Male genital claspers influence female mate acceptance in the stick insect Clitarchus hookeri

Chromosome-level assembly of the brown planthopper genome with a characterized Y chromosome

Hundreds of insect genome sequences have been reported; however, most sequencing projects have not determined the sex chromosomes, and no Y chromosomes from a heterometabolous insect have been identified and characterized to date. The brown planthopper (Nilaparvata lugens Stål) is the most economically damaging pest to rice and is also an ideal research subject for paddy ecology and functional genomics. We previously assembled a draft female genome mainly using second-generation sequencing technologies, with a contig N50 of only 24 kb, due to the large size and excessive repetitive regions in the N. lugens genome. Here, we utilize third-generation sequencing technologies and Hi-C data to generate a high-quality male N. lugens assembly with a contig N50 of 1.01 Mb, a scaffold N50 of 69.96 Mb and more than 95.6% of the assembled bases located on 16 chromosomes. Fourteen autosomes and two sex chromosomes (X + Y) were identified, filling in the gap related to the Y chromosome in heterometabolous insects. A total of 18,021 protein-coding genes and 6423 long-noncoding RNAs were predicted with full-length cDNA sequencing data. All 315 of the Y chromosome genes (Y-genes) were derived from autosomal and X-chromosome duplications. Large-scale RNA interference (RNAi) experiments were conducted against the N. lugens Y-genes, demonstrating that 7 Y-genes were essential for normal BPH development or male organ development, suggesting the importance of Y-genes. The first identified Y chromosome in heterometabolous insects will help gain more insight into sex determination, fertility and chromosome evolution.

Comparative morphology of clasping structures in predator stink bugs (Hemiptera: Pentatomidae: Asopinae): Insights into their function and evolution

Parameres are male genital structures found in many insects which are often used as clasping devices to exert dominance in copula. The asopines have evolved a remarkable additional pair of similar structures, often denominated processes, which combines with the parameres in a tweezers-like system. Processes in similar positions have also been found in other subfamilies of Pentatomidae, but smaller and less developed. Using scanning electron microscopy, we document the among-species variability found in the clasping structures in asopines. We first revealed a vast diversity of ornamentations, such as sensilla and microsculpture. When present, these ornamentations are invariably found on both, the parameres and processes, and often on the corresponding female parts, the valvifers VIII, indicating a functional role of attachment and sensory perception in copula for the ornamentations. We also show that the processes are drastically different between the Asopinae and non-asopines. Therefore, we suggest the term "pseudoclasper" for the Asopinae processes and "superior process of dorsal rim" for the remaining examined taxa. The pseudoclaspers are directly connected to and attached in the same place as the parameres in asopines, while the superior processes of dorsal rim and parameres are completely disconnected in other pentatomids. These results indicate a non-homologous origin between pseudoclaspers and superior processes of dorsal rim in Pentatomidae.

Copulatory function and development shape modular architecture of genitalia differently in males and females

Genitalia are multitasking structures whose development is mediated by numerous regulatory pathways. This multifactorial nature provides an avenue for multiple sources of selection. As a result, genitalia tend to evolve as modular systems comprising semi-independent subsets of structures, yet the processes that give rise to those patterns are still poorly understood. Here, we ask what are the relative roles of development and function in shaping modular patterns of genitalia within populations and across species of stink-bugs. We found that male genitalia are less integrated, more modular, and primarily shaped by functional demands. In contrast, females show higher integration, lower modularity, and a predominant role of developmental processes. Further, interactions among parts of each sex are more determinant to modularity than those between the sexes, and patterns of modularity are equivalent between and within species. Our results strongly indicate that genitalia have been subjected to sex-specific selection, although male and female genitalia are homologous and functionally associated. Moreover, modular patterns are seemingly constant in the evolutionary history of stink-bugs, suggesting a scenario of multivariate stabilizing selection within each sex. Our study demonstrates that interactions among genital parts of the same sex may be more fundamental to genital evolution than previously thought.

How Does the Male Penisfilum Enter the Female Copulatory Pore in Hangingflies?

Hangingflies are characterized by the interesting nuptial feeding behavior and unusual belly-to-belly hanging mating position. However, the mating behavior and the copulatory mechanism remain poorly known for Bittacidae, especially how the elongated male penisfilum enters the copulatory pore of the female. In this study, the mating behavior and copulatory mechanism of Terrobittacus implicatus (Huang and Hua, 2006) were investigated to reveal the functional morphology of hangingfly genitalia. The results show that the male provides a prey as a nuptial gift to the female and twists his abdomen about 180° to form a belly-to-belly hanging mating position. During the penisfilum-entering process, the male epandrial lobes clamp the female subgenital plate with the aid of the female abdomen swelling. Then the male locates the female copulatory pore through his upper branch of the proctiger and inserts his penisfilum into the female spermathecal duct in cooperation with the short setae on the groove of the proctiger. The female subgenital plate where the epandrial lobes clamp is strongly sclerotized and melanized. The copulatory mechanism of Terrobittacus is briefly discussed.

Cretaceous winged stick insects clarify the early evolution of Phasmatodea

Wingless and shorter winged stick insects are very common today, but most known extinct stick insects had fully developed wings, leading to contentious affinities among the extinct winged and extant groups. We report herein three male winged stick insects, assigned to Pterophasmatidae fam. nov., from mid-Cretaceous Myanmar (Burmese) amber. Pterophasmatidae fam. nov. are regarded as transitional taxa from extinct winged to modern wingless and shorter winged stick insects based on their similar tegmina venation with extinct Susumanioidea and some body features the same as extant Phasmatodea. However, their symmetric phallic organs comprising two consistent phallomeres are different from those of all living groups. Phylogenetic analyses suggest that the extinct winged taxa, including the new family, are the stem groups of modern stick and leaf insects, and all of them constitute the clade of Phasmatodea. New findings indicate winged and wingless stick insects' morphologies diversified significantly during or before the mid-Cretaceous.

Divergence in female damselfly sensory structures is consistent with a species recognition function but shows no evidence of reproductive character displacement

Males and females transmit and receive signals prior to mating that convey information such as sex, species identity, or individual condition. In some animals, tactile signals relayed during physical contact between males and females before and during mating appear to be important for mate choice or reproductive isolation. This is common among odonates, when a male grasps a female's thorax with his terminal appendages prior to copulation, and the female subsequently controls whether copulation occurs by bending her abdomen to complete intromission. It has been hypothesized that mechanosensory sensilla on the female thoracic plates mediate mating decisions, but is has been difficult to test this idea. Here, we use North American damselflies in the genus Enallagma (Odonata: Coenagrionidae) to test the hypothesis that variation in female sensilla traits is important for species recognition. Enallagma anna and E. carunculatum hybridize in nature, but experience strong reproductive isolation as a consequence of divergence in male terminal appendage morphology. We quantified several mechanosensory sensilla phenotypes on the female thorax among multiple populations of both species and compared divergence in these traits in sympatry versus allopatry. Although these species differed in features of sensilla distribution within the thoracic plates, we found no strong evidence of reproductive character displacement among the sensilla traits we measured in regions of sympatry. Our results suggest that species-specific placement of female mechanoreceptors may be sufficient for species recognition, although other female sensory phenotypes might have diverged in sympatry to reduce interspecific hybridization.

Exaggerated male legs increase mating success by reducing disturbance to females in the cave wētā Pachyrhamma waitomoensis

Mate guarding is a widespread behaviour resulting from sperm competition and conflict over optimal remating rates. It is a key way in which males exhibit differential mating investment, and represents a complex interplay between mating effort, intrasexual competition, opportunity costs and sexual conflict. Nevertheless, although there are many examples of exaggerated male structures used to fight rivals, few animals have developed specialized male morphological adaptations for directly sheltering females from disturbance by non-rivals. Here we report on the use of sexually dimorphic, elongated male hind legs, which are used to guard females in the New Zealand cave wētā Pachyrhamma waitomoensis (Orthoptera: Rhaphidophoridae). We found that male hind legs alongside the female failed to deter rivals from accessing her or disrupting copulation. However, they did reduce the disturbance to females from other, non-rival animals such as juveniles and heterospecifics. Males with longer hind legs were more effective in reducing disturbance, and remained with females for longer. Longer guarding periods also led to higher numbers of matings between pairs. Models of males with artificially altered hind leg dimensions also showed a benefit to greater leg length, and artificially altering the disturbance rate to females also had a significant effect on pair duration. Our results indicate that nuisance disturbance to females may play an important role in driving sexual selection on male leg length and its exaggeration in this species.

Assembling large genomes: analysis of the stick insect (Clitarchus hookeri) genome reveals a high repeat content and sex-biased genes associated with reproduction

BACKGROUND

Stick insects (Phasmatodea) have a high incidence of parthenogenesis and other alternative reproductive strategies, yet the genetic basis of reproduction is poorly understood. Phasmatodea includes nearly 3000 species, yet only the genome of Timema cristinae has been published to date. Clitarchus hookeri is a geographical parthenogenetic stick insect distributed across New Zealand. Sexual reproduction dominates in northern habitats but is replaced by parthenogenesis in the south. Here, we present a de novo genome assembly of a female C. hookeri and use it to detect candidate genes associated with gamete production and development in females and males. We also explore the factors underlying large genome size in stick insects.

RESULTS

The C. hookeri genome assembly was 4.2 Gb, similar to the flow cytometry estimate, making it the second largest insect genome sequenced and assembled to date. Like the large genome of Locusta migratoria, the genome of C. hookeri is also highly repetitive and the predicted gene models are much longer than those from most other sequenced insect genomes, largely due to longer introns. Miniature inverted repeat transposable elements (MITEs), absent in the much smaller T. cristinae genome, is the most abundant repeat type in the C. hookeri genome assembly. Mapping RNA-Seq reads from female and male gonadal transcriptomes onto the genome assembly resulted in the identification of 39,940 gene loci, 15.8% and 37.6% of which showed female-biased and male-biased expression, respectively. The genes that were over-expressed in females were mostly associated with molecular transportation, developmental process, oocyte growth and reproductive process; whereas, the male-biased genes were enriched in rhythmic process, molecular transducer activity and synapse. Several genes involved in the juvenile hormone synthesis pathway were also identified.

CONCLUSIONS

The evolution of large insect genomes such as L. migratoria and C. hookeri genomes is most likely due to the accumulation of repetitive regions and intron elongation. MITEs contributed significantly to the growth of C. hookeri genome size yet are surprisingly absent from the T. cristinae genome. Sex-biased genes identified from gonadal tissues, including genes involved in juvenile hormone synthesis, provide interesting candidates for the further study of flexible reproduction in stick insects.

Mechanical and tactile incompatibilities cause reproductive isolation between two young damselfly species

External male reproductive structures have received considerable attention as a cause of reproductive isolation (RI), because the morphology of these structures often evolves rapidly between populations. This rapid evolution presents the potential for mechanical incompatibilities with heterospecific female structures during mating and could thus prevent interbreeding between nascent species. Although such mechanical incompatibilities have received little empirical support as a common cause of RI, the potential for mismatch of reproductive structures to cause RI due to incompatible species-specific tactile cues has not been tested. We tested the importance of mechanical and tactile incompatibilities in RI between Enallagma anna and E. carunculatum, two damselfly species that diverged within the past ∼250,000 years and currently hybridize in a sympatric region. We quantified 19 prezygotic and postzygotic RI barriers using both naturally occurring and laboratory-reared damselflies. We found incomplete mechanical isolation between the two pure species and between hybrid males and pure species females. Interestingly, in mating pairs for which mechanical isolation was incomplete, females showed greater resistance and refusal to mate with hybrid or heterospecific males compared to conspecific males. This observation suggests that tactile incompatibilities involving male reproductive structures can influence female mating decisions and form a strong barrier to gene flow in early stages of speciation.