Mechanical incompatibility caused by modifications of multiple male genital structures using genomic introgression in Drosophila

Developmental evolution in fast-forward: insect male genital diversification

Insect male genitalia are among the fastest evolving structures of animals. Studying these changes among closely related species represents a powerful approach to dissect developmental processes and genetic mechanisms underlying phenotypic diversification and the underlying evolutionary drivers. Here, we review recent breakthroughs in understanding the developmental and genetic bases of the evolution of genital organs among Drosophila species and other insects. This work has helped reveal how tissue and organ size evolve and understand the appearance of morphological novelties, and how these phenotypic changes are generated through altering gene expression and redeployment of gene regulatory networks. Future studies of genital evolution in Drosophila and a wider range of insects hold great promise to help understand the specification, differentiation, and diversification of organs more generally.

The development of extremely large male genitalia under spatial limitation

Extensive research in evolutionary biology has focused on the exaggeration of sexual traits; however, the developmental basis of exaggerated sexual traits has only been determined in a few cases. The evolution of exaggerated sexual traits may involve the relaxation of constraints or developmental processes mitigating constraints. Ground beetles in the subgenus Ohomopterus (genus Carabus) have species-specific genitalia that show coevolutionary divergence between the sexes. Here, we examined the morphogenesis of the remarkably enlarged male and female genitalia of Carabus uenoi by X-ray microcomputed tomography. The morphogenetic processes generating the male and female genitalia at the pupal stage were qualitatively similar to those in closely related species with standard genital sizes. Higher growth rates contributed to the exaggeration of both the male and female genital parts of C. uenoi, possibly related to a gene network commonly upregulated in both sexes. Additionally, the length of the copulatory piece (CP), the enlarged male genital part stored in the aedeagus (AD), reached close to that of the AD at the later developmental stages and thereafter decelerated to grow in parallel with the AD, suggesting a structural constraint on the CP by the outer AD. Then, unlike related species, the lengths of the CP and AD increased at eclosion, suggesting a mechanism leading to further elongation of the male genitalia. These observations suggest that a developmental process allows continuous growth of the male genitalia even under the spatial limitation. These results revealed the spatio-temporal dynamics of the development of exaggerated genital structures under structural constraints.

Are males just passive? Coupling mechanism of the Brazilian cave insects with inverted genitalia

Species of the Brazilian cave barklouse genus Neotrogla (Psocodea: "Psocoptera": Trogiomorpha: Prionoglarididae: Sensitibillini) are known to have a "female penis (gynosome)" that functions as an intromittent organ inserted into the membranous pouches in the simple male genital chamber during copulation to receive semen. However, the functions of other male and female genital structures and the copulatory processes of Neotrogla were completely unknown to date. Based on µCT observation of the male and female postabdomen and connected muscles both before and in copula, we clarified the functions of the male and female genital structures. In addition, based on the analyses of the established 3D models, we concluded that precise and rigid contact of multiple genital structures, and step-by-step releases of each holding mechanism achieved by the cooperation of both sexes are involved in the copulatory processes. The coevolution between the male and female genital structures in Neotrogla may provide a new example for the evolution of tolerance traits.

Reproductive isolation via divergent genital morphology due to cascade reinforcement in Ohomopterus ground beetles

Secondary contact between incipient species and selection against maladaptive hybridization can drive reinforcement between populations in contact and result in reproductive character displacement (RCD). Resultant divergence in mating traits within a species may generate downstream reproductive isolation between populations with displaced and non-displaced traits, referred to as the cascade reinforcement hypothesis. We examined this hypothesis using three allopatric populations of the ground beetle Carabus maiyasanus with a genital lock-and-key system. This species shows RCD in male and female genital morphologies in populations in contact with the sister species C. iwawakianus. In a reciprocal mating experiment using three allopatric populations with differences in male and female genital sizes, insemination failure increased as the difference in genital size increased. Based on the reproductive isolation index, insemination failure was the major postmating-prezygotic isolation barrier, at least in one population pair with comparable total isolation to those of other species pairs. By contrast, there was only incomplete premating isolation among populations. These results suggest that RCD in genital morphologies drives incipient allopatric speciation, supporting the cascade reinforcement hypothesis. These findings provide insight into the roles of interspecific interactions and subsequent trait diversification in speciation processes.

Molecular and Developmental Signatures of Genital Size Macro-Evolution in Bugs

Our understanding of the genetic architecture of phenotypic traits has experienced drastic growth over the last years. Nevertheless, the majority of studies associating genotypes and phenotypes have been conducted at the ontogenetic level. Thus, we still have an elusive knowledge of how these genetic-developmental architectures evolve themselves and how their evolution is mirrored in the phenotypic change across evolutionary time. We tackle this gap by reconstructing the evolution of male genital size, one of the most complex traits in insects, together with its underlying genetic architecture. Using the order Hemiptera as a model, spanning over 350 million years of evolution, we estimate the correlation between genitalia and three features: development rate, body size, and rates of DNA substitution in 68 genes associated with genital development. We demonstrate that genital size macro-evolution has been largely dependent on body size and weakly influenced by development rate and phylogenetic history. We further revealed significant correlations between mutation rates and genital size for 19 genes. Interestingly, these genes have diverse functions and participate in distinct signaling pathways, suggesting that genital size is a complex trait whose fast evolution has been enabled by molecular changes associated with diverse morphogenetic processes. Our data further demonstrate that the majority of DNA evolution correlated with the genitalia has been shaped by negative selection or neutral evolution. Thus, in terms of sequence evolution, changes in genital size are predominantly facilitated by relaxation of constraints rather than positive selection, possibly due to the high pleiotropic nature of the morphogenetic genes.

Genital coupling and copulatory wounding in the Drosophila auraria species complex (Diptera: Drosophilidae)

Animal genitalia have changed substantially and rapidly during evolution, and functionally interacting anatomical structures complementarily match between the sexes. Several hypotheses have been proposed to explain how such structure-matching evolved. A test of these hypotheses would require a detailed analysis of male and female genitalia among closely related species and a comparison of the functional aspects of the interacting structures between the sexes. Therefore, here we document genital coupling and copulatory wounds in the four species of the Drosophila auraria complex. The position of the protrusion of the median gonocoxite of males relative to the female terminalia differed among the species, which may reflect differences in protrusion morphology. Species-specific female structures were discovered on the membrane between the genitalia and analia and on the vaginal wall. The former makes contact with the protrusion, and the latter makes contact with appendages of the aedeagus. Copulatory wounds, which are produced during copulation, were seen at three locations on females: depressions near the genital orifice, the membrane between the genitalia and analia, and the vaginal wall. Some of the copulatory wounds were located at sites that could potentially make contact with the species-specific structures that we identified. We speculate that the female structures that differ between species of the D. auraria complex evolved in concert with the genitalia of male conspecifics.

Effects of body size divergence on male mating tactics in the ground beetle Carabus japonicus

Animal body size is involved in reproduction in various ways. Carabus japonicus exhibits considerable variation in adult body size across geographical locations depending on the larval environment. To investigate the effects of body size divergence on male mating traits, spermatophore deposition and weight, copulation duration, and post-copulatory mounting were observed using male-female pairs from C. japonicus populations with different body sizes. Then, variables with high predictive power on the mating traits were identified from individual characteristics. When the male was slightly smaller than his mate, spermatophore deposition likely succeeded, suggesting that mechanical size-assortative insemination determined male body size. Although male reproductive organ size was positively correlated with male body size, spermatophore weight was not significantly affected by male body size, whereas copulation duration decreased with increasing male body size. Enlarged males, with a high capacity for spermatophore production, could increase paternity by decreasing copulation duration and increasing mating frequency. Such shifts in mating tactics would alter selection pressures of intra- and intersexual interactions (e.g., sperm competition and sexual conflict). Genital dimensions also affected mating traits other than copulatory duration. Thus, ecological heterogeneity has the potential to lead to divergences in sexual traits, such as genital morphology, through body size divergence.

Unraveling the Genetic Basis for the Rapid Diversification of Male Genitalia between Drosophila Species

In the last 240,000 years, males of the Drosophila simulans species clade have evolved striking differences in the morphology of their epandrial posterior lobes and claspers (surstyli). These appendages are used for grasping the female during mating and so their divergence is most likely driven by sexual selection. Mapping studies indicate a highly polygenic and generally additive genetic basis for these morphological differences. However, we have limited understanding of the gene regulatory networks that control the development of genital structures and how they evolved to result in this rapid phenotypic diversification. Here, we used new D. simulans/D. mauritiana introgression lines on chromosome arm 3L to generate higher resolution maps of posterior lobe and clasper differences between these species. We then carried out RNA-seq on the developing genitalia of both species to identify the expressed genes and those that are differentially expressed between the two species. This allowed us to test the function of expressed positional candidates during genital development in D. melanogaster. We identified several new genes involved in the development and possibly the evolution of these genital structures, including the transcription factors Hairy and Grunge. Furthermore, we discovered that during clasper development Hairy negatively regulates tartan (trn), a gene known to contribute to divergence in clasper morphology. Taken together, our results provide new insights into the regulation of genital development and how this has evolved between species.

Interspecific introgression reveals a role of male genital morphology during the evolution of reproductive isolation in Drosophila

Rapid divergence in genital structures among nascent species has been posited to be an early‐evolving cause of reproductive isolation, although evidence supporting this idea as a widespread phenomenon remains mixed. Using a collection of interspecific introgression lines between two Drosophila species that diverged approximately 240,000 years ago, we tested the hypothesis that even modest divergence in genital morphology can result in substantial fitness losses. We studied the reproductive consequences of variation in the male epandrial posterior lobes between Drosophila mauritiana and Drosophila sechellia and found that divergence in posterior lobe morphology has significant fitness costs on several prefertilization and postcopulatory reproductive measures. Males with divergent posterior lobe morphology also significantly reduced the life span of their mates. Interestingly, one of the consequences of genital divergence was decreased oviposition and fertilization, which suggests that a sensory bias for posterior lobe morphology could exist in females, and thus, posterior lobe morphology may be the target of cryptic female choice in these species. Our results provide evidence that divergence in genitalia can in fact give rise to substantial reproductive isolation early during species divergence, and they also reveal novel reproductive functions of the external male genitalia in Drosophila.

An Atlas of Transcription Factors Expressed in Male Pupal Terminalia of Drosophila melanogaster

During development, transcription factors and signaling molecules govern gene regulatory networks to direct the formation of unique morphologies. As changes in gene regulatory networks are often implicated in morphological evolution, mapping transcription factor landscapes is important, especially in tissues that undergo rapid evolutionary change. The terminalia (genital and anal structures) of Drosophila melanogaster and its close relatives exhibit dramatic changes in morphology between species. While previous studies have identified network components important for patterning the larval genital disc, the networks governing adult structures during pupal development have remained uncharted. Here, we performed RNA-seq in whole Drosophila melanogaster male terminalia followed by in situ hybridization for 100 highly expressed transcription factors during pupal development. We find that the male terminalia are highly patterned during pupal stages and that specific transcription factors mark separate structures and substructures. Our results are housed online in a searchable database (https://flyterminalia.pitt.edu/) as a resource for the community. This work lays a foundation for future investigations into the gene regulatory networks governing the development and evolution of Drosophila terminalia.

tartan underlies the evolution of Drosophila male genital morphology

The morphology of male genitalia evolves rapidly, probably driven by sexual selection. However, little is known about the genes underlying genitalia differences between species. Identifying these genes is key to understanding how sexual selection acts to produce rapid phenotypic change. We have found that the gene tartan underlies differences between male Drosophila mauritiana and Drosophila simulans in the size and bristle number of the claspers—genital projections that grasp the female during copulation. Moreover, since tartan encodes a protein that is involved in cell interactions, this may represent an alternative developmental mechanism for morphological change. Therefore, our study provides insights into the genetic and developmental bases for the rapid evolution of male genitalia and organ size more generally.

Male genital structures are among the most rapidly evolving morphological traits and are often the only features that can distinguish closely related species. This process is thought to be driven by sexual selection and may reinforce species separation. However, while the genetic bases of many phenotypic differences have been identified, we still lack knowledge about the genes underlying evolutionary differences in male genital organs and organ size more generally. The claspers (surstyli) are periphallic structures that play an important role in copulation in insects. Here, we show that divergence in clasper size and bristle number between Drosophila mauritiana and Drosophila simulans is caused by evolutionary changes in tartan (trn), which encodes a transmembrane leucine-rich repeat domain protein that mediates cell–cell interactions and affinity. There are no fixed amino acid differences in trn between D. mauritiana and D. simulans, but differences in the expression of this gene in developing genitalia suggest that cis-regulatory changes in trn underlie the evolution of clasper morphology in these species. Finally, analyses of reciprocal hemizygotes that are genetically identical, except for the species from which the functional allele of trn originates, determined that the trn allele of D. mauritiana specifies larger claspers with more bristles than the allele of D. simulans. Therefore, we have identified a gene underlying evolutionary change in the size of a male genital organ, which will help to better understand not only the rapid diversification of these structures, but also the regulation and evolution of organ size more broadly.

The evolution of female genitalia

Female genitalia have been largely neglected in studies of genital evolution, perhaps due to the long-standing belief that they are relatively invariable and therefore taxonomically and evolutionarily uninformative in comparison with male genitalia. Contemporary studies of genital evolution have begun to dispute this view, and to demonstrate that female genitalia can be highly diverse and covary with the genitalia of males. Here, we examine evidence for three mechanisms of genital evolution in females: species isolating 'lock-and-key' evolution, cryptic female choice and sexual conflict. Lock-and-key genital evolution has been thought to be relatively unimportant; however, we present cases that show how species isolation may well play a role in the evolution of female genitalia. Much support for female genital evolution via sexual conflict comes from studies of both invertebrate and vertebrate species; however, the effects of sexual conflict can be difficult to distinguish from models of cryptic female choice that focus on putative benefits of choice for females. We offer potential solutions to alleviate this issue. Finally, we offer directions for future studies in order to expand and refine our knowledge surrounding female genital evolution.

A standardized nomenclature and atlas of the male terminalia of Drosophila melanogaster

Animal terminalia represent some of the most diverse and rapidly evolving structures in the animal kingdom, and for this reason have been a mainstay in the taxonomic description of species. The terminalia of Drosophila melanogaster, with its wide range of experimental tools, have recently become the focus of increased interest in the fields of development, evolution, and behavior. However, studies from different disciplines have often used discrepant terminologies for the same anatomical structures. Consequently, the terminology of genital parts has become a barrier to integrating results from different fields, rendering it difficult to determine what parts are being referenced. We formed a consortium of researchers studying the genitalia of D. melanogaster to help establish a set of naming conventions. Here, we present a detailed visual anatomy of male genital parts, including a list of synonymous terms, and suggest practices to avoid confusion when referring to anatomical parts in future studies. The goal of this effort is to facilitate interdisciplinary communication and help newcomers orient themselves within the exciting field of Drosophila genitalia.

Digest: The importance of genital morphology in Drosophila copulation

Drosophila is a common model organism in the study of reproductive isolation. In their 2018 work, Tanaka et al. used introgression to substitute D. mauritiana genomic segments into a D. simulans genetic background, creating lines with modified genital structures. These changes were found to significantly alter the copulation duration and motility of mating pairs by influencing genital coupling.

An innovative ovipositor for niche exploitation impacts genital coevolution between sexes in a fruit-damaging Drosophila

Limited attention has been given to ecological factors influencing the coevolution of male and female genitalia. The innovative ovipositor of Drosophila suzukii, an invading fruit pest, represents an appealing case to document this phenomenon. The serrated saw-like ovipositor is used to pierce the hard skin of ripening fruits that are not used by other fruit flies that prefer soft decaying fruits. Here, we highlight another function of the ovipositor related to its involvement in genital coupling during copulation. We compared the morphology and coupling of male and female genitalia in this species to its sibling species, Drosophila subpulchrella, and to an outgroup species, Drosophila biarmipes These comparisons and a surgical manipulation indicated that the shape of male genitalia in D. suzukii has had to be adjusted to ensure tight coupling, despite having to abandon the use of a hook-like structure, paramere, because of the more linearly elongated ovipositor. This phenomenon demonstrates that ecological niche exploitation can directly affect the mechanics of genital coupling and potentially cause incompatibility among divergent forms. This model case provides new insights towards elucidating the importance of the dual functions of ovipositors in other insect species that potentially induce genital coevolution and ecological speciation.