Two intromittent organs in Zorotypus caudelli (Insecta,Zoraptera): the paradoxical coexistence of an extremely long tube and a large spermatophore

Very unusual genitalia of the species Zorotypus caudelli are described. It contains the unique configuration of two different intromittent organs, one of them strongly elongated. Hyper elongated genitalia are known in different groups of insects. Males have to accommodate these unwieldy structures in the limited spaces of the abdomen and manipulate them acutely during copulation. A crucial question is how do species with elongated genitalia cope with these requirements? To investigate this, we studied key features enabling storage, insertion, and withdrawal of the elongated genitalia. The co‐existence of an elongated narrow tube and a bulky spermatophore is a highly unusual and apparently paradoxical condition. However, we demonstrate that the tube is not involved in sperm transmission, whereas the large spermatophore is transferred to females by a membranous fold of the genitalia. The movement of the spermatophore is caused by haemolymph pressure, which likely also promotes the insertion of both intromittent organs. A comparison with the genital anatomy and reproductive mode in related groups suggests that the elongated tube and its accommodating pouch is a de novo structure, and that the ancestral sperm transport via spermatophore is a preadaptive condition for the acquisition of this unusual structure. © 2014 The Linnean Society of London, Biological Journal of the Linnean Society, 2014, 112 , 40–54.     

Genital variation in a dimorphic moth Selenia tetralunaria (Lepidoptera, Geometridae)

Insect genitals vary greatly among species and provide a key tool for species-level taxonomy. Insects differing in the genitalia are often treated as discrete, reproductively isolated species. This principle dates back to the lock-and-key hypothesis, which states that genitalia vary between species in order to provide a mechanical reproductive isolation system. Thus, the hypothesis assumes low within-species variability in genital traits. However, recent studies suggest that sexual selection may be responsible for the evolution of insect genitalia. We studied allometry and genital size and shape variation in a dimorphic moth Selenia tetralunaria . We found that the genitalia showed negative allometry in relation to body size as reported in many insect and spider species. This allometry was stronger in internal genital structures than it was in external genitalia. We also found that there was more variation in internal compared with external genitalia. Finally, we found that the shape of genital structures differed between morphs in all three examined areas. S. tetralunaria is among the first reported cases of genitally dimorphic insect species. Considerable variation in internal genitalia and especially the presence of genital shape differences between morphs were not consistent with the predictions of the lock-and-key hypothesis.  © 2006 The Linnean Society of London, Biological Journal of the Linnean Society , 2006, 87 , 297–307.     

Intraspecific and interspecific variation of female genitalia in two species of watersnake

The morphological differences in female genitalia within and between species are little studied and poorly understood, yet understanding patterns of variation in female genitalia can provide insights into mechanisms of genital evolution. The present study aimed to explore the patterns of intraspecific and interspecific variation in female genitalia in two sister taxa of watersnake (Nerodia sipedon and Nerodia fasciata) that have similar genital shape. We used a geometric morphometric (GM) approach to study variation in shape of the vagina between and within two sister species. We examined genital shape in female watersnakes ranging from small, sexually immature females to large reproductive females that had recently given birth. We found that shape variation of genitalia is strongly correlated with body size, where larger but not smaller females have a bifurcation in the vagina. However, we also found significant shape variation in the structure of the vagina between the two species, where N. fasciata has narrower genitalia with more prominent bifurcation, whereas N. sipedon has wider genitalia with less marked bifurcation. Using GM allowed us to detect significant differences in genital shape that were not apparent upon visual examination alone, suggesting that shape variation in female genitalia may be greater than previously assumed. Additional study of morphological differences in male reproductive organs for these species would help to determine whether there has been genital co‐evolution, and potentially mechanical reproductive isolation, in these two closely‐related and occasionally sympatric species. © 2013 The Linnean Society of London, Biological Journal of the Linnean Society, 2014, 111, 183–191.     

Phenotypic and genetic variation in male genitalia in the seedbug, Lygaeus equestris (Heteroptera)

Male genitalia evolve through sexual selection and, in insects, tend to show negative static allometry, low phenotypic variation, and are usually relatively small. Much less is known about the genetic variation and heritability of male genitalia. Additionally, in instances where the intromittent organ is greatly elongated, it is unclear whether typical patterns of genital scaling and variation also apply. In the present study, we investigated the allometry, variation, and heritability of male genital length in the seedbug, Lygaeus equestris , a species with a greatly elongated intromittent organ (i.e. almost as long as male body size). We found that genital length was negatively allometric, in spite of its great length, and was no more variable than nongenital traits. Additionally, genital length was significantly heritable and had considerable evolvability.  © 2009 The Linnean Society of London, Biological Journal of the Linnean Society , 2009, 98 , 400–405.     

Mosaic pattern of genital divergence in three populations of Schistocerca lineata Scudder, 1899 (Orthoptera: Acrididae: Cyrtacanthacridinae)

Sexual selection theory predicts that genital structures in isolated populations are likely to diverge, but male genitalia are often species-specific, which led to the idea that male genitalia are relatively invariable within species. Previous allometric studies collectively suggested that male genitalia are intraspecifically invariable in size compared with external body parts. We investigated whether male genitalia are invariable in shape in three populations of a grasshopper Schistocerca lineata Scudder, 1899, using two independent methods of geometric morphometric analyses. Specifically, we focused on the idea that male genitalia are complex structures consisting of many functionally different components, and studied how these individual parts diverge among three populations. Individual components of male genitalia show different population-level divergence, resulting in the mosaic pattern of genital divergence. Individual components diverge independently from each other. Body size is positively correlated with genitalia size, but is significantly correlated with the shape of only one of the three genital structures we measured. Thus, different components of male genitalia may be influenced by different evolutionary processes. This study is the first to show that components of complex genitalia evolve separately within a species.  © 2008 The Linnean Society of London, Biological Journal of the Linnean Society , 2008, 94 , 289–301.     

Divergence in genital morphology may contribute to mechanical reproductive isolation in a millipede

Genitalia appear to evolve rapidly and divergently in taxa with internal fertilization. The current consensus is that intense directional sexual selection drives the rapid evolution of genitalia. Recent research on the millipede Antichiropus variabilis suggests that the male genitalia are currently experiencing stabilizing selection – a pattern of selection expected for lock‐and‐key structures that enforce mate recognition and reproductive isolation. Here, we investigate how divergence in genital morphology affects reproductive compatibility among isolated populations of A. variabilis. Females from a focal population were mated first to a male from their own population and, second, to a male from one of two populations with divergent genital morphology. We observed variation in mating behavior that might indicate the emergence of precopulatory reproductive barriers: males from one divergent population took significantly longer to recognize females and exhibited mechanical difficulty in genital insertion. Moreover, we observed very low paternity success for extra‐population males who were successful in copulating. Our data suggest that divergence in genital shape may be contributing to reproductive isolation, and incipient speciation among isolated populations of A. variabilis.     

The spermatozoon of arthropoda. XXIV. Sperm metamorphosis in the diplopod Polyxenus

Specimens, representative of each of the major taxa of mosquitoes, were fixed in copula and the external genitalia examined by scanning electron microscopy. The periphery of the basin-like everted aedeagus of Aedus aegypti precisely matches that of the everted atrial membrane of the female. These structures are appressed during coitus and sealed by pressure of the paraprocts, aedeagal pouch and proctiger. When everted, the aedeagus of male Culex pipiens reveals a ridged dome that surrounds the genital opening. This dome seals itself laterally into a gutter formed by pad-like extensions of the female's genital lips and is sealed dorsally by pressure of the aedeagal apodeme. The aedeagus of another culicine species, Wyeomyia smithii, bears the gonopore at the apex of a spined tube. This tube is inserted between the female's genital lips and is sealed within the genital atrium. The aedeagus of the toxorhynchitine species Toxorhynchitis brevipalpus is immobile and is inserted deep within the genital atrium of the female where it is sealed by pressure of the atrial walls. Males of each of these mosquitoes deliver a mixture of semen and sperm to the copulatory bursa of the female. After withdrawal of the aedeagus, sperm is transferred to the spermathecae. In contrast, sperm of Anopheles quadrimaculatus are delivered directly to the spermathecal duct. The tube-like aedeagus is positioned by its leaflets during sperm transfer and is driven deep into the atrium, where a mixture of semen and sperm is ejaculated. The significance of mechanical barriers to mating between species is discussed.     

Evolution of a giant intromittent organ in Scydmaeninae (Coleoptera: Staphylinidae): Functional morphology of the male postabdomen in Mastigini

We compared the postabdominal architecture of Mastigini with extremely long (Stenomastigus) or short (Palaeostigus) aedeagus. A novel mode of copulation was discovered: males of Stenomastigus insert a paramere between the female's abdomen and elytra, and the intromission is stabilized by several structures of both sexes. The intrinsic aedeagal mechanism is indicated as responsible for inflating the endophallus, and the long flagellum does not penetrate the ductus spermathecae during copulation. The structure of the flagellum suggests that it is primarily responsible for the sperm transfer. Asymmetrical postabdominal rotators of the aedeagus were only found in Stenomastigus; they presumably facilitate the withdrawal of the genitalia; their origin as bundles separated from larger muscles is postulated. We discuss a scenario in which the evolution of elongated genitalia was facilitated by the lack of structural constraints and existing preadaptations. Benefits of stabilizing the copulation and intromission are indicated as the driving force for the evolution of extremely long aedeagi, while the short aedeagi might have the advantage of freedom of movements facilitating the initiation of copulation by males. Disruptive selection is suggested as a working hypothesis to further investigate mechanisms that have played a role in the evolution of genital structures of Mastigini.     

The male postabdomen of Stolotermes inopinus: a termite with unusually well-developed external genitalia (Dictyoptera: Isoptera: Stolotermitinae)

Klass, K.‐D., Thorne, B. L. and Lenz, M. 2000. The male postabdomen of Stolotermes inopinus: a termite with unusually well‐developed external genitalia (Dictyoptera: Isoptera: Stolotermitinae). —Acta Zoologica (Stockholm) 81 : 121–130 Stolotermes inopinus has large external male genitalia (phallic lobe), which contrast with the small genital papillae or lack of external genitalia of other Isoptera. As in the genital papilla of Mastotermesdarwiniensis, a ventral sclerite pair is present, the gonopore is located ventroterminally on the phallic lobe, and the genital area is entirely symmetrical – suggesting that this may be the groundplan condition of Isoptera. The relations of the phallic lobe to surrounding components like the subgenital plate, paraprocts, and certain muscles and nerves indicate that the lobe of S. inopinus is homologous with the phallomeres of other Dictyoptera. The bilateral symmetry and simple structure, however, are in strong contrast to the asymmetry and high complexity found in male genitalia of Blattaria and Mantodea. The postabdominal nervous system of S. inopinus resembles that of the cockroach Periplaneta americana. Indications are given that the Stolotermitinae are related to the Kalotermitidae, Rhinotermitidae, and Termitidae rather than to the Termopsinae.     

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.     

Functional morphology of Trichadenotecnum male and female genitalia analyzed using μCT (Insecta: Psocodea: Psocomorpha)

Although the great genital diversity of the barklouse genus Trichadenotecnum has been described in previous studies, the specific function of the genital structures during the copulation process has received less investigative attention. We reconstructed 3D-models of each structure of the male and female genitalia of Trichadenotecnum incognitum in copula and those of uncopulated male and female of Trichadenotecnum pseudomedium. By comparing the changes in male and female genital structures and related muscles in copulated and uncopulated states, the function of each genital structure can be described. During the copulation, we found that the female subgenital plate was hooked into the male body by the distal process on the male paraproct and was fixed by the male epiproct, hypandrium and phallosome. In addition, sexual coevolution was suggested by tightly contacting structures, that is, thorny male hypandrium and thickened membrane around the female spermapore plate. These results not only give us a new understanding copulation process of Trichadenotecnum, but also explain the reasons why genital structures are so divers in the genus.     

Comparative copulation anatomy of the Drosophila melanogaster species complex (Diptera: Drosophilidae)

The Drosophila melanogaster species complex consists of four species: D. melanogaster, D. simulans, D. sechellia and D. mauritiana. To identify these closely related species, researchers often examine the male genitalia, especially species‐specific shapes of the posterior process, as the most reliable and easily observable character. However, compared to genetic aspects, the evolutionary significance of the posterior process and other genital parts remains largely unexplained. By comparing genital coupling among these species, we revealed that the posterior processes, which are hidden under the female abdominal tergite VII when genital coupling is established, mesh with different parts of the intersegmental membrane between the tergite VIII and the oviscapts and that this membrane region broadens in a species‐specific manner. Furthermore, in D. simulans and D. sechellia, this membrane region is likely to incur wounds from the sharply pointed tip of the posterior process. On the basis of the use and functions of these and other genital parts, we discuss possible evolutionary forces underlying the diversification of genitalia in this group.     

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.     

Towards a phylogeny of the flesh flies (Diptera: Sarcophagidae): morphology and phylogenetic implications of the acrophallus in the subfamily Sarcophaginae

The morphology of the acrophallus, the distal portion of the male phallus carrying the phallotreme, was studied in 72 exemplar species representing 56 genera and subgenera of the flesh fly subfamily Sarcophaginae. For 42 of those species, scanning electron microscopy was used to clarify the phallic morphology. Terms used to describe the male genitalia were updated based on new interpretations of homology. Male genitalic characters, combined with other morphological characters of adult males and females and of larvae, were used to construct a phylogeny. The monophyly of the subfamily was supported, and some generic‐level sister‐group relationships proposed in the literature, but without previous cladistic analyses, were also supported. The genus Blaesoxipha Loew, as currently recognized, was not monophyletic in our analysis. The genus Helicobia Coquillett is synonymized with Sarcophaga Meigen syn. nov. and treated as a subgenus of the latter. The Sarcophaga subgenera Neobellieria Blanchard and Mehria Enderlein were not monophyletic. Many of the clades in the analysis were supported primarily or exclusively by male genitalic character states, highlighting the importance of the male genitalia as a source of morphological characters for sarcophagine phylogeny. © 2010 The Linnean Society of London, Zoological Journal of the Linnean Society, 2010, 158 , 740–778.     

Genital morphology of female goblin spiders (Arachnida: Araneae: Oonopidae) with functional implications

Spider genital morphology usually provides the best characters for taxonomy. Furthermore, functional genital morphology helps to understand the evolution of complex genitalia and their role in the context of sexual selection. The genital systems of most haplogyne spider families are poorly investigated with respect to their morphology. The present study investigates the female genitalia of the oonopids Oonops pulcher, Oonopinus kilikus, and Pseudotriaeris sp. by means of light microscopy and SEM. The male palps are briefly described. Females of O. pulcher store spermatozoa in an anterior and a posterior receptaculum (PRe). The genitalia resemble the primitive dysderoid genitalia supporting the hypothesis that the subfamily Oonopinae contains more basal oonopids. In O. kilikus, the anterior receptaculum is reduced to a sclerite. Spermatozoa are stored in a PRe. The receptacula of Pseudotriaeris sp. are reduced to sclerites. Spermatozoa in the uterus internus indicate that fertilization happens there or in the ovary. The anterior sclerite might serve females to lock the uterus during copulation as suggested for other gamasomorphines. The male palp of O. kilikus is simple, whereas the palps of O. pulcher and Pseudotriaeris sp. appear more complex. Complicated structures on the palp of Pseudotriaeris sp. indicate that males exert copulatory courtship.     

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.     

Goings‐on inside a worm: functional hypotheses derived from sexual conflict thinking

Different interests between mating partners regarding the fate of their gametes can lead to sexual conflicts in many species. Although these conflicts can sometimes be dealt with pre‐copulatorily (e.g. by choosing with which partners to mate), they often extend beyond copulation. Post‐copulatory sexual conflicts are expected to be particularly strong in simultaneous hermaphrodites because an individual may have to accept sperm in order to obtain an opportunity to donate sperm, reducing the effectiveness of pre‐copulatory conflict resolution. The present study investigates the post‐copulatory interactions between male and female sexual traits of a highly promiscuous simultaneous hermaphrodite, the free‐living flatworm Macrostomum lignano. Using light and electron microscopy, we show the different levels of complexity of the sperm and the genitalia, and derive hypotheses about how the different traits may represent evolutionary responses to such sexual conflicts. © 2010 The Linnean Society of London, Biological Journal of the Linnean Society, 2010, 99 , 370–383.     

Mating behavior and genital damage during copulation in the leaf beetle Lema coronata (Chrysomelidae: Criocerinae)

The criocerine leaf beetle Lema coronata Baly has extremely long genitalia that reach more than twice the body length in both sexes. We observed mating behavior of this species in the laboratory and inspected the male genital morphology using a scanning electron microscope. The males did not perform pre-copulatory courtship and post-copulatory guarding of the mates, and copulation lasted only for about 30 min. The surface of male genitalia is smooth without any special structures at the tip. A fragment of broken male genitalia was detected in the spermathecal duct of one female. We discuss the adaptive significance of male genital damage and the selective factor of elongated genitalia.