Development of the genitalia in Drosophila melanogaster

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 developmental and genetic basis of male genitalia evolution in Drosophilids

Reproductive organs are among the most variable and rapidly evolving structures in the animal kingdom, probably due to sexual selection. In insects, the diverse morphology of male genitalia is often one of the few visible characteristics that can reliably distinguish closely related species, making it crucial for taxonomic classification. Consistent with this, males of the model organism Drosophila melanogaster and its closely related species display remarkable variations in genital morphology. This variation has made the male genitalia of Drosophilids an invaluable system for dissecting the genetic and developmental pathways responsible for morphological evolution, providing insights into how new structures emerge and how gene regulatory networks are co-opted during this process. In this review, we highlight recent studies that have uncovered developmental processes, novel genes, and regulatory networks that contribute to the morphogenesis and evolution of these extraordinary structures. These studies mark a significant advancement in our understanding of the mechanisms driving the evolution of complex organs.

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.

Male genitalia, hierarchical homology, and the anatomy of the bullet ant (Paraponera clavata; Hymenoptera, Formicidae)

The male genitalia of insects are among the most variable, complex, and informative character systems for evolutionary analysis and taxonomic purposes. Because of these general properties, many generations of systematists have struggled to develop a theory of homology and alignment of parts. This struggle continues to the present day, where fundamentally different models and nomenclatures for the male genitalia of Hymenoptera, for example, are applied. Here, we take a multimodal approach to digitalize and comprehensively document the genital skeletomuscular anatomy of the bullet ant (Paraponera clavata; Hymenoptera: Formicidae), including hand dissection, synchrotron radiation microcomputed tomography, microphotography, scanning electron microscopy, confocal laser scanning microscopy, and 3D-printing. Through this work, we generate several new concepts for the structure and form of the male genitalia of Hymenoptera, such as for the endophallic sclerite (=fibula ducti), which we were able to evaluate in detail for the first time for any species. Based on this phenomic anatomical study and comparison with other Holometabola and Hexapoda, we reconsider the homologies of insect genitalia more broadly, and propose a series of clarifications in support of the penis-gonopod theory of male genital identity. Specifically, we use the male genitalia of Paraponera and insects more broadly as an empirical case for hierarchical homology by applying and refining the 5-category classification of serial homologs from DiFrisco et al. (2023) (DLW23) to all of our formalized concepts. Through this, we find that: (1) geometry is a critical attribute to account for in ontology, especially as all individually identifiable attributes are positionally indexed hence can be recognized as homomorphic; (2) the definition of "structure" proposed by DLW23 is difficult to apply, and likely heterogeneous; and (3) formative elements, or spatially defined foldings or in- or evaginations of the epidermis and cuticle, are an important yet overlooked class of homomorphs. We propose a morphogenetic model for male and female insect genitalia, and a model analogous to gene-tree species-tree mappings for the hierarchical homology of male genitalia specifically. For all of the structures evaluated in the present study, we provide 3D-printable models - with and without musculature, and in various states of digital dissection - to facilitate the development of a tactile understanding. Our treatment of the male genitalia of P. clavata serves as a basic template for future phenomic studies of male insect genitalia, which will be substantially improved with the development of automation and collections-based data processing pipelines, that is, collectomics. The Hymenoptera Anatomy Ontology will be a critical resource to include in this effort, and in best practice concepts should be linked.

A developmental atlas of male terminalia across twelve species of Drosophila

How complex morphologies evolve is one of the central questions in evolutionary biology. Observing the morphogenetic events that occur during development provides a unique perspective on the origins and diversification of morphological novelty. One can trace the tissue of origin, emergence, and even regression of structures to resolve murky homology relationships between species. Here, we trace the developmental events that shape some of the most diverse organs in the animal kingdom-the male terminalia (genitalia and analia) of Drosophilids. Male genitalia are known for their rapid evolution with closely related species of the Drosophila genus demonstrating vast variation in their reproductive morphology. We used confocal microscopy to monitor terminalia development during metamorphosis in twelve related species of Drosophila. From this comprehensive dataset, we propose a new staging scheme for pupal terminalia development based on shared developmental landmarks, which allows one to align developmental time points between species. We were able to trace the origin of different substructures, find new morphologies and suggest possible homology of certain substructures. Additionally, we demonstrate that posterior lobe is likely originated prior to the split between the Drosophila melanogaster and the Drosophila yakuba clade. Our dataset opens up many new directions of research and provides an entry point for future studies of the Drosophila male terminalia evolution and development.

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

The model organism Drosophila melanogaster has become a focal system for investigations of rapidly evolving genital morphology as well as the development and functions of insect reproductive structures. To follow up on a previous paper outlining unifying terminology for the structures of the male terminalia in this species, we offer here a detailed description of the female terminalia of D. melanogaster. Informative diagrams and micrographs are presented to provide a comprehensive overview of the external and internal reproductive structures of females. We propose a collection of terms and definitions to standardize the terminology associated with the female terminalia in D. melanogaster and we provide a correspondence table with the terms previously used. Unifying terminology for both males and females in this species will help to facilitate communication between various disciplines, as well as aid in synthesizing research across publications within a discipline that has historically focused principally on male features. Our efforts to refine and standardize the terminology should expand the utility of this important model system for addressing questions related to the development and evolution of animal genitalia, and morphology in general.

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.

Filopodia-based contact stimulation of cell migration drives tissue morphogenesis

Cells migrate collectively to form tissues and organs during morphogenesis. Contact inhibition of locomotion (CIL) drives collective migration by inhibiting lamellipodial protrusions at cell-cell contacts and promoting polarization at the leading edge. Here, we report a CIL-related collective cell behavior of myotubes that lack lamellipodial protrusions, but instead use filopodia to move as a cohesive cluster in a formin-dependent manner. We perform genetic, pharmacological and mechanical perturbation analyses to reveal the essential roles of Rac2, Cdc42 and Rho1 in myotube migration. These factors differentially control protrusion dynamics and cell-matrix adhesion formation. We also show that active Rho1 GTPase localizes at retracting free edge filopodia and that Rok-dependent actomyosin contractility does not mediate a contraction of protrusions at cell-cell contacts, but likely plays an important role in the constriction of supracellular actin cables. Based on these findings, we propose that contact-dependent asymmetry of cell-matrix adhesion drives directional movement, whereas contractile actin cables contribute to the integrity of the migrating cell cluster.

Specification and Patterning of Drosophila Appendages

Appendages are external projections of the body that serve the animal for locomotion, feeding, or environment exploration. The appendages of the fruit fly Drosophilamelanogaster are derived from the imaginal discs, epithelial sac-like structures specified in the embryo that grow and pattern during larva development. In the last decades, genetic and developmental studies in the fruit fly have provided extensive knowledge regarding the mechanisms that direct the formation of the appendages. Importantly, many of the signaling pathways and patterning genes identified and characterized in Drosophila have similar functions during vertebrate appendage development. In this review, we will summarize the genetic and molecular mechanisms that lead to the specification of appendage primordia in the embryo and their posterior patterning during imaginal disc development. The identification of the regulatory logic underlying appendage specification in Drosophila suggests that the evolutionary origin of the insect wing is, in part, related to the development of ventral appendages.

Myotube migration to cover and shape the testis of Drosophila depends on Heartless, Cadherin/Catenin, and myosin II

During Drosophila metamorphosis, nascent testis myotubes migrate from the prospective seminal vesicle of the genital disc onto pupal testes and then further to cover the testes with multinucleated smooth-like muscles. Here we show that DWnt2 is likely required for determination of testis-relevant myoblasts on the genital disc. Knock down of fibroblast growth factor receptor (FGFR) heartless by RNAi and a dominant-negative version revealed multiple functions of Heartless, namely regulation of the amount of myoblasts on the genital disc, connection of seminal vesicles and testes, and migration of muscles along the testes. Live imaging indicated that the downstream effector Stumps is required for migration of testis myotubes on the testis towards the apical tip. After myoblast fusion, myosin II is needed for migration of nascent testis myotubes, in which Thisbe-dependent fibroblast growth factor (FGF) signaling is activated. Cadherin-N is essential for connecting these single myofibers and for creating a firm testis muscle sheath that shapes and stabilizes the testis tubule. Based on these results, we propose a model for the migration of testis myotubes in which nascent testis myotubes migrate as a collective onto and along the testis, dependent on FGF-regulated expression of myosin II.

Summary:Drosophila testes and mammalian seminiferous tubules are surrounded by a muscle layer. Drosophila myotubes migrate towards testes in dependence of the FGF receptor Heartless, myosin II and Cadherin-N.

TGF-β Family Signaling in Drosophila

The transforming growth factor β (TGF-β) family signaling pathway is conserved and ubiquitous in animals. In Drosophila, fewer representatives of each signaling component are present compared with vertebrates, simplifying mechanistic study of the pathway. Although there are fewer family members, the TGF-β family pathway still regulates multiple and diverse functions in Drosophila. In this review, we focus our attention on several of the classic and best-studied functions for TGF-β family signaling in regulating Drosophila developmental processes such as embryonic and imaginal disc patterning, but we also describe several recently discovered roles in regulating hormonal, physiological, neuronal, innate immunity, and tissue homeostatic processes.

The legacy of Drosophila imaginal discs

The study of Drosophila imaginal discs has contributed to a number of discoveries in developmental and cellular biology. In addition to the elucidation of the role of tissue compartments and organ-specific master regulator genes during development, imaginal discs have also become well established as models for studying cellular interactions and complex genetic pathways. Here, we review key discoveries resulting from investigations of these epithelial precursor organs, ranging from cell fate determination and transdetermination to tissue patterning. Furthermore, the design of increasingly sophisticated genetic tools over the last decades has added value to the use of imaginal discs as model systems. As a result of tissue-specific genetic screens, several components of developmentally regulated signaling pathways were identified and epistasis revealed the levels at which they function. Discs have been widely used to assess cellular interactions in their natural tissue context, contributing to a better understanding of growth regulation, tissue regeneration, and cancer. With the continuous implementation of novel tools, imaginal discs retain significant potential as model systems to address emerging questions in biology and medicine.

Intersex (ix) mutations of Drosophila melanogaster cause nonrandom cell death in genital disc and can induce tumours in genitals in response to decapentaplegic (dpp(disk)) mutations

In Drosophila melanogaster, the intersex (ix) is a terminally positioned gene in somatic sex determination hierarchy and function with the female specific product of double sex (DSX(F)) to implement female sexual differentiation. The null phenotype of ix is to transform diplo-X individuals into intersexes while leaving haplo-X animals unaffected. This study on the effect of different intersex mutations on genital disc development provides the following major results: (i) similar range of a characteristic array of morphological structures (from almost double sex terminalia to extreme reduction of terminal appendages) was displayed by the terminalia of XX ix(1)/ix(1) , XX ix(2)/ix(2) and XX ix(5)/ix(5) individuals; (ii) an increased number of apoptotic cells were found to occur in a localized manner in mature third instar larval genital discs of ix individuals; (iii) ix mutations can induce high frequency of neoplastic tumours in genitals in the presence of decapentaplegic (dpp(disk)) mutations; and (iv) heteroallelic combinations of dpp(disk) mutations can also induce tumours in intersex genitals with variable expressivity. On the basis of these findings, we suggest that: (i) loss of function of ix causes massive cell death in both male and female genital primordia of genital discs, resulting phenotype mimicking in male and female characteristics in genitals; and (ii) at the discs, the apoptotic cells persist as 'undead' cells that can induce oncogenic transformation in the neighbouring disc cells when dpp signalling is blocked or reduced by dpp(disk) mutations.

A new level of plasticity: Drosophila smooth-like testes muscles compensate failure of myoblast fusion

The testis of Drosophila resembles an individual testis tubule of mammals. Both are surrounded by a sheath of smooth muscles, which in Drosophila are multinuclear and originate from a pool of myoblasts that are set aside in the embryo and accumulate on the genital disc later in development. These muscle stem cells start to differentiate early during metamorphosis and give rise to all muscles of the inner male reproductive system. Shortly before the genital disc and the developing testes connect, multinuclear nascent myotubes appear on the anterior tips of the seminal vesicles. Here, we show that adhesion molecules are distinctly localized on the seminal vesicles; founder cell (FC)-like myoblasts express Dumbfounded (Duf) and Roughest (Rst), and fusion-competent myoblast (FCM)-like cells mainly express Sticks and stones (Sns). The smooth but multinuclear myotubes of the testes arose by myoblast fusion. RNAi-mediated attenuation of Sns or both Duf and Rst severely reduced the number of nuclei in the testes muscles. Duf and Rst probably act independently in this context. Despite reduced fusion in all of these RNAi-treated animals, myotubes migrated onto the testes, testes were shaped and coiled, muscle filaments were arranged as in the wild type and spermatogenesis proceeded normally. Hence, the testes muscles compensate for fusion defects so that the myofibres encircling the adult testes are indistinguishable from those of the wild type and male fertility is guaranteed.

Summary:Drosophila testes muscles arise from stem cells and can compensate for fusion defects to safeguard fertility; this plasticity may compensate for the observed lack of satellite cells in Drosophila.

A Flippase-Mediated GAL80/GAL4 Intersectional Resource for Dissecting Appendage Development in Drosophila

Drosophila imaginal discs provide an ideal model to study processes important for cell signaling and cell specification, tissue differentiation, and cell competition during development. One challenge to understanding genetic control of cellular processes and cell interactions is the difficulty in effectively targeting a defined subset of cells in developing tissues in gene manipulation experiments. A recently developed Flippase-induced intersectional GAL80/GAL4 repression method incorporates several gene manipulation technologies in Drosophila to enable such fine-scale dissection in neural tissues. In particular, this approach brings together existing GAL4 transgenes, newly developed enhancer-trap flippase transgenes, and GAL80 transgenes flanked by Flippase recognition target sites. The combination of these tools enables gene activation/repression in particular subsets of cells within a GAL4 expression pattern. Here, we expand the utility of a large collection of these enhancer-trap flippase transgenic insertion lines by characterizing their expression patterns in third larval instar imaginal discs. We screened 521 different enhancer-trap flippase lines and identified 28 that are expressed in imaginal tissues, including two transgenes that show sex-specific expression patterns. Using a line that expresses Flippase in the wing imaginal disc, we demonstrate the utility of this intersectional approach for studying development by knocking down gene expression of a key member of the planar cell polarity pathway. The results of our experiments show that these enhancer-trap flippase lines enable fine-scale manipulation in imaginal discs.

Dpp-expressing and non-expressing cells: two different populations of growing cells in Drosophila

There are different models that explain growth during development. One model is based on insect and amphibian regeneration studies. This model proposes that growth is directed by pattern, and growth takes place by intercalation at a growth discontinuity; therefore, proliferation should surround the discontinuity. Currently, this model, apart from regenerative studies on mostly adult patterning, has not found supporting evidence in Drosophila that shows proliferation surrounding a discontinuity. Despite this lack of evidence, the importance of discontinuities has been shown in different experiments, even under wt conditions, more specifically in the formation of the leg joints because of the occurrence of cell death at their boundaries. Here, we show the existence of a sharp discontinuity in Decapentaplegic (Dpp) in the genital discs at the third larvae stage (L3), which determines the upregulation in the Jun-NH2-Terminal-Kinase (JNK) pathway, reaper (rpr), head involution defective (hid) and active caspases from its boundaries. The proliferation and cell death surrounding the discontinuity suggest that growth can proceed by intercalation and competitive death takes place in this area. Finally, we show that the Rpr, Grim and Hid (RGH) products are a few of the factors that define the growth discontinuity because they are negative regulators of growth, a new function that is unique from their known functions in apoptosis.

Genetic architecture and functional characterization of genes underlying the rapid diversification of male external genitalia between Drosophila simulans and Drosophila mauritiana

Male sexual characters are often among the first traits to diverge between closely related species and identifying the genetic basis of such changes can contribute to our understanding of their evolutionary history. However, little is known about the genetic architecture or the specific genes underlying the evolution of male genitalia. The morphology of the claspers, posterior lobes, and anal plates exhibit striking differences between Drosophila mauritiana and D. simulans. Using QTL and introgression-based high-resolution mapping, we identified several small regions on chromosome arms 3L and 3R that contribute to differences in these traits. However, we found that the loci underlying the evolution of clasper differences between these two species are independent from those that contribute to posterior lobe and anal plate divergence. Furthermore, while most of the loci affect each trait in the same direction and act additively, we also found evidence for epistasis between loci for clasper bristle number. In addition, we conducted an RNAi screen in D. melanogaster to investigate if positional and expression candidate genes located on chromosome 3L, are also involved in genital development. We found that six of these genes, including components of Wnt signaling and male-specific lethal 3 (msl3), regulate the development of genital traits consistent with the effects of the introgressed regions where they are located and that thus represent promising candidate genes for the evolution these traits.

Sex- and tissue-specific functions of Drosophila doublesex transcription factor target genes

Primary sex-determination "switches" evolve rapidly, but Doublesex (DSX)-related transcription factors (DMRTs) act downstream of these switches to control sexual development in most animal species. Drosophila dsx encodes female- and male-specific isoforms (DSX(F) and DSX(M)), but little is known about how dsx controls sexual development, whether DSX(F) and DSX(M) bind different targets, or how DSX proteins direct different outcomes in diverse tissues. We undertook genome-wide analyses to identify DSX targets using in vivo occupancy, binding site prediction, and evolutionary conservation. We find that DSX(F) and DSX(M) bind thousands of the same targets in multiple tissues in both sexes, yet these targets have sex- and tissue-specific functions. Interestingly, DSX targets show considerable overlap with targets identified for mouse DMRT1. DSX targets include transcription factors and signaling pathway components providing for direct and indirect regulation of sex-biased expression.

Transcriptomics of two evolutionary novelties: how to make a sperm-transfer organ out of an anal fin and a sexually selected "sword" out of a caudal fin

Swords are exaggerated male ornaments of swordtail fishes that have been of great interest to evolutionary biologists ever since Darwin described them in the Descent of Man (1871). They are a novel sexually selected trait derived from modified ventral caudal fin rays and are only found in the genus Xiphophorus. Another phylogenetically more widespread and older male trait is the gonopodium, an intromittent organ found in all poeciliid fishes, that is derived from a modified anal fin. Despite many evolutionary and behavioral studies on both traits, little is known so far about the molecular mechanisms underlying their development. By investigating transcriptomic changes (utilizing a RNA-Seq approach) in response to testosterone treatment in the swordtail fish, Xiphophorus hellerii, we aimed to better understand the architecture of the gene regulatory networks underpinning the development of these two evolutionary novelties. Large numbers of genes with tissue-specific expression patterns were identified. Among the "sword genes" those involved in embryonic organ development, sexual character development and coloration were highly expressed, while in the gonopodium rather more morphogenesis-related genes were found. Interestingly, many genes and genetic pathways are shared between both developing novel traits derived from median fins: the sword and the gonopodium. Our analyses show that a larger set of gene networks was co-opted during the development and evolution of the "older" gonopodium than in the "younger," and morphologically less complex trait, the sword. We provide a catalog of candidate genes for future efforts to dissect the development of those sexually selected exaggerated male traits in swordtails.

Ectopic expression of the male BmDSX affects formation of the chitin plate in female Bombyx mori

Mating structures are involved in successful copulation, intromission, and/or insemination. These structures enable tight coupling between external genitalia of two sexes. During Bombyx mori copulation, the double harpagones in the external genitalia of males clasp the female chitin plate, which is derived from the larval eighth abdominal segment; abnormal development of the female chitin plate affects copulation. We report that ERK phosphorylation (p-ERK) and expression of Abdominal-B (Abd-B) in the posterior abdomen of the female adult is lower than in the male. Ectopic expression of the male-specific spliced form of B. mori doublesex (Bmdsx(M)) in females, however, up-regulates Abd-B and spitz (spi) expression, increasing EGFR signaling activity, and thus forming an abnormal chitin plate and reduced female copulation. These findings indicate that Bmdsx affects the development of the eighth abdominal segment by regulating the activity of EGFR signaling and the expression of Abd-B, resulting in an extra eighth abdominal segment (A8) in males versus the loss of this segment in adult females.

A survey of 6,300 genomic fragments for cis-regulatory activity in the imaginal discs of Drosophila melanogaster

Over 6,000 fragments from the genome of Drosophila melanogaster were analyzed for their ability to drive expression of GAL4 reporter genes in the third-instar larval imaginal discs. About 1,200 reporter genes drove expression in the eye, antenna, leg, wing, haltere, or genital imaginal discs. The patterns ranged from large regions to individual cells. About 75% of the active fragments drove expression in multiple discs; 20% were expressed in ventral, but not dorsal, discs (legs, genital, and antenna), whereas ∼23% were expressed in dorsal but not ventral discs (wing, haltere, and eye). Several patterns, for example, within the leg chordotonal organ, appeared a surprisingly large number of times. Unbiased searches for DNA sequence motifs suggest candidate transcription factors that may regulate enhancers with shared activities. Together, these expression patterns provide a valuable resource to the community and offer a broad overview of how transcriptional regulatory information is distributed in the Drosophila genome.

NR5A nuclear receptor Hr39 controls three-cell secretory unit formation in Drosophila female reproductive glands

BACKGROUND

Secretions within the adult female reproductive tract mediate sperm survival, storage, activation, and selection. Drosophila female reproductive gland secretory cells reside within the adult spermathecae and parovaria, but their development remains poorly characterized.

RESULTS

With cell-lineage tracing, we found that precursor cells downregulate lozenge and divide stereotypically to generate three-cell secretory units during pupal development. The NR5A-class nuclear hormone receptor Hr39 is essential for precursor cell division and secretory unit formation. Moreover, ectopic Hr39 in multiple tissues generates reproductive gland-like primordia. Rarely, in male genital discs these primordia can develop into sperm-filled testicular spermathecae.

CONCLUSION

Drosophila spermathecae provide a powerful model for studying gland development. Hr39 functions as a master regulator of a program that may have been conserved throughout animal evolution for the production of female reproductive glands and other secretory tissues.

Shape--but not size--codivergence between male and female copulatory structures in Onthophagus beetles

Genitalia are among the fastest evolving morphological traits in arthropods. Among the many hypotheses aimed at explaining this observation, some explicitly or implicitly predict concomitant male and female changes of genital traits that interact during copulation (i.e., lock and key, sexual conflict, cryptic female choice and pleiotropy). Testing these hypotheses requires insights into whether male and female copulatory structures that physically interact during mating also affect each other's evolution and patterns of diversification. Here we compare and contrast size and shape evolution of male and female structures that are known to interact tightly during copulation using two model systems: (a) the sister species O. taurus (1 native, 3 recently established populations) and O. illyricus, and (b) the species-complex O. fracticornis-similis-opacicollis. Partial Least Squares analyses indicated very little to no correlation between size and shape of copulatory structures, both in males and females. Accordingly, comparing shape and size diversification patterns of genitalia within each sex showed that the two components diversify readily - though largely independently of each other - within and between species. Similarly, comparing patterns of divergence across sexes showed that relative sizes of male and female copulatory organs diversify largely independent of each other. However, performing this analysis for genital shape revealed a signature of parallel divergence. Our results therefore suggest that male and female copulatory structures that are linked mechanically during copulation may diverge in concert with respect to their shapes. Furthermore, our results suggest that genital divergence in general, and co-divergence of male and female genital shape in particular, can evolve over an extraordinarily short time frame. Results are discussed in the framework of the hypotheses that assume or predict concomitant evolutionary changes in male and female copulatory organs.

The genetic basis of rapidly evolving male genital morphology in Drosophila

The external genitalia are some of the most rapidly evolving morphological structures in insects. The posterior lobe of the male genital arch shows striking differences in both size and shape among closely related species of the Drosophila melanogaster species subgroup. Here, we dissect the genetic basis of posterior lobe morphology between D. mauritiana and D. sechellia, two island endemic species that last shared a common ancestor ∼300,000 years ago. We test a large collection of genome-wide homozygous D. mauritiana genetic introgressions, which collectively cover ∼50% of the genome, for their morphological effects when placed in a D. sechellia genetic background. We find several introgressions that have large effects on posterior lobe morphology and that posterior lobe size and posterior lobe shape can be separated genetically for some of the loci that specify morphology. Using next generation sequencing technology, we perform whole transcriptome gene expression analyses of the larval genital imaginal disc of D. mauritiana, D. sechellia, and two D. mauritiana-D. sechellia hybrid introgression genotypes that each have large effects on either posterior lobe size or posterior lobe shape. Many of the genes we identify as differentially expressed are expressed at levels similar to D. mauritiana in one introgression hybrid, but are expressed at levels similar to D. sechellia in the other introgression hybrid. However, we also find that both introgression hybrids express some of the same genes at levels similar to D. mauritiana, and notably, that both introgression hybrids possess genes in the insulin receptor signaling pathway, which are expressed at D. mauritiana expression levels. These results suggest the possibility that the insulin signaling pathway might integrate size and shape genetic inputs to establish differences in overall posterior lobe morphology between D. mauritiana and D. sechellia.

The female-specific doublesex isoform regulates pleiotropic transcription factors to pattern genital development in Drosophila

Regulatory networks driving morphogenesis of animal genitalia must integrate sexual identity and positional information. Although the genetic hierarchy that controls somatic sexual identity in the fly Drosophila melanogaster is well understood, there are very few cases in which the mechanism by which it controls tissue-specific gene activity is known. In flies, the sex-determination hierarchy terminates in the doublesex (dsx) gene, which produces sex-specific transcription factors via alternative splicing of its transcripts. To identify sex-specifically expressed genes downstream of dsx that drive the sexually dimorphic development of the genitalia, we performed genome-wide transcriptional profiling of dissected genital imaginal discs of each sex at three time points during early morphogenesis. Using a stringent statistical threshold, we identified 23 genes that have sex-differential transcript levels at all three time points, of which 13 encode transcription factors, a significant enrichment. We focus here on three sex-specifically expressed transcription factors encoded by lozenge (lz), Drop (Dr) and AP-2. We show that, in female genital discs, Dsx activates lz and represses Dr and AP-2. We further show that the regulation of Dr by Dsx mediates the previously identified expression of the fibroblast growth factor Branchless in male genital discs. The phenotypes we observe upon loss of lz or Dr function in genital discs explain the presence or absence of particular structures in dsx mutant flies and thereby clarify previously puzzling observations. Our time course of expression data also lays the foundation for elucidating the regulatory networks downstream of the sex-specifically deployed transcription factors.

Apoptosis controls the speed of looping morphogenesis in Drosophila male terminalia

In metazoan development, the precise mechanisms that regulate the completion of morphogenesis according to a developmental timetable remain elusive. The Drosophila male terminalia is an asymmetric looping organ; the internal genitalia (spermiduct) loops dextrally around the hindgut. Mutants for apoptotic signaling have an orientation defect of their male terminalia, indicating that apoptosis contributes to the looping morphogenesis. However, the physiological roles of apoptosis in the looping morphogenesis of male terminalia have been unclear. Here, we show the role of apoptosis in the organogenesis of male terminalia using time-lapse imaging. In normal flies, genitalia rotation accelerated as development proceeded, and completed a full 360° rotation. This acceleration was impaired when the activity of caspases or JNK or PVF/PVR signaling was reduced. Acceleration was induced by two distinct subcompartments of the A8 segment that formed a ring shape and surrounded the male genitalia: the inner ring rotated with the genitalia and the outer ring rotated later, functioning as a 'moving walkway' to accelerate the inner ring rotation. A quantitative analysis combining the use of a FRET-based indicator for caspase activation with single-cell tracking showed that the timing and degree of apoptosis correlated with the movement of the outer ring, and upregulation of the apoptotic signal increased the speed of genital rotation. Therefore, apoptosis coordinates the outer ring movement that drives the acceleration of genitalia rotation, thereby enabling the complete morphogenesis of male genitalia within a limited developmental time frame.

Developmental decoupling of alternative phenotypes: insights from the transcriptomes of horn-polyphenic beetles

Developmental mechanisms play an important role in determining the costs, limits, and evolutionary consequences of phenotypic plasticity. One issue central to these claims is the hypothesis of developmental decoupling, where alternate morphs result from evolutionarily independent developmental pathways. We address this assumption through a microarray study that tests whether differences in gene expression between alternate morphs are as divergent as those between sexes, a classic example of developmental decoupling. We then examine whether genes with morph-biased expression are less conserved than genes with shared expression between morphs, as predicted if developmental decoupling relaxes pleiotropic constraints on divergence. We focus on the developing horns and brains of two species of horned beetles with impressive sexual- and morph-dimorphism in the expression of horns and fighting behavior. We find that patterns of gene expression were as divergent between morphs as they were between sexes. However, overall patterns of gene expression were also highly correlated across morphs and sexes. Morph-biased genes were more evolutionarily divergent, suggesting a role of relaxed pleiotropic constraints or relaxed selection. Together these results suggest that alternate morphs are to some extent developmentally decoupled, and that this decoupling has significant evolutionary consequences. However, alternative morphs may not be as developmentally decoupled as sometimes assumed and such hypotheses of development should be revisited and refined.

Coupling of apoptosis and L/R patterning controls stepwise organ looping

Handed asymmetry in organ shape and positioning is a common feature among bilateria, yet little is known about the morphogenetic mechanisms underlying left-right (LR) organogenesis. We utilize the directional 360° clockwise rotation of genitalia in Drosophila to study LR-dependent organ looping. Using time-lapse imaging, we show that rotation of genitalia by 360° results from an additive process involving two ring-shaped domains, each undergoing 180° rotation. Our results show that the direction of rotation for each ring is autonomous and strictly depends on the LR determinant myosin ID (MyoID). Specific inactivation of MyoID in one domain causes rings to rotate in opposite directions and thereby cancels out the overall movement. We further reveal a specific pattern of apoptosis at the ring boundaries and show that local cell death is required for the movement of each domain, acting as a brake-releaser. These data indicate that organ looping can proceed through an incremental mechanism coupling LR determination and apoptosis. Furthermore, they suggest a model for the stepwise evolution of genitalia posture in Diptera, through the emergence and duplication of a 180° LR module.

The Drosophila serine protease homologue Scarface regulates JNK signalling in a negative-feedback loop during epithelial morphogenesis

In Drosophila melanogaster, dorsal closure is a model of tissue morphogenesis leading to the dorsal migration and sealing of the embryonic ectoderm. The activation of the JNK signal transduction pathway, specifically in the leading edge cells, is essential to this process. In a genome-wide microarray screen, we identified new JNK target genes during dorsal closure. One of them is the gene scarface (scaf), which belongs to the large family of trypsin-like serine proteases. Some proteins of this family, like Scaf, bear an inactive catalytic site, representing a subgroup of serine protease homologues (SPH) whose functions are poorly understood. Here, we show that scaf is a general transcriptional target of the JNK pathway coding for a secreted SPH. scaf loss-of-function induces defects in JNK-controlled morphogenetic events such as embryonic dorsal closure and adult male terminalia rotation. Live imaging of the latter process reveals that, like for dorsal closure, JNK directs the dorsal fusion of two epithelial layers in the pupal genital disc. Genetic data show that scaf loss-of-function mimics JNK over-activity. Moreover, scaf ectopic expression aggravates the effect of the JNK negative regulator puc on male genitalia rotation. We finally demonstrate that scaf acts as an antagonist by negatively regulating JNK activity. Overall, our results identify the SPH-encoding gene scaf as a new transcriptional target of JNK signalling and reveal the first secreted regulator of the JNK pathway acting in a negative-feedback loop during epithelial morphogenesis.

Left-right asymmetry in Drosophila

Seminal studies of left-right (L/R) patterning in vertebrate models have led to the discovery of roles for the nodal pathway, ion flows and cilia in this process. Although the molecular mechanisms underlying L/R asymmetries seen in protostomes are less well understood, recent work using Drosophila melanogaster as a novel genetic model system to study this process has identified a number of mutations affecting directional organ looping. The genetic analysis of this, the most evolutionary conserved feature of L/R patterning, revealed the existence of a L/R pathway that involves the actin cytoskeleton and an associated type I myosin. In this review, we describe this work in the context of Drosophila development, and discuss the implications of these results for our understanding of L/R patterning in general.

The creation of sexual dimorphism in the Drosophila soma

Animals have evolved a fascinating array of mechanisms for conducting sexual reproduction. These include producing the sex-specific gametes, as well as mechanisms for attracting a mate, courting a mate, and getting the gametes together. These processes require that males and females take on dramatically different forms (sexual dimorphism). Here, we will explore the problem of how sex is determined in Drosophila, and pay particular attention to how information about sexual identity is used to instruct males and females to develop differently. Along the way, we will highlight new work that challenges some of the traditional views about sex determination. In Drosophila, it is commonly thought that every cell decides its own sex based on its sex chromosome constitution (XX vs. XY). However, we now know that many cell types undergo nonautonomous sex determination, where they are told what sex to be through signals from surrounding cells, independent of their own chromosomal content. Further, it now appears that not all cells even "know" their sex, since key members of the sex determination pathway are not expressed in all cells. Thus, our understanding of how sex is determined, and how sexual identity is used to create sexual dimorphism, has changed considerably.

Allocation and specification of the genital disc precursor cells in Drosophila

The adult structures of Drosophila melanogaster are derived from larval imaginal discs, which originate as clusters of cells within the embryonic ectoderm. The genital imaginal disc is composed of three primordia (female genital, male genital, and anal primordia) that originate from the embryonic tail segments A8, A9, and A10, respectively, and produce the sexually dimorphic genitalia and analia. We show that the genital disc precursor cells (GDPCs) are first detectable during mid-embryogenesis as a 22-cell cluster in the ventral epidermis. Analysis of mutant and double mutant phenotypes of embryonic patterning genes in the GDPCs, together with their expression patterns in these cells, revealed the following with respect to the origins and specification of the GDPCs. The allocation of the GDPCs from the ventral epidermis requires the function of ventral patterning genes, including the EGF receptor and the spitz group of genes. The ventral localization of the GDPCs is further restricted by the action of dorsal patterning genes. Along the anterior-posterior axis, several segment polarity genes (wingless, engrailed, hedgehog, and patched) are required for the proper allocation of the GDPCs. These segment polarity genes are expressed in some, but not all of the GDPCs, indicating that anterior and posterior compartments are not fully established in the GDPCs. In addition, we found that the three primordia of the larval genital disc have already been specified in the GDPCs by the coordinated actions of the homeotic (Hox) genes, abdominal-A, Abdominal-B, and caudal. By identifying how these different patterning networks regulate the allocation and primordial organization of the 22 embryonic precursors of the compound genital disc, we demonstrate that at least some of the organization of the larval disc originates as positional information in the embryo, thus providing a context for further studies on the development of the genital disc.

Comparative organization of follicle, accessory cells and spawning anlagen in dynamic semelparous clutch manipulators, the urochordate Oikopleuridae

BACKGROUND INFORMATION

The urochordate appendicularians play a key trophic role in marine ecosystems and are the second largest component of zooplankton after copepods. Part of their success is due to their ability to undergo rapid population blooms in response to changes in primary productivity. Nonetheless, the reproductive biology of this important group remains poorly understood.

RESULTS

In the present study, we investigated the organization of male and female germ and accessory somatic cells in the Oikopleuridae. We found that the structure of the ovary had been previously misconstrued as consisting of germ and accessory 'cells' interspersed together, whereas, in fact, the germline exists as a giant transparent syncytium. Somatic follicle cells, integral to regulation of the temporal progression of gametogenesis, could be classified into three types in females and two in males, and we characterized functional gap junctions between follicle cells and the germline syncytium in both sexes. The number of follicle cells per oocyte produced was much reduced in comparison with many commonly studied model organisms. We further identified a novel anlagen that permits spawning of the animal via rupture of the gonad wall, which is obligatory for the release of oocytes, but optional for the release of sperm that usually occurs via the spermiduct.

CONCLUSIONS

The organization of the female germline in the Oikopleuridae shares some features of meroistic oogenesis with the arthropod Drosophila, but the process of synchronous oogenesis in these semelparous organisms remains quite distinctive with respect to that previously characterized in the animal kingdom and certainly within the chordate phylum.

Assigning sex to pre-adult stalk-eyed flies using genital disc morphology and X chromosome zygosity

BACKGROUND

In stalk-eyed flies (Diopsidae) the eyes and antennae are laterally displaced at the ends of elongated eyestalks. Eyespan and the degree of sexual dimorphism in eyespan vary considerably between species and several sexually dimorphic species show sexual selection through female mate preference for males with exaggerated eyespan. The genes on which selection acts to regulate eyespan remain to be identified. This could be achieved by comparing gene expression during eyestalk development in males and females if the sex of pre-adult flies could be reliably assigned. Here we describe two techniques, one morphological and one microsatellite-based, that identify the sex of stalk-eyed fly larvae and pupae.

RESULTS

We showed that genital discs of the stalk-eyed fly Teleopsis dalmanni have two highly distinct morphologies, compact ("C") and lobed ("L"). Segment composition (revealed by Engrailed expression) was consistent with C morphology being typical of males and L morphology of females. We confirmed the proposed association between disc morphology and sex by evaluating the combined heterozygosity of four X-linked microsatellite markers. We demonstrated that individuals with C genital discs had hemizygous (male) genotypes while those with L discs were heterozygous (female) genotypes. Similar dimorphism in genital disc morphology was observed in eight other species spanning three representative Diopsid genera. In every case the segment composition supported C morphology being male and L morphology female. We assigned larval sex by C or L morphology and compared cell division frequencies in male and female eye-antennal discs in two species (T. dalmanni and Diasemopsis meigenii) sexually dimorphic for eyespan. The number of mitotic (anti-H3-labelled) cells did not differ between the sexes in either species.

CONCLUSION

We have made novel use of two complementary techniques for identifying the sex of pre-adult stalk-eyed flies. These procedures will facilitate studies of the evolution of sexually dimorphic development in a variety of other species. Morphology and En expression in male and female genital discs are highly conserved within each genus of Diopsidae. Finally, sexual dimorphism for eyespan in two Diopsid species is unlikely to be due to an increased rate of cell division at the third larval instar in males.

A 3' cis-regulatory region controls wingless expression in the Drosophila eye and leg primordia

The precise regulation of wingless (wg) expression in the Drosophila eye disc is key to control the anteroposterior and dorsoventral patterning of this disc. Here, we identify an eye disc-specific wg cis-regulatory element that functions as a regulatory rheostat. Pannier (Pnr), a transcription factor previously proposed to act as an upstream activator of wg, is sufficient to activate the eye disc enhancer but required for wg expression only in the peripodial epithelium of the disc. We propose that this regulation of wg by Pnr appeared associated to the development of the peripodial epithelium in higher dipterans and was added to an existing mechanism regulating the deployment of wingless in the dorsal region of the eye primordium. In addition, our analysis identifies a separate ventral disc enhancer that lies adjacent to the eye-specific one, and thus altogether, they define a 1-kb genomic region where disc-specific enhancers of the wg gene are located.

Requirement of Abdominal-A and Abdominal-B in the developing genitalia of Drosophila breaks the posterior downregulation rule

The genitalia of Drosophila derive from the genital disc and require the activity of the Abdominal-B (Abd-B) Hox gene. This gene encodes two different proteins, Abd-B M and Abd-B R. We show here that the embryonic genital disc, like the larval genital disc, is formed by cells from the eighth (A8), ninth (A9) and tenth (A10) abdominal segments, which most likely express the Abd-B M, Abd-B R and Caudal products, respectively. Abd-B m is needed for the development of A8 derivatives such as the external and internal female genitalia, the latter also requiring abdominal-A (abd-A), whereas Abd-B r shapes male genitalia (A9 in males). Although Abd-B r represses Abd-B m in the embryo, in at least part of the male A9 such regulation does not occur. In the male A9, some Abd-B m(-)r(-) or Abd-B r(-) clones activate Distal-less and transform part of the genitalia into leg or antenna. In the female A8, many Abd-B m(-)r(-) mutant clones produce similar effects, and also downregulate or eliminate abdominal-A expression. By contrast, although Abd-B m is the main or only Abd-B transcript present in the female A8, Abd-B m(-) clones induced in this primordium do not alter Distal-less or abd-A expression, and transform the A8 segment into the A4. The relationship between Abd-B and abd-A in the female genital disc is opposite to that of the embryonic epidermis, and contravenes the rule that posteriorly expressed Hox genes downregulate more anterior ones.

Mapping the proteome of Drosophila melanogaster: analysis of embryos and adult heads by LC-IMS-MS methods

Multidimensional separations combined with mass spectrometry are used to study the proteins that are present in two states of Drosophila melanogaster: the whole embryo and the adult head. The approach includes the incorporation of a gas-phase separation dimension in which ions are dispersed according to differences in their mobilities and is described as a means of providing a detailed analytical map of the proteins that are present. Overall, we find evidence for 1133 unique proteins. In total, 780 are identified in the head, and 660 are identified in the embryo. Only 307 proteins are in common to both developmental stages, indicating that there are significant differences in these proteomes. A comparison of the proteome to a database of mRNAs that are found from analysis by cDNA approaches (i.e., transcriptome) also shows little overlap. All of this information is discussed in terms of the relationship between the predicted genome, and measured transcriptomes and proteomes. Additionally, the merits and weaknesses of current technologies are assessed in some detail.

Signaling circuitries in development: insights from the retinal determination gene network

Context-specific integration of information received from the Notch, Transforming growth factor beta, Wingless/Wnt, Hedgehog and Epidermal growth factor receptor signaling pathways sets the stage for deployment of the retinal determination gene network (RDGN), a group of transcription factors that collectively directs the formation of the eye and other tissues. Recent investigations have revealed how these transcription factors are regulated by their interactions with each other and with effectors of the above signaling pathways. Further study of the RDGN may provide insights into how common cues can generate context-specific responses, a key aspect of developmental regulation that remains poorly understood.