidopteran pests. The sex of Trichogramma is determined by its ploidy as well as certain sex ratio distorters, such as the endosymbiotic bacteria Wolbachia spp. and the paternal sex ratio (PSR) chromosome. The sex determination systems of hymenopterans, such as Trichogramma spp., involve cascades of the genes transformer (tra), transformer-2 (tra2), and doublesex (dsx) and are associated with sex-specific tra and dsx splicing. First, these genes and their sex-specific variants must be identified to elucidate the interactions between the sex ratio disorders and the sex determination mechanism of Trichogramma. Methods: Here, we characterized the sex determination genes tra, tra2, and dsx in Trichogramma dendrolimi. Sex-specific tra and dsx variants were detected in cDNA samples obtained from both male and female Trichogramma wasps. They were observed in the early embryos (1-10 h), late embryos (12-20 h), larvae (32 h and 48 h), pre-pupae (96 h), and pupae (144 h, 168 h, 192 h, and 216 h) of both male and female T. dendrolimi offspring. Results: We detected female-specific tra variants throughout the entire early female offspring stage. The male-specific variant began to express at 9-10 h as the egg was not fertilized. However, we did not find any maternally derived, female-specific tra variant in the early male embryo. This observation suggests that the female-specific tra variant expressed in the female embryo at 1-9 h may not have originated from the maternal female wasp. Discussion: The present study might be the first to identify the sex determination genes and sex-specific gene splicing in Trichogramma wasps. The findings of this study lay the foundation for investigating the sex determination mechanisms of Trichogramma and other wasps. They also facilitate sex identification in immature T. dendrolimi and the application of this important egg parasitoid in biological insect pest control programs.

Intersex is required for female sexual development in Hermetia illucens

Sex-determination pathways are extremely diverse. Understanding the mechanism of sex determination in insects is important for genetic manipulation of the pest population and for breeding of economically valuable insects. Although sex determination has been well characterized in the model species Drosophila melanogaster, little is known about this pathway in Stratiomyidae. In the present study, we first identified the Drosophila intersex (ix) homolog in Hermetia illucens, also known as the black soldier fly, which belongs to the Stratiomyidae family and which is an important insect for the conversion of various organic wastes. Phylogenetic analyses and multiple sequence alignment revealed that Hiix is conserved compared with Drosophila. We showed that Hiix is highly expressed in internal genitalia. Disruption of the Hiix gene using CRISPR/Cas9 resulted in female-specific defects in external genitalia and abnormal and undersized ovaries. Taken together, our study furthers our understanding of sex determination in insects and could facilitate breeding of H. illucens.

Transformer gene regulates feminization under two complementary sex determination loci in the ant, Vollenhovia emeryi

Organisms that reproduce sexually have evolved well-organized mechanisms to determine two sexes. Some hymenopterans (such as ants, bees, and wasps) have a complementary sex-determination system in which heterozygosity at one CSD locus induces female development, whereas hemi- or homozygosity at the locus induces male development. This system can generate a high cost of inbreeding, as individuals that are homozygous at the locus become sterile, diploid males. On the other hand, some hymenopterans have evolved a multi-locus, complementary, sex-determination system in which heterozygosity in at least one CSD locus induces female development. This system effectively reduces the proportion of sterile diploid males; however, how these multiple
primary signals based on CSD pass through a molecular cascade to regulate downstream genes has remained unclear. To clarify this matter, we used a backcross to investigate the molecular cascade in the ant, Vollenhovia emeryi, with two CSD loci. Here we show by gene disruption that transformer (tra) is necessary for proper feminization. Expression analysis of tra and doublesex (dsx) showed that heterozygosity in at least one of the two CSD loci is sufficient to promote female sex determination. Analysis of overexpression suggested that female-type Tra protein promotes splicing of tra pre-mRNA to female isoform by a positive-regulatory-feedback loop. Our data also showed that tra affects splicing of dsx. We conclude that two-loci sex determination system in V. emeryi evolved based on tra-dsx splicing cascade that is well conserved in other insect species. Finally, we suggest a cascade model to arrive at a binary determination of sex under multiple primary signals.

Biotic and abiotic factors that affect parasitism in Trichogramma pintoi (Hymenoptera: Trichogrammatidae) as a biocontrol agent against Heortia vitessoides (Lepidoptera: Pyralidae)

The parasitoid wasp, Trichogramma pintoi, is a promising candidate for inundative release against Heortia vitessoides. Parasitoid females can regulate the sex of their offspring in response to environmental and biological factors. In pest control programs utilizing these parasitoids, male overproduction is not conducive to success. To optimize the production of T. pintoi as an egg parasitoid of H. vitessoides, factors affecting the rates of parasitism and eclosion and the percentage of females among T. pintoi offspring, such as temperature, photoperiod, host age, host density, maternal age, maternal density, and food, were investigated. The proportion of T. pintoi female offspring was significantly affected by temperature, photoperiod, host density, maternal age, and maternal density. The female offspring percentage decreased in response to host density (160 eggs), maternal age (≥ 4 days old), maternal density (≥ 4 females), photoperiods (24:0 and 18:6 L:D), and extremely low temperature (15 °C). However, host age and female diet did not affect the proportion of female offspring. According to the present work, female parasitoid production can be maximized under laboratory conditions of 25 °C, 75% relative humidity, and a photoperiod of 0:24 h (L:D) via exposure of forty 1-day-old H. vitessoides eggs for 24 h or eighty 1-day-old H. vitessoides eggs to a newly emerged, mated female fed a 10% sucrose solution until the female dies. These findings will guide mass production efforts for this parasitoid.

Theme and variation in the evolution of insect sex determination

The development of dimorphic adult sexes is a critical process for most animals, one that is subject to intense selection. Work in vertebrate and insect model species has revealed that sex determination mechanisms vary widely among animal groups. However, this variation is not uniform, with a limited number of conserved factors. Therefore, sex determination offers an excellent context to consider themes and variations in gene network evolution. Here we review the literature describing sex determination in diverse insects. We have screened public genomic sequence databases for orthologs and duplicates of 25 genes involved in insect sex determination, identifying patterns of presence and absence. These genes and a 3.5 reference set of 43 others were used to infer phylogenies and compared to accepted organismal relationships to examine patterns of congruence and divergence. The function of candidate genes for roles in sex determination (virilizer, female-lethal-2-d, transformer-2) and sex chromosome dosage compensation (male specific lethal-1, msl-2, msl-3) were tested using RNA interference in the milkweed bug, Oncopeltus fasciatus. None of these candidate genes exhibited conserved roles in these processes. Amidst this variation we wish to highlight the following themes for the evolution of sex determination: (1) Unique features within taxa influence network evolution. (2) Their position in the network influences a component's evolution. Our analyses also suggest an inverse association of protein sequence conservation with functional conservation.

Nasonia-microbiome associations: a model for evolutionary hologenomics research

In recent years, with the development of microbial research technologies, microbiota research has received widespread attention. The parasitoid wasp genus Nasonia is a good model organism for studying insect behavior, development, evolutionary genetics, speciation, and symbiosis. This review describes key advances and progress in the field of the Nasonia-microbiome interactions. We provide an overview of the advantages of Nasonia as a model organism for microbiome studies, list research methods to study the Nasonia microbiome, and discuss recent discoveries in Nasonia microbiome research. This summary of the complexities of Nasonia-microbiome relationships will help to contribute to a better understanding of the interactions between animals and their microbiomes and establish a clear research direction for Nasonia-microbiome interactions in the future.

Sexually Dimorphic Gene Expression in X and Y Sperms Instructs Sexual Dimorphism of Embryonic Genome Activation in Yellow Catfish (Pelteobagrus fulvidraco)

Paternal factors play an important role in embryonic morphogenesis and contribute to sexual dimorphism in development. To assess the effect of paternal DNA on sexual dimorphism of embryonic genome activation, we compared X and Y sperm and different sexes of embryos before sex determination. Through transcriptome sequencing (RNA-seq) and whole-genome bisulfite sequencing (WGBS) of X and Y sperm, we found a big proportion of upregulated genes in Y sperm, supported by the observation that genome-wide DNA methylation level is slightly lower than in X sperm. Cytokine-cytokine receptor interaction, TGF-beta, and toll-like receptor pathways play important roles in spermatogenesis. Through whole-genome re-sequencing (WGRS) of parental fish and RNA-seq of five early embryonic stages, we found the low-blastocyst time point is a key to maternal transcriptome degradation and zygotic genome activation. Generally, sexual differences emerged from the bud stage. Moreover, through integrated analysis of paternal SNPs and gene expression, we evaluated the influence of paternal inheritance on sexual dimorphism of genome activation. Besides, we screened out gata6 and ddx5 as potential instructors for early sex determination and gonad development in yellow catfish. This work is meaningful for revealing the molecular mechanisms of sex determination and sexual dimorphism of fish species.

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.

Sex-based de novo transcriptome assemblies of the parasitoid wasp Encarsia suzannae, a host of the manipulative heritable symbiont Cardinium hertigii

Parasitoid wasps in the genus Encarsia are commonly used as biological pest control agents of whiteflies and armored scale insects in greenhouses or the field. They are also hosts of the bacterial endosymbiont Cardinium hertigii, which can cause reproductive manipulation phenotypes, including parthenogenesis, feminization, and cytoplasmic incompatibility (the last is mainly studied in Encarsia suzannae). Despite their biological and economic importance, there are no published Encarsia genomes and only one public transcriptome. Here, we applied a mapping-and-removal approach to eliminate known contaminants from previously-obtained Illumina sequencing data. We generated de novo transcriptome assemblies for both female and male E. suzannae which contain 45,986 and 54,762 final coding sequences, respectively. Benchmarking Single-Copy Orthologs results indicate both assemblies are highly complete. Preliminary analyses revealed the presence of homologs of sex-determination genes characterized in other insects and putative venom proteins. Our male and female transcriptomes will be valuable tools to better understand the biology of Encarsia and their evolutionary relatives, particularly in studies involving insects of only one sex.

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.

Safety Evaluation of Chemical Insecticides to Tetrastichus howardi (Hymenoptera: Eulophidae), a Pupal Parasitoid of Spodoptera frugiperda (Lepidoptera: Noctuidae) Using Three Exposure Routes

Spodoptera frugiperda has become a major pest in many crops worldwide. The main control strategies are biological and chemical controls. However, pesticides have varying degrees of toxicity to parasitic wasps in the field. To integrate chemical and biological controls, we evaluated the safety of insecticides to Tetrastichus howardi, an important pupal parasitoid of S. frugiperda. This study assessed the toxicity of six major control insecticides (emamectin benzoate, chlorfenapyr, indoxacarb, chlorantraniliprole, bisultap, and lufenuron) to T. howardi based on risk quotient. The results showed that indoxacarb had the lowest risk quotient (RQ = 7.43). Then the side effects of three sublethal concentrations (LC20, LC30, LC40) of indoxacarb were tested using three methods (1. Adult exposure to pesticide residues on the glass tube; 2. Adult exposure to pesticide residues on the host; 3. Larval exposure to pesticides through host exposure). Overall, T. howardi had a lower parasitism rate and emergence rate with the higher pesticide concentrations. Furthermore, among three methods, the adult exposure to pesticide residues on the glass tube was the most efficient in inhibiting the parasitism rate, and impairing the emergence rate and the offspring female/male ratio. This study guides a more scientific and comprehensive application of pesticides and releases natural enemies in the field.

Silencing Doublesex expression triggers three-level pheromonal feminization in Nasonia vitripennis males

Doublesex (Dsx) has a conserved function in controlling sexual morphological differences in insects, but our knowledge of its role in regulating sexual behaviour is primarily limited to Drosophila. Here, we show with the parasitoid wasp Nasonia vitripennis that males whose Dsx gene had been silenced (NvDsx-i) underwent a three-level pheromonal feminization: (i) NvDsx-i males were no longer able to attract females from a distance, owing to drastically reduced titres of the long-range sex pheromone; (ii) NvDsx-i males were courted by wild-type males as though they were females, which correlated with a lower abundance of alkenes in their cuticular hydrocarbon (CHC) profiles. Supplementation with realistic amounts of synthetic (Z)-9-hentriacontene (Z9C31), the most significantly reduced alkene in NvDsx-i males, to NvDsx-i males interrupted courtship by wild-type conspecific males. Supplementation of female CHC profiles with Z9C31 reduced courtship and mating attempts by wild-type males. These results prove that Z9C31 is crucial for sex discrimination in N. vitripennis; and (iii) Nvdsx-i males were hampered in eliciting female receptivity and thus experienced severely reduced mating success, suggesting that they are unable to produce the to-date unidentified oral aphrodisiac pheromone reported in N. vitripennis males. We conclude that Dsx is a multi-level key regulator of pheromone-mediated sexual communication in N. vitripennis.

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.

Genetic, morphometric, and molecular analyses of interspecies differences in head shape and hybrid developmental defects in the wasp genus Nasonia

Males in the parasitoid wasp genus Nasonia have distinct, species-specific, head shapes. The availability of fertile hybrids among the species, along with obligate haploidy of males, facilitates analysis of complex gene interactions in development and evolution. Previous analyses showed that both the divergence in head shape between Nasonia vitripennis and Nasonia giraulti, and the head-specific developmental defects of F2 haploid hybrid males, are governed by multiple changes in networks of interacting genes. Here, we extend our understanding of the gene interactions that affect morphogenesis in male heads. Use of artificial diploid male hybrids shows that alleles mediating developmental defects are recessive, while there are diverse dominance relationships among other head shape traits. At the molecular level, the sex determination locus doublesex plays a major role in male head shape differences, but it is not the only important factor. Introgression of a giraulti region on chromsome 2 reveals a recessive locus that causes completely penetrant head clefting in both males and females in a vitripennis background. Finally, a third species (N. longicornis) was used to investigate the timing of genetic changes related to head morphology, revealing that most changes causing defects arose after the divergence of N. vitripennis from the other species, but prior to the divergence of N. giraulti and N. longicornis from each other. Our results demonstrate that developmental gene networks can be dissected using interspecies crosses in Nasonia, and set the stage for future fine-scale genetic dissection of both head shape and hybrid developmental defects.

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.

Conservation insights from wild bee genetic studies: Geographic differences, susceptibility to inbreeding, and signs of local adaptation

Conserving bees are critical both ecologically and economically. Genetic tools are valuable for monitoring these vital pollinators since tracking these small, fast-flying insects by traditional means is difficult. By surveying the current state of the literature, this review discusses how recent advances in landscape genetic and genomic research are elucidating how wild bees respond to anthropogenic threats. Current literature suggests that there may be geographic differences in the vulnerability of bee species to landscape changes. Populations of temperate bee species are becoming more isolated and more genetically depauperate as their landscape becomes more fragmented, but tropical bee species appear unaffected. These differences may be an artifact of historical differences in land-use, or it suggests that different management plans are needed for temperate and tropical bee species. Encouragingly, genetic studies on invasive bee species indicate that low levels of genetic diversity may not lead to rapid extinction in bees as once predicted. Additionally, next-generation sequencing has given researchers the power to identify potential genes under selection, which are likely critical to species' survival in their rapidly changing environment. While genetic studies provide insights into wild bee biology, more studies focusing on a greater phylogenetic and life-history breadth of species are needed. Therefore, caution should be taken when making broad conservation decisions based on the currently few species examined.

Evolution of sexual development and sexual dimorphism in insects

Most animal species consist of two distinct sexes. At the morphological, physiological, and behavioral levels the differences between males and females are numerous and dramatic, yet at the genomic level they are often slight or absent. This disconnect is overcome because simple genetic differences or environmental signals are able to direct the sex-specific expression of a shared genome. A canonical picture of how this process works in insects emerged from decades of work on Drosophila. But recent years have seen an explosion of molecular-genetic and developmental work on a broad range of insects. Drawing these studies together, we describe the evolution of sexual dimorphism from a comparative perspective and argue that insect sex determination and differentiation systems are composites of rapidly evolving and highly conserved elements.

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.