Compartmental organization of the Drosophila genital imaginal discs

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.

Developmental expression patterns of toolkit genes in male accessory gland of Drosophila parallels those of mammalian prostate

Conservation of genetic toolkits in disparate phyla may help reveal commonalities in organ designs transcending their extreme anatomical disparities. A male accessory sexual organ in mammals, the prostate, for instance, is anatomically disparate from its analogous, phylogenetically distant counterpart – the male accessory gland (MAG) – in insects like Drosophila. It has not been ascertained if the anatomically disparate Drosophila MAG shares developmental parallels with those of the mammalian prostate. Here we show that the development of Drosophila mesoderm-derived MAG entails recruitment of similar genetic toolkits of tubular organs like that seen in endoderm-derived mammalian prostate. For instance, like mammalian prostate, Drosophila MAG morphogenesis is marked by recruitment of fibroblast growth factor receptor (FGFR) – a signalling pathway often seen recruited for tubulogenesis – starting early during its adepithelial genesis. A specialisation of the individual domains of the developing MAG tube, on the other hand, is marked by the expression of a posterior Hox gene transcription factor, Abd-B, while Hh-Dpp signalling marks its growth. Drosophila MAG, therefore, reveals the developmental design of a unitary bud-derived tube that appears to have been co-opted for the development of male accessory sexual organs across distant phylogeny and embryonic lineages.

This article has an associated First Person interview with the first author of the paper.

Summary: We show genetic toolkit conservation between Drosophila MAG and mammalian prostate may suggest a common modular developmental design.

Structural and transcriptional evidence of mechanotransduction in the Drosophila suzukii ovipositor

Drosophila suzukii is an invasive pest that prefers to lay eggs in ripening fruits, whereas most closely related Drosophila species exclusively use rotten fruit as oviposition site. This behaviour is allowed by an enlarged and serrated ovipositor that can pierce intact fruit skin, and by multiple contact sensory systems (mechanosensation and taste) that detect the optimal egg-laying substrates. Here, we tested the hypothesis that bristles present in the D. suzukii ovipositor tip contribute to these sensory modalities. Analysis of the bristle ultrastructure revealed that four different types of cuticular elements (conical pegs type 1 and 2, chaetic and trichoid sensilla) are present on the tip of each ovipositor plate. All of them have a poreless shaft and are innervated at their base by a single neuron that ends in a distal tubular body, thus resembling mechanosensitive structures. Fluorescent labelling in D. suzukii and D. melanogaster revealed that pegs located on the ventral side of the ovipositor tip are innervated by a single neuron in both species. RNA-sequencing profiled gene expression, notably sensory receptor genes of the terminalia of D. suzukii and of three other Drosophila species with changes in their ovipositor structure (from serrated to blunt ovipositor: Drosophila subpulchrella, Drosophila biarmipes and D. melanogaster). Our results revealed few species-specific transcripts and an overlapping expression of candidate mechanosensitive genes as well as the presence of some chemoreceptor transcripts. These experimental evidences suggest a mechanosensitive function for the D. suzukii ovipositor, which might be crucial across Drosophila species independently from ovipositor shape.

Sex and the Single Fly: A Perspective on the Career of Bruce S. Baker

Bruce Baker, a preeminent Drosophila geneticist who made fundamental contributions to our understanding of the molecular genetic basis of sex differences, passed away July 1, 2018 at the age of 72. Members of Bruce's laboratory remember him as an intensely dedicated, rigorous, creative, deep-thinking, and fearless scientist. His trainees also remember his strong commitment to teaching students at every level. Bruce's career studying sex differences had three major epochs, where the laboratory was focused on: (1) sex determination and dosage compensation, (2) the development of sex-specific structures, and (3) the molecular genetic basis for sex differences in behavior. Several members of the Baker laboratory have come together to honor Bruce by highlighting some of the laboratory's major scientific contributions in these areas.

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.

Left-right asymmetric cell intercalation drives directional collective cell movement in epithelial morphogenesis

Morphogenetic epithelial movement occurs during embryogenesis and drives complex tissue formation. However, how epithelial cells coordinate their unidirectional movement while maintaining epithelial integrity is unclear. Here we propose a novel mechanism for collective epithelial cell movement based on Drosophila genitalia rotation, in which epithelial tissue rotates clockwise around the genitalia. We found that this cell movement occurs autonomously and requires myosin II. The moving cells exhibit repeated left–right-biased junction remodelling, while maintaining adhesion with their neighbours, in association with a polarized myosin II distribution. Reducing myosinID, known to cause counter-clockwise epithelial-tissue movement, reverses the myosin II distribution. Numerical simulations revealed that a left–right asymmetry in cell intercalation is sufficient to induce unidirectional cellular movement. The cellular movement direction is also associated with planar cell-shape chirality. These findings support a model in which left–right asymmetric cell intercalation within an epithelial sheet drives collective cellular movement in the same direction.

Coordinated epithelial movement during embryogenesis drives complex tissue formation, but how this movement is coordinated to maintain epithelial integrity is not clear. Here the authors show that left-right asymmetry in cell intercalation drives clockwise rotation of epithelia in Drosophila genital development.

The making of a pest: the evolution of a fruit-penetrating ovipositor in Drosophila suzukii and related species

Evolutionary innovation can allow a species access to a new ecological niche, potentially reducing competition with closely related species. While the vast majority of Drosophila flies feed on rotting fruit and other decaying matter, and are harmless to human activity, Drosophila suzukii, which has a morphologically modified ovipositor, is capable of colonizing live fruit that is still in the process of ripening, causing massive agricultural damage. Here, we conducted the first comparative analysis of this species and its close relatives, analysing both ovipositor structure and fruit susceptibility. We found that the ovipositor of the species most closely related to D. suzukii, Drosophila subpulchrella, has a similar number of enlarged, evolutionarily derived bristles, but a notably different overall shape. Like D. suzukii, D. subpulchrella flies are capable of puncturing the skin of raspberries and cherries, but we found no evidence that they could penetrate the thicker skin of two varieties of grapes. More distantly related species, one of which has previously been mistaken for D. suzukii, have blunt ovipositors with small bristles. While they did not penetrate fruit skin in any of the assays, they readily colonized fruit interiors where the skin was broken. Our results suggest that considering evolutionary context may be beneficial to the management of invasive species.

A small cohort of FRU(M) and Engrailed-expressing neurons mediate successful copulation in Drosophila melanogaster

BACKGROUND

In Drosophila, male flies require the expression of the male-specific Fruitless protein (FRU(M)) within the developing pupal and adult nervous system in order to produce male courtship and copulation behaviors. Recent evidence has shown that specific subsets of FRU(M) neurons are necessary for particular steps of courtship and copulation. In these neurons, FRU(M) function has been shown to be important for determining sex-specific neuronal characteristics, such as neurotransmitter profile and morphology.

RESULTS

We identified a small cohort of FRU(M) interneurons in the brain and ventral nerve cord by their co-expression with the transcription factor Engrailed (En). We used an En-GAL4 driver to express a fru(M) RNAi construct in order to selectively deplete FRU(M) in these En/FRU(M) co-expressing neurons. In courtship and copulation tests, these males performed male courtship at wild-type levels but were frequently sterile. Sterility was a behavioral phenotype as these En-fru(M)RNAi males were less able to convert a copulation attempt into a stable copulation, or did not maintain copulation for long enough to transfer sperm and/or seminal fluid.

CONCLUSIONS

We have identified a population of interneurons necessary for successful copulation in Drosophila. These data confirm a model in which subsets of FRU(M) neurons participate in independent neuronal circuits necessary for individual steps of male behavior. In addition, we have determined that these neurons in wild-type males have homologues in females and fru mutants, with similar placement, projection patterns, and neurochemical profiles.

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.

Drosophila as a model of wound healing and tissue regeneration in vertebrates

Understanding the molecular basis of wound healing and regeneration in vertebrates is one of the main challenges in biology and medicine. This understanding will lead to medical advances allowing accelerated tissue repair after wounding, rebuilding new tissues/organs and restoring homeostasis. Drosophila has emerged as a valuable model for studying these processes because the genetic networks and cytoskeletal machinery involved in epithelial movements occurring during embryonic dorsal closure, larval imaginal disc fusion/regeneration, and epithelial repair are similar to those acting during wound healing and regeneration in vertebrates. Recent studies have also focused on the use of Drosophila adult stem cells to maintain tissue homeostasis. Here, we review how Drosophila has contributed to our understanding of these processes, primarily through live-imaging and genetic tools that are impractical in mammals. Furthermore, we highlight future research areas where this insect may provide novel insights and potential therapeutic strategies for wound healing and regeneration.

Establishment and maintenance of compartmental boundaries: role of contractile actomyosin barriers

During animal development, tissues and organs are partitioned into compartments that do not intermix. This organizing principle is essential for correct tissue morphogenesis. Given that cell sorting defects during compartmentalization in humans are thought to cause malignant invasion and congenital defects such as cranio-fronto-nasal syndrome, identifying the molecular and cellular mechanisms that keep cells apart at boundaries between compartments is important. In both vertebrates and invertebrates, transcription factors and short-range signalling pathways, such as EPH/Ephrin, Hedgehog, or Notch signalling, govern compartmental cell sorting. However, the mechanisms that mediate cell sorting downstream of these factors have remained elusive for decades. Here, we review recent data gathered in Drosophila that suggest that the generation of cortical tensile forces at compartmental boundaries by the actomyosin cytoskeleton could be a general mechanism that inhibits cell mixing between compartments.

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.

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.

Rapid antagonistic coevolution between primary and secondary sexual characters in horned beetles

Different structures may compete during development for a shared and limited pool of resources to sustain growth and differentiation. The resulting resource allocation trade-offs have the potential to alter both ontogenetic outcomes and evolutionary trajectories. However, little is known about the evolutionary causes and consequences of resource allocation trade-offs in natural populations. Here, we explore the significance of resource allocation trade-offs between primary and secondary sexual traits in shaping early morphological divergences between four recently separated populations of the horned beetle Onthophagus taurus as well as macroevolutionary divergence patterns across 10 Onthophagus species. We show that resource allocation trade-offs leave a strong signature in morphological divergence patterns both within and between species. Furthermore, our results suggest that genital divergence may, under certain circumstances, occur as a byproduct of evolutionary changes in secondary sexual traits. Given the importance of copulatory organ morphology for reproductive isolation our findings begin to raise the possibility that secondary sexual trait evolution may promote speciation as a byproduct. We discuss the implications of our results on the causes and consequences of resource allocation trade-offs in insects.

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.

Disruption of sphingolipid metabolism elicits apoptosis-associated reproductive defects in Drosophila

Sphingolipid signaling is thought to regulate apoptosis via mechanisms that are dependent on the concentration of ceramide relative to that of sphingosine-1-phosphate (S1P). This study reports defects in reproductive structures and function that are associated with enhanced apoptosis in Drosophila Sply05091 mutants that lack functional S1P lyase and thereby accumulate sphingolipid long chain base metabolites. Analyses of reproductive structures in these adult mutants unmasked multiple abnormalities, including supernumerary spermathecae, degenerative ovaries, and severely reduced testes. TUNEL assessment revealed increased cell death in mutant egg chambers at most oogenic stages and in affected mutant testes. These reproductive abnormalities and elevated gonadal apoptosis were also observed, to varying degrees, in other mutants affecting sphingolipid metabolism. Importantly, the reproductive defects seen in the Sply05091 mutants were ameliorated both by a second site mutation in the lace gene that restores long chain base levels towards normal and by genetic disruption of the proapoptotic genes reaper, hid and grim. These data thus provide the first evidence in Drosophila that accumulated sphingolipids trigger elevated levels of apoptosis via the modulation of known signaling pathways.

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.

Rap-GEF/Rap signaling restricts the formation of supernumerary spermathecae in Drosophila melanogaster

Sperm storage in the female is a key factor for reproductive success in a variety of organisms, including Drosophila melanogaster. The spermathecae (SP) are the Drosophila organs for long-term storage. While wild-type female flies have two SP, occasionally, three or four SP have been observed in mutant flies. However, the molecular mechanism of SP formation is unknown. Here we show that loss of function of a Drosophila Rap-GEF (GEF26) result in an occurrence of the supernumerary SP; females have three SP (varies from 11 to 62% in different allele combinations) instead of the normal two SP. In addition, the Gef26 mutant flies also have ectopic wing veins and extra mechanosensory organs. The supernumerary SP phenotype of the Gef26 mutation can be enhanced by the Drosophila Rap mutations and rescued by overexpressing the cell adhesion molecule DE-cadherin. These data suggest that the Rap-GEF/Rap signaling controls the formation of supernumerary spermathecae through modulating cell adhesion in Drosophila.

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.

Type ID unconventional myosin controls left-right asymmetry in Drosophila

Breaking left-right symmetry in Bilateria embryos is a major event in body plan organization that leads to polarized adult morphology, directional organ looping, and heart and brain function. However, the molecular nature of the determinant(s) responsible for the invariant orientation of the left-right axis (situs choice) remains largely unknown. Mutations producing a complete reversal of left-right asymmetry (situs inversus) are instrumental for identifying mechanisms controlling handedness, yet only one such mutation has been found in mice (inversin) and snails. Here we identify the conserved type ID unconventional myosin 31DF gene (Myo31DF) as a unique situs inversus locus in Drosophila. Myo31DF mutations reverse the dextral looping of genitalia, a prominent left-right marker in adult flies. Genetic mosaic analysis pinpoints the A8 segment of the genital disc as a left-right organizer and reveals an anterior-posterior compartmentalization of Myo31DF function that directs dextral development and represses a sinistral default state. As expected of a determinant, Myo31DF has a trigger-like function and is expressed symmetrically in the organizer, and its symmetrical overexpression does not impair left-right asymmetry. Thus Myo31DF is a dextral gene with actin-based motor activity controlling situs choice. Like mouse inversin, Myo31DF interacts and colocalizes with beta-catenin, suggesting that situs inversus genes can direct left-right development through the adherens junction.

BMP signaling is required for controlling somatic stem cell self-renewal in the Drosophila ovary

BMP signaling is essential for promoting self-renewal of mouse embryonic stem cells and Drosophila germline stem cells and for repressing stem cell proliferation in the mouse intestine and skin. However, it remains unknown whether BMP signaling can promote self-renewal of adult somatic stem cells. In this study, we show that BMP signaling is necessary and sufficient for promoting self-renewal and proliferation of somatic stem cells (SSCs) in the Drosophila ovary. BMP signaling is required in SSCs to directly control their maintenance and division, but is dispensable for proliferation of their differentiated progeny. Furthermore, BMP signaling is required to control SSC self-renewal, but not survival. Moreover, constitutive BMP signaling prolongs the SSC lifespan. Therefore, our study clearly demonstrates that BMP signaling directly promotes SSC self-renewal and proliferation in the Drosophila ovary. Our work further suggests that BMP signaling could promote self-renewal of adult stem cells in other systems.

Absence asymmetry: the evolution of monorchid beetles (Insecta: Coleoptera: Carabidae)

Asymmetrical monorchy, or the complete absence of one testis coupled with the presence of its bilateral counterpart, is reported for 174 species of the carabid beetle tribes Abacetini, Harpalini, and Platynini (Insecta: Coleoptera: Carabidae) based on a survey of over 820 species from throughout the family. This condition was not found in examined individuals of any other carabid beetle tribes, or of other adephagan beetle families. One monorchid taxon within Platynini exhibits symmetrical vasa deferentia at the beginning of the pupal stadium, suggesting that developmental arrest of the underdeveloped vas deferens takes place in pupation. The point at which development of the testis is interrupted is unknown. Complete absence of one organ of a bilateral pair--absence asymmetry--is rarely found in any animal clade and among insects is otherwise only known for testes in the minute-sized beetles of the family Ptiliidae, ovaries in Scarabaeinae dung beetles, and ovaries of some aphids. Based on current phylogenetic hypotheses for Carabidae, testis loss has occurred independently at least three times, and up to five origins are possible, given the variation within Abacetini. Clear phylogenetic evidence for multiple independent origins suggests an adaptive or functional cause for this asymmetry. A previously posited taxon-specific hypothesis wherein herbivory in the tribe Harpalini led to testis loss is rejected. Optimal visceral packing of the beetle abdomen is suggested as a general explanation. Specifically, based on the function of various organ systems, we hypothesize that interaction of internal organs and pressure to optimize organ size and space usage in each system led to the multiple origins and maintenance of the monorchid condition. Testes are the only redundant and symmetrically paired structures not thought to be developmentally linked to other symmetrical structures in the abdomen. Among all possible organs, they are the most likely--although the observed frequency is very small--to bypass constraints that maintain bilateral symmetry, resulting in absence asymmetry. However, based solely on our observations of gross morphology of internal organs, no function conclusively explains the ontogenetic loss of one testis in these taxa. Unlike the analogous absence asymmetry of organs in other animal groups, no dramatic body-form constraint--e.g., snakes and lung loss, ptiliid beetles' small body-size and relatively giant sperm--or adaptive scenario of improved locomotory performance--e.g., birds and ovary loss due to flight constraints-applies to these carabid beetles. We tentatively suggest that testis loss is driven wholly by an interaction among the internal organs of these beetles, possibly due to selective pressure to maximize the comparatively large accessory glands found in these taxa. However, as the ordering of these evolutionary events of testis loss and accessory gland size increase is not known, large accessory glands might have secondarily evolved to compensate for a decreased testicular output.

Intimate relationships with their neighbors: tales of stem cells in Drosophila reproductive systems

Stem cells have the unique potential to self-renew and to supply differentiated cells that replenish lost cells throughout an organism's lifetime. This unique property makes stem cells powerful therapeutic tools for future regenerative medicine. However, the molecular mechanisms of stem cell regulation are still poorly understood in many stem cell systems. Stem cell function has been shown recently to be controlled by concerted actions of extrinsic signals from its regulatory niche and intrinsic factors inside the stem cell. Stem cells in the Drosophila reproductive systems provide excellent models to understand the fundamental mechanisms underlying stem cell regulation, including the relationships between stem cells and their niches. Within the past few years, much progress in understanding stem cells in Drosophila has been made, and the knowledge gained from studying these stem cells greatly advances our understanding of stem cells in other systems, including humans. In this review, we summarize the recent progress and describe future challenges in understanding the molecular mechanisms controlling stem cell self-renewal, division, and differentiation in the Drosophila reproductive systems.

Dynamics and function of intron sequences of the wingless gene during the evolution of the Drosophila genus

To understand the function and evolution of genes with complex patterns of expression, such as the Drosophila wingless gene, it is essential to know how their transcription is regulated. However, extracting the relevant regulatory information from a genome is still a complex task. We used a combination of comparative genomics and functional approaches to identify putative regulatory sequences in two introns (1 and 3) of the wingless gene and to infer their evolution. Comparison of the sequences obtained from several Drosophila species revealed colinear and well-conserved sequence blocks in both introns. Drosophila willistoni showed a rate of evolution, in both introns, faster than expected from its phylogenetic position. Intron 3 appeared to be composed of two separate modules, one of them lost in the willistoni group. We tested whether sequence conservation in noncoding regions is a reliable indicator of regulatory function and, if this function is conserved, by analyzing D. melanogaster transgenic reporter lines harboring intron 3 sequences from D. melanogaster (Sophophora subgenus) and the species from the Drosophila subgenus presenting the most divergent sequence, D. americana. The analysis indicated that intron 3 contains pupal enhancers conserved during the evolution of the genus, despite the fact that only 30% of the D. melanogaster intron 3 sequences lie in conserved blocks. Additional analysis of D. melanogaster transgenic reporter lines harboring intron 3 sequences from D. willistoni revealed the absence of an abdomen-specific expression pattern, probably due to the above-mentioned loss of a regulatory module in this species.

The ken and barbie gene encoding a putative transcription factor with a BTB domain and three zinc finger motifs functions in terminalia development of Drosophila

Mutations in the ken and barbie locus are accompanied by the malformation of terminalia in adult Drosophila. Male and female genitalia often remain inside the body, and the same portions of genitalia and analia are missing in a fraction of homozygous flies. Rotated and/or duplicated terminalia are also observed. Terminalia phenotypes are enhanced by mutations in the gap gene tailless, the homeobox gene caudal, and the decapentaplegic gene that encodes a TGFbeta-like morphogen. The ken and barbie gene encodes a protein with three CCHH-type zinc finger motifs that are conserved in several transcription factors such as Krüppel and BCL-6. All defects in ken and barbie mutants are fully rescued by the expression of a wild-type genomic construct, which establishes the causality between phenotypes and the gene.

Development of the genitalia in Drosophila melanogaster

The imaginal discs of Drosophila melanogaster are an excellent material with which to analyze how signaling pathways and Hox genes control growth and pattern formation. The study of one of these discs, the genital disc, offers, in addition, the possibility of integrating the sex determination pathway into this analysis. This disc, whose growth and shape are sexually dimorphic, gives rise to the genitalia and analia, the more posterior structures of the fruit fly. Male genitalia, which develop from the ninth abdominal segment, and female genitalia, which develop mostly from the eighth one, display a characteristic array of structures. We will review here some recent findings about the development of these organs. As in other discs, different signaling pathways establish the positional information in the genital primordia. The Hox and sex determination genes modify these signaling routes at different levels to specify the particular growth and differentiation of male and female genitalia.

Sex comes in from the cold: the integration of sex and pattern

There has recently been a revolution in our understanding of how the Drosophila sex-determination hierarchy generates somatic sexual dimorphism. Most significantly, the sex hierarchy has been shown to modulate the activities of well-known signaling molecules (FGF, Wnt and TGF beta proteins) and transcription factors (BAB and DAC) to direct various sex-specific aspects of growth and differentiation. As some of the genes encoding these proteins are also the targets of Hox gene action, these and other findings are revealing the levels at which the sex determination and Hox patterning pathways are integrated to control growth, morphogenesis and differentiation.

Sex-specific deployment of FGF signaling in Drosophila recruits mesodermal cells into the male genital imaginal disc

A central issue in developmental biology is how the deployment of generic signaling proteins produces diverse specific outcomes. We show that Drosophila FGF is used, only in males, to recruit mesodermal cells expressing its receptor to become part of the genital imaginal disc. Male-specific deployment of FGF signaling is controlled by the sex determination regulatory gene doublesex. The recruited mesodermal cells become epithelial and differentiate into parts of the internal genitalia. Our results provide exceptions to two basic tenets of imaginal disc biology-that imaginal disc cells are derived from the embryonic ectoderm and belong to either an anterior or posterior compartment. The recruited mesodermal cells migrate into the disc late in development and are neither anterior nor posterior.

The sex determination gene doublesex regulates the A/P organizer to direct sex-specific patterns of growth in the Drosophila genital imaginal disc

Each Drosophila genital imaginal disc contains primordia for both male and female genitalia and analia. The sexually dimorphic development of this disc is governed by the sex-specific expression of doublesex (dsx). We present data that substantially revises our understanding of how dsx controls growth and differentiation in the genital disc. The classical view of genital disc development is that in each sex, dsx autonomously "represses" the development of the inappropriate genital primordium while allowing the development of the appropriate primordium. Instead, we show that dsx regulates the A/P organizer to control growth of each genital primordium, and then directs each genital primordium to differentiate defined adult structures in both sexes.

The development of the Drosophila genital disc

The imaginal discs of Drosophila melanogaster, which form the adult epidermal structures, are a good experimental model for studying morphogenesis. The genital disc forms the terminalia, which are the most sexually dimorphic structures of the fly. Both sexes of Drosophila have a single genital disc formed by three primordia. The female genital primordium is derived from 8(th) abdominal segment and is located anteriorly, the anal primordium (10 and 11(th) abdominal segments) is located posteriorly, and the male genital primordium from the 9(th) abdominal segment lies between them. In both sexes, only two of these three primordia develop to form the adult terminalia. The anal primordium develops in both sexes but, depending on the genetic sex, will form either male or female analia. However, only one of the genital primordia develops in each sex, forming either the male or the female genitalia. This depends on the genetic sex of the fly. Therefore, the genital disc is a very good experimental model of how the sex-determination and homeotic genes - which determine cell identity - interact to direct the development of a population of cells into male or female terminalia. It has been proposed that the sexually dimorphic development of the genital disc is the result of an integrated genetic input, made up by the sex-determination gene doublesex and the homeotic gene Abdominal-B. This input acts by modulating the response to Hedgehog, Wingless, and Decapentaplegic morphogenetic signals.

The Drosophila sex determination hierarchy modulates wingless and decapentaplegic signaling to deploy dachshund sex-specifically in the genital imaginal disc

The integration of multiple developmental cues is crucial to the combinatorial strategies for cell specification that underlie metazoan development. In the Drosophila genital imaginal disc, which gives rise to the sexually dimorphic genitalia and analia, sexual identity must be integrated with positional cues, in order to direct the appropriate sexually dimorphic developmental program. Sex determination in Drosophila is controlled by a hierarchy of regulatory genes. The last known gene in the somatic branch of this hierarchy is the transcription factor doublesex (dsx); however, targets of the hierarchy that play a role in sexually dimorphic development have remained elusive. We show that the gene dachshund (dac) is differentially expressed in the male and female genital discs, and plays sex-specific roles in the development of the genitalia. Furthermore, the sex determination hierarchy mediates this sex-specific deployment of dac by modulating the regulation of dac by the pattern formation genes wingless (wg) and decapentaplegic (dpp). We find that the sex determination pathway acts cell-autonomously to determine whether dac is activated by wg signaling, as in females, or by dpp signaling, as in males.

Sex determination genes control the development of the Drosophila genital disc, modulating the response to Hedgehog, Wingless and Decapentaplegic signals

In both sexes, the Drosophila genital disc contains the female and male genital primordia. The sex determination gene doublesex controls which of these primordia will develop and which will be repressed. In females, the presence of Doublesex(F) product results in the development of the female genital primordium and repression of the male primordium. In males, the presence of Doublesex(M) product results in the development and repression of the male and female genital primordia, respectively. This report shows that Doublesex(F) prevents the induction of decapentaplegic by Hedgehog in the repressed male primordium of female genital discs, whereas Doublesex(M) blocks the Wingless pathway in the repressed female primordium of male genital discs. It is also shown that Doublesex(F) is continuously required during female larval development to prevent activation of decapentaplegic in the repressed male primordium, and during pupation for female genital cytodifferentiation. In males, however, it seems that Doublesex(M) is not continuously required during larval development for blocking the Wingless signaling pathway in the female genital primordium. Furthermore, Doublesex(M) does not appear to be needed during pupation for male genital cytodifferentiation. Using dachshund as a gene target for Decapentaplegic and Wingless signals, it was also found that Doublesex(M) and Doublesex(F) both positively and negatively control the response to these signals in male and female genitalia, respectively. A model is presented for the dimorphic sexual development of the genital primordium in which both Doublesex(M) and Doublesex(F) products play positive and negative roles.

How Drosophila appendages develop

Just a glance at the body of the fruit fly Drosophila reveals that it has a main body part--the trunk--and a number of specialized appendages such as legs, wings, halteres and antennae. How do Drosophila appendages develop, what gives each appendage its unique identity, and what can the fruit fly teach us about appendage development in vertebrates?

The Hox gene Abdominal-B antagonizes appendage development in the genital disc of Drosophila

In Drosophila, the Hox gene Abdominal-B is required to specify the posterior abdomen and the genitalia. Homologues of Abdominal-B in other species are also needed to determine the posterior part of the body. We have studied the function of Abdominal-B in the formation of Drosophila genitalia, and show here that absence of Abdominal-B in the genital disc of Drosophila transforms male and female genitalia into leg or, less frequently, into antenna. These transformations are accompanied by the ectopic expression of genes such as Distal-less or dachshund, which are normally required in these appendages. The extent of wild-type and ectopic Distal-less expression depends on the antagonistic activities of the Abdominal-B gene, as a repressor, and of the decapentaplegic and wingless genes as activators. Absence of Abdominal-B also changes the expression of Homothorax, a Hox gene co-factor. Our results suggest that Abdominal-B forms genitalia by modifying an underlying positional information and repressing appendage development. We propose that the genital primordia should be subdivided into two regions, one of them competent to be transformed into an appendage in the absence of Abdominal-B.

Complex organization of promoter and enhancer elements regulate the tissue- and developmental stage-specific expression of the Drosophila melanogaster Gld gene

The Drosophila melanogaster Gld gene has multiple and diverse developmental and physiological functions. We report herein that interactions among proximal promoter elements and a cluster of intronically located enhancers and silencers specify the complex regulation of Gld that underlies its diverse functions. Gld expression in nonreproductive tissues is largely determined by proximal promoter elements with the exception of the embryonic labium where Gld is activated by an enhancer within the first intron. A nuclear protein, GPAL, has been identified that binds the Gpal elements in the proximal promoter region. Regulation of Gld in the reproductive organs is particularly complex, involving interactions among the Gpal proximal promoter elements, a unique TATA box, three distinct enhancer types, and one or more silencer elements. The three somatic reproductive organ enhancers each activate expression in male and female pairs of reproductive organs. One of these pairs, the male ejaculatory duct and female oviduct, are known to be developmentally homologous. We report evidence that the other two pairs of organs are developmentally homologous as well. A comprehensive model to explain the full developmental regulation of Gld and its evolution is presented.

Wnt gene expression in sea urchin development: heterochronies associated with the evolution of developmental mode

The Wnt genes encode a large family of conserved secreted proteins that are widely involved in animal development. The variety and ubiquity of this ancient family suggest that Wnt genes may have been important in the evolution of animal development, including early development. To test this hypothesis, we have characterized the expression of several Wnt genes in closely related sea urchins that exhibit radically different modes of early development. Wnt-1, -4, and -5 genes exhibit several conserved molecular and developmental characteristics, both within sea urchins and with Wnt genes examined in other animals (Ferkowicz et al. 1998). Here, we demonstrate that sea urchin Wnt-5 transcripts are specifically detected by in situ hybridization in discrete embryonic, larval, and developing adult tissues and processes: (1) in a band of vegetal ectoderm in mesenchyme blastula stage embryos, (2) in the larval ciliary bands, (3) in tissues that form the early adult rudiment (left coelomic pouch and overlying vestibular ectoderm), and (4) in the developing adult radial nervous system. We find that the sites of Wnt-5 transcript accumulation are conserved in species exhibiting either indirect- or direct-developmental modes, suggesting that Wnt-5 function(s) have been conserved in sea urchin development. However, dramatic heterochronic changes in Wnt-5 gene expression have occurred in the direct-developing species that parallel the accelerated morphological changes that occur during direct development. These results suggest that heterochronic changes in the expression of conserved developmental regulatory genes, such as the Wnt family members, are agents of evolutionary change in animal development.

On the range of hedgehog signaling

Hedgehog (Hh) is a secreted signaling protein that regulates the development of many organ systems. It can travel from its site of synthesis, a process that involves covalent attachment of cholesterol to its carboxyl terminus, proteins with putative sterol sensing domains in both sending and receiving cells, and glycosaminoglycans. Understanding how the movement of Hh is controlled and propelled will be key to understanding how it carries out its essential roles.

Hedgehog signal transduction in the posterior compartment of the Drosophila wing imaginal disc

Drosophila Hedgehog (Hh) is secreted by Posterior (P) compartment cells and induces Anterior (A) cells to create a developmental organizer at the AP compartment border. Hh signaling converts Fused (Fu) to a hyperphosphorylated form, Fu*. We show that A border cells of wing imaginal discs contain Fu*. Unexpectedly, P cells also produce Fu*, in a Hh-dependent and Ptc-independent manner. Increasing Ptc, the putative Hh receptor expressed specifically by A cells, reduced Fu*. These results are consistent with proposals that Ptc downregulates Hh signaling and suggest that a receptor other than Ptc mediates Hh signaling in P cells of imaginal discs. We conclude that Hh signals in these P cells and that the outputs of the pathway are blocked by transcriptional repression.

The battle of the sexes

The sex determining gene, Sry, determines the sex of the organism by initiating development of a testis rather than an ovary from the cells of the bipotential gonad. In the 10 years since the discovery of Sry, new genes and cellular pathways that operate in the establishment of the gonadal primordium and the initiation of testis development have been discovered. Experiments defining mechanisms downstream of Sry are providing clear examples of how a regulatory transcription factor initiates cellular processes including proliferation and cell migration, which in turn influence architectural patterning, fate commitment, and differentiation of cells within an organ.

Wingless, decapentaplegic and EGF receptor signaling pathways interact to specify dorso-ventral pattern in the adult abdomen of Drosophila

Adult abdominal segments of Drosophila are subdivided along the dorso-ventral axis into a dorsal tergite, a ventral sternite and ventro-lateral pleural cuticle. We report that this pattern is largely specified during the pupal stage by Wingless (Wg), Decapentaplegic (Dpp) and Drosophila EGF Receptor (DER) signaling. Expression of wg and dpp is activated at the posterior edge of the anterior compartment by Hedgehog signaling. Within this region, wg and dpp are expressed in domains that are mutually exclusive along the dorso-ventral axis: wg is expressed in the sternite and medio-lateral tergite, whereas dpp expression is confined to the pleura and the dorsal midline. Neither gene is expressed in the lateral tergite. Shirras and Couso (1996, Dev. Biol. 175, 24–36) have shown that tergite and sternite cell fates are specified by Wg signaling. We find that DER acts synergistically with Wg to promote tergite and sternite identities, and that Wg and DER activities are opposed by Dpp signaling, which promotes pleural identity. Wg and Dpp interact antagonistically at two levels. First, their expression is confined to complementary domains by mutual transcriptional repression. Second, Wg and Dpp compete directly with one another by exerting opposite effects on cell fate. DER signaling does not affect the expression of wg or dpp, indicating that it interacts with Wg and Dpp at the level of cell fate determination. Within the tergite, the requirements for Wg and DER function are roughly complementary: Wg is required mainly in the medial region, whereas DER is most important laterally. Finally, we show that Dpp signaling at the dorsal midline controls dorso-ventral patterning within the tergite by promoting pigmentation in the medial region.

Drosophila terminalia as an appendage-like structure

In Drosophila, the homeotic gene Distal-less (Dll) has a fundamental role in the establishment of the identity of ventral appendages such as the leg and antenna. This study reports the expression pattern of Dll in the genital disc, the requirement of Dll activity for the development of the terminalia and the activation of Dll by the combined action of the morphogenetic signals Wingless (Wg) and Decapentaplegic (Dpp). During the development of the two components of the anal primordium - the hindgut and the analia - only the latter is dependent on Dll and hedgehog (hh) functions. The hindgut is defined by the expression of the homeobox gene even-skipped. The lack of Dll function in the anal primordia transforms the anal tissue into hindgut by the extension of the eve domain. Meanwhile targeted ectopic Dll represses eve expression and hindgut formation. The Dll requirement for the development of both anal plates in males and only for the dorsal anal plate in females, provides further evidence for the previously held idea that the analia arise from two primordia. In addition, evaluation was made of the requirement for the optomotor-blind (omb) gene which, as in the leg and antenna, is located downstream to Dpp. These results suggest that the terminalia show similar behaviour to the leg disc or the antennal part of the eye-antennal disc consistent with both the proposed ventral origin of the genital disc and the evolutive consideration of the terminalia as an ancestral appendage.

decapentaplegic is essential for the maintenance and division of germline stem cells in the Drosophila ovary

Stem cells are thought to occupy special local environments, or niches, established by neighboring cells that give them the capability for self-renewal. Each ovariole in the Drosophila ovary contains two germline stem cells surrounded by a group of differentiated somatic cells that express hedgehog and wingless. Here we show that the BMP2/4 homolog decapentaplegic (dpp) is specifically required to maintain female germline stem cells and promote their division. Overexpression of dpp blocks germline stem cell differentiation. Conversely, mutations in dpp or its receptor (saxophone) accelerate stem cell loss and retard stem cell division. We constructed mutant germline stem cell clones to show that the dpp signal is directly received by germline stem cells. Thus, dpp signaling helps define a niche that controls germline stem cell proliferation.