Gradual acquisition of the developmental capacity to differentiate adult structures by the genital disc of Drosophila melanogaster

Summary Diplo-X flies homozygous for the transform-er-2 ^ts (tra-2 ^ts) mutation develop into females at 16° C, while they develop into males at 29° C (Belote and Baker 1982). By means of this conditional mutation, we have carried out a detailed analysis of the development of the genital disc. Temperature shifts between 16 and 29° C, in both directions, and temperature pulses at 29° C, have been applied during the larval growth of tra-2 ^ts homozygous diplo-X flies, and the external derivatives of the genital disc have been analysed. Genital discs shifted from 16 to 29° C rapidly lose their capacity to differentiate female genital structures, while they become able to differentiate male genital structures whose inventory is more complete the earlier in larval development the temperature shift is carried out; moreover, duplicated male genital structures were observed. In the shift from 29 to 16° C, the genital disc loses its capacity to differentiate male genital structures, while it becomes able to differentiate female genital structures. The inventory of male structures is smaller, and the inventory of the female structures is more complete, the earlier in larval development the temperature is shifted. No duplicated female or male genital structures were observed in the downshift experiment. With respect to the analia, the shift from 16 to 29° C resulted in the quick formation of pure male anal plates, while in the opposite shift the formation of pure female anal plates occurred gradually. Moreover, the time course for the dorsal and ventral anal plates to show normal female phenotype was different: when the dorsal anal plates were completely normal, it was still possible to find incomplete ventral anal plates. In the pulse experiment at 29° C, the genital disc is able to differentiate both female and male genital structures, although the inventory of the latter ones was not complete. In addition, the capacity of the genital disc to differentiate male genital structures depended on the duration of the temperature pulse. The anal plates were always female, although they showed a reduction in their size, the ventral female anal plate being more affected than the dorsal one. No male anal plates were observed. The results have revealed that the genital disc follows a sequence in its capacity to differentiate female or male adult structures. We suggest that this sequence reflects the sequence of determination events occurring in the genital disc during its larval growth. In addition, results shown here provide evidence for the existence in the female genital primordium of a set of cells capable of giving rise either to female genital structures (ventral vaginal plates) or to male genital structures (hypandrium and penis apparatus). We also present evidence supporting the previous idea of two primordia for the anal plates.     

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.     

Molecular phylogeny reveals the repeated evolution of complex male genital traits in the New Zealand moth genus Izatha (Lepidoptera: Xyloryctidae)

Male genitalia are among the most rapidly evolving and divergent morphological structures and sexual selection is known to drive this phenomenon in many taxa. Because of their diversity, even within a single genus, genital characters are frequently used to infer relationships among closely‐related species. Moths within the genus Izatha (Xyloryctidae) are ideal candidates for investigating the phylogenetic patterns of genital evolution as they display great variation in male genital structure and complexity. We determined the evolutionary relationships among 31 species of Izatha by constructing a molecular phylogeny of the genus based on the mitochondrial cytochrome oxidase subunit I gene and the isocitrate dehydrogenase and carbamoylphosphate synthase domain protein nuclear genes. This allowed estimations of ancestral male genital character states and patterns of male genital diversification using maximum‐likelihood models. The genus is divided into two well‐supported clades and two poorly supported clades at the root of the phylogeny with incomplete phylogenetic resolution within two species groups, likely due to rapid speciation. Izatha display a number of apomorphic phallic traits including cornuti (sclerotized spines) which are either discharged into the female during copulation (deciduous cornuti) or fixed to the male phallus (compound and fish‐hook cornuti). Within the genus, there is a reduction of secondary genital characters – the uncus and gnathos – but an elaboration of another grasping structure, the juxta; the potential origin and functionality of these male genital traits are discussed. Overall, some male genital characters provided a good indication of species relationships; however, several parts of the complex male genitalia of Izatha show evidence of homoplasy and convergence highlighting the problems of using these traits in determining species relationships. Additionally, this convergence has highlighted that complex genital structures may evolve repeatedly and independently within a lineage.     

Mating positions and the evolution of asymmetric insect genitalia

Genital asymmetry is a recurring phenomenon in insect morphology and current data suggest that it has arisen multiple times independently in several neopteran orders. Various explanations have been proposed, including space constraints, ecological constraints, sexual selection via antagonistic coevolution, and sexual selection via changed mating positions. Each of these hypotheses may best explain individual cases, but only the last seems to account for the large majority of insect genital asymmetries. Here I summarize the basic assumptions and evolutionary steps implied in this model and review the evidence for each of them. Several components of this scenario can be easily tested, for example by including genital asymmetries and mating positions in phylogenetic analyses. Others require in-depth analyses of the function of asymmetric genital structures, targeted comparative analyses (e.g., of taxa with sex-role reversal, taxa with reversal to symmetry, etc.), and of female genital neuroanatomy.     

Sexual selection and the evolution of genital shape and complexity in water striders

Animal genitalia show two striking but incompletely understood evolutionary trends: a great evolutionary divergence in the shape of genitalic structures, and characteristic structural complexity. Both features are thought to result from sexual selection, but explicit comparative tests are hampered by the fact that it is difficult to quantify both morphological complexity and divergence in shape. We undertake a comparative study of multiple nongenitalic and male genital traits in a clade of 15 water strider species to quantify complexity and shape divergence. We show that genital structures are more complex and their shape more divergent among species than nongenital traits. Further, intromittent genital traits are more complex and have evolved more divergently than nonintromittent genital traits. More importantly, shape and complexity of nonintromittent genital traits show correlated evolution with indices of premating sexual selection and intromittent genital traits with postmating sexual selection, suggesting that the evolution of different components of genital morphology are shaped independently by distinct forms of sexual selection. Our quantitative results provide direct comparative support for the hypothesis that sexual selection is associated with morphological complexity in genitalic traits and highlight the importance of quantifying morphological shape and complexity, rather than size in studies of genital evolution.     

Sex-specific control of growth and differentiation in the Drosophila genital disc, studied using a temperature-sensitive transformer-2 mutation

Mutations of the transformer-2 (tra-2) locus of Drosophila melanogaster cause chromosomally female (XX) animals to develop as males, but have no effect on the development of chromosomally male (XY) animals. In the female genital disc, such mutations cause repression of growth and inhibition of differentiation in the female genital primordium, while allowing growth and differentiation of the otherwise repressed male genital primordium. We used a temperature-sensitive mutation of this locus (tra-2^ts1) to switch development from one sexual pathway to the other. Following development at the male-determining temperature (29°C), subsequent culture of the XX;tra-2^ts1 genital disc in vivo at the female-determining temperature (16°C) allowed the previously repressed female genital primordium to develop and form female genital structures, whereas the formation of male genital elements was grossly disturbed. Conversely, following development at the female-determining temperature, subsequent culture in vivo at the male-determining temperature allowed the formerly repressed male genital primordium to grow and produce male genital structures, and repressed the formation of female elements from the already fully developed female genital primordium. The experiments indicate that the tra-2 product has to operate during the culture period in order to maintain the female state of sex determination, i.e., to promote the development of female structures, as well as to repress that of male structures. The experimental treatments, as well as the results of temperature shifts on developing larvae, resulted in sexual transformation of the anal plates, and clarified the sexual homologies of these structures. In both genitalia and analia, a switch from the female to the male developmental pathway was accomplished more rapidly and effectively than the reverse change.     

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.     

Morphology and histology of male and female reproductive systems in the inseminating species Scoloplax distolothrix (Ostariophysi: Siluriformes: Scoloplacidae)

The morphology and histology of male and female reproductive systems were examined in Scoloplax distolothrix. Internal insemination was documented in this species by the presence of sperm within the ovaries. Mature males and females have elongated genital papillae, exhibiting a tubular shape in males and a plain heart‐shape with two median protuberances in females. The testes are two elongated structures that converge ventrally, under the intestine, towards the genital papilla. They are joined at the caudal end, forming an ovoid single chamber for sperm storage. Secretory regions were not observed. In the lumen of the testicular tubules, spermatozoa can be tightly packed along their lengths, but do not constitute a spermatozeugmata. The lumen of the sperm storage chamber and spermatic duct are filled with free spermatozoa without the accompanying secretions. The ovaries are bird‐wing shaped, saccular structures that converge ventrally under the intestine, towards the genital papilla. They are joined at the caudal end, forming a tubular chamber possibly destined for oocyte storage. An oviduct with an irregular outline connects the chamber to the tubular region of the genital papilla. No distinct sperm storage structure was found in the ovaries. The unique male and female genital papillae suggest that these structures are associated with the reproductive mode in scoloplacids, representing evidence for insemination. The occurrence of free spermatozoa, without the accompanying secretions and not arranged in a spermatozeugmata can be associated with the presence of a tubular male genital papilla for sperm transfer to the female genital tract. This reinforces the idea that sperm packets are not necessary for all inseminating species. The male reproductive system in scoloplacids is very different from that in auchenipterids, a second catfish family with insemination, which indicates that the occurrence of insemination is not connected to the internal morphology of reproductive organs. J. Morphol., 2008. © 2008 Wiley‐Liss, Inc.     

Multiple sexual selection pressures drive the rapid evolution of complex morphology in a male secondary genital structure

The genitalia of internally fertilizing taxa represent a striking example of rapid morphological evolution. Although sexual selection can shape variation in genital morphology, it has been difficult to test whether multiple sexual selection pressures combine to drive the rapid evolution of individual genital structures. Here, we test the hypothesis that both pre‐ and postcopulatory sexual selection can act in concert to shape complex structural variation in secondary genital morphology. We genetically modified the size and shape of the posterior lobes of Drosophila melanogaster males and tested the consequences of morphological variation on several reproductive measures. We found that the posterior lobes are necessary for genital coupling and that they are also the targets of multiple postcopulatory processes that shape quantitative variation in morphology, even though these structures make no direct contact with the external female genitalia or internal reproductive organs during mating. We also found that males with smaller and less structurally complex posterior lobes suffer substantial fitness costs in competitive fertilization experiments. Our results show that sexual selection mechanisms can combine to shape the morphology of a single genital structure and that the posterior lobes of D. melanogaster are the targets of multiple postcopulatory selection pressures.     

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.     

Female genital configuration in the classification of Lepidoptera

In Coleoptera, Neuroptera, and Megaloptera, and the panorpoid orders Diptera, Trichoptera, and Mecoptera the common oviduct is ventral to, or opens into, the vagina or genital chamber. In Lepidoptera, in the superfamilies Micropterigoidea, Eriocranioidea, Incurvarioidea, and Nepticuloidea, the common oviduct enters the copulatory chamber ventrally; in Mnesarchaeidae, Hepialoidea, and all Ditrysia auct. the common oviduct is dorsal to the copulatory chamber, and the vagina (that region posterior to the entry of the spermatheca) opens separately from the genital ostium. Lepidoptera are unique in the complex and various arrangements of the ectodermal elements in the female genitalia.

The arrangements of, and connections between rectal and genital structures in representatives of Megaloptera, Neuroptera, and the panorpoid orders are re‐examined for comparison with the systems found in Zeugloptera, Dacnonypha, Monotrysia, and Ditrysia auct.

The Zeugloptera are here included in Lepidoptera because they have a circumcloacal chamber. Other female zeuglopteran genital structures intergrade with those of Dacnonypha, here restricted to Eriocraniidae, Agathiphagidae, Lophocoronidae, and the divergent Acanthopteroctetes. Dacnonypha have a less specialised spermathecal and vaginal structure than do most of the Monotrysia, here restricted to Incurvarioidea and Nepticuloidea (including Tischeriidae); many Monotrysia lack a cloaca, whereas it is always present in Dacnonypha.

There is no basis for retaining either Mnesarchaeidae or Hepialoidea in Monotrysia auct., as the dorsal common oviduct and the two genital openings indicate that these groups are ditrysian. They are here regarded as exoporian Ditrysia, a group characterised by the lack of a free, tubular ductus seminalis. A fixed gutter or channel between ostium and ovipore characterises the Hepialoidea, and the absence of this channel (ostium and ovipore opposable within an external genital pouch) characterises Mnesarchaeidae.

The endoporian Ditrysia all have a free, tubular ductus seminalis; where a cloaca is present it is incomplete, i.e., combines ovipore and rectum but never copulatory structures, in contrast to the complete cloaca found in Zeugloptera, Dacnonypha, and many Monotrysia. The endoporian Ditrysia comprise all other superfamilies, i.e., about 97% of species of Lepidoptera.     

The principal structure of male genital sclerites and muscles of bombycoid moths, with special reference to Anthelidae (Lepidoptera: Bombycoidea)

Male genital structures and muscles of bombycoid moths have repeatedly been misidentified in the literature. Furthermore, the genital structures of some bombycoid families, such as the poorly known Australo-New Guinean Anthelidae, have essentially remained unstudied. Based on comparative morphology, this study details the principal arrangements of male genital sclerites and muscles in all bombycoid families, with particular focus on basic structures and their modifications in Anthelidae. Emphasis is placed on the homology of and fusions between these structures and their function, providing a basis for the interpretation of modifications in future phylogenetic and taxonomic studies. This includes the unique fusion of gnathos and valvae in several bombycoid families, the arrangement and extent of the fused tegumen and vinculum, as well as the homology of the "transtilla". Further, a modification of the valve adductor muscle (the segment IX sternum to valva muscle, m4) widely regarded as a synapomorphy of Bombycoidea is demonstrated to be non-existent, as is the presumed presence of the valve abductor muscle (the segment IX tergum to valva muscle, m2) in Saturniidae.     

The evolution of male genitalia: functional integration of genital sclerites in the dung beetle Onthophagus taurus

It is now widely recognized that sexual selection has been important in the rapid and divergent evolution of male genital morphology. However, distinguishing among putative mechanisms of sexual selection acting on male genital morphology represents a considerable challenge. Although there is growing evidence that variation in the size and/or shape of male genital structures can determine a male's success in gaining fertilizations, our knowledge of the functional morphology of male genitalia remains limited. Here we examine the functional morphology of genital sclerites that are known to influence paternity in the dung beetle Onthophagus taurus . We show that three of the sclerites form a functionally integrated unit that generates the tubular-shaped spermatophore and delivers its opening to the female's spermathecal duct. A fourth sclerite acts as a holdfast device during copulation. Our observations shed light on the mechanism by which these sclerites influence a male's paternity, and their patterns of phenotypic and genetic (co)variation.  © 2008 The Linnean Society of London, Biological Journal of the Linnean Society , 2008, 93 , 257–266.     

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.     

Experimental modifications imply a stimulatory function for male tsetse fly genitalia,supporting cryptic female choice theory

One of the most sweeping of all patterns in morphological evolution is that animal genitalia tend to diverge more rapidly than do other structures. Abundant indirect evidence supports the cryptic female choice (CFC) explanation of this pattern, which supposes that male genitalia often function to court females during copulation; but direct experimental demonstrations of a stimulatory function have been lacking. In this study, we altered the form of two male genital structures that squeeze the female’s abdomen rhythmically in Glossina pallidipes flies. As predicted by theory, this induced CFC against the male: ovulation and sperm storage decreased, while female remating increased. Further experiments showed that these effects were due to changes in tactile stimuli received by the female from the male’s altered genitalia, and were not due to other possible changes in the males due to alteration of their genital form. Stimulation from male genital structures also induces females to permit copulation to occur. Together with previous studies of tsetse reproductive physiology, these data constitute the most complete experimental confirmation that sexual selection (probably by CFC) acts on the stimulatory properties of male genitalia.     

Mode of transfer of spermatozoa in Orthoptera Tettigoniidae

A morphological and ultrastructural study was carried out on the spermatophore and spermatodoses of some species of Orthoptera Tettigoniidae. From the results concerning the spermatophore it emerged that this structure has a morphological and ultrastructural organization represented by a dilated ampulla and a peduncle or neck. From the examination of freshly deposited spermatophores and those at various time intervals thereafter, it was seen that these structures other than allowing gamete transfer, represent the site where spermatodesms, organized in the male genital tracts, undergo reorganization to acquire their definitive morphological and structural characteristics as found in the female genital tracts. The spermatodoses, in the same way as the spermatophore, represent capsules containing spermatodesms, which are originated in the spermatheca, their specific morphology seems to diversify according to the species considered. As regards their role, it is hypothesized that these structures represent a long-term conservation mechanism for spermatozoa inside the seminal receptacle.     

Condition dependence of male and female genital structures in the seed beetle Callosobruchus maculatus (Coleoptera: Bruchidae)

Theory predicts that costly secondary sexual traits will evolve heightened condition dependence, and many studies have reported strong condition dependence of signal and weapon traits in a variety of species. However, although genital structures often play key roles in intersexual interactions and appear to be subject to sexual or sexually antagonistic selection, few studies have examined the condition dependence of genital structures, especially in both sexes simultaneously. We investigated the responses of male and female genital structures to manipulation of larval diet quality (new versus once‐used mung beans) in the bruchid seed beetle Callosobruchus maculatus. We quantified effects on mean relative size and static allometry of the male aedeagus, aedeagal spines, flap and paramere and the female reproductive tract and bursal spines. None of the male traits showed a significant effect of diet quality. In females, we found that longer bursal spines (relative to body size) were expressed on low‐quality diet. Although the function of bursal spines is poorly understood, we suggest that greater bursal spine length in low‐condition females may represent a sexually antagonistic adaptation. Overall, we found no evidence that genital traits in C. maculatus are expressed to a greater extent when nutrients are more abundant. This suggests that, even though some genital traits appear to function as secondary sexual traits, genital traits do not exhibit heightened condition dependence in this species. We discuss possible reasons for this finding.