Drosophila pachea asymmetric lobes are part of a grasping device and stabilize one-sided mating

Piezo-mediated mechanosensation contributes to stabilizing copulation posture and reproductive success in Drosophila males

In internal fertilization animals, reproductive success depends on maintaining copulation until gametes are transported from male to female. In Drosophila melanogaster, mechanosensation in males likely contributes to copulation maintenance, but its molecular underpinning remains to be identified. Here we show that the mechanosensory gene piezo and its' expressing neurons are responsible for copulation maintenance. An RNA-seq database search and subsequent mutant analysis revealed the importance of piezo for maintaining male copulation posture. piezo-GAL4-positive signals were found in the sensory neurons of male genitalia bristles, and optogenetic inhibition of piezo-expressing neurons in the posterior side of the male body during copulation destabilized posture and terminated copulation. Our findings suggest that the mechanosensory system of male genitalia through Piezo channels plays a key role in copulation maintenance and indicate that Piezo may increase male fitness during copulation in flies.

Resolving between novelty and homology in the rapidly evolving phallus of Drosophila

The genitalia present some of the most rapidly evolving anatomical structures in the animal kingdom, possessing a variety of parts that can distinguish recently diverged species. In the Drosophila melanogaster group, the phallus is adorned with several processes, pointed outgrowths, that are similar in size and shape between species. However, the complex three-dimensional nature of the phallus can obscure the exact connection points of each process. Previous descriptions based upon adult morphology have primarily assigned phallic processes by their approximate positions in the phallus and have remained largely agnostic regarding their homology relationships. In the absence of clearly identified homology, it can be challenging to model when each structure first evolved. Here, we employ a comparative developmental analysis of these processes in eight members of the melanogaster species group to precisely identify the tissue from which each process forms. Our results indicate that adult phallic processes arise from three pupal primordia in all species. We found that in some cases the same primordia generate homologous structures whereas in other cases, different primordia produce phenotypically similar but remarkably non-homologous structures. This suggests that the same gene regulatory network may have been redeployed to different primordia to induce phenotypically similar traits. Our results highlight how traits diversify and can be redeployed, even at short evolutionary scales.

No evidence for asymmetric sperm deposition in a species with asymmetric male genitalia

Background

Asymmetric genitalia have repeatedly evolved in animals, yet the underlying causes for their evolution are mostly unknown. The fruit fly Drosophila pachea has asymmetric external genitalia and an asymmetric phallus with a right-sided phallotrema (opening for sperm release). The complex of female and male genitalia is asymmetrically twisted during copulation and males adopt a right-sided copulation posture on top of the female. We wished to investigate if asymmetric male genital morphology and a twisted gentitalia complex may be associated with differential allocation of sperm into female sperm storage organs.

Methods

We examined the internal complex of female and male reproductive organs by micro-computed tomography and synchrotron X-ray tomography before, during and after copulation. In addition, we monitored sperm aggregation states and timing of sperm transfer during copulation by premature interruption of copulation at different time-points.

Results

The asymmetric phallus is located at the most caudal end of the female abdomen during copulation. The female reproductive tract, in particular the oviduct, re-arranges during copulation. It is narrow in virgin females and forms a broad vesicle at 20 min after the start of copulation. Sperm transfer into female sperm storage organs (spermathecae) was only in a minority of examined copulation trials (13/64). Also, we found that sperm was mainly transferred early, at 2-4 min after the start of copulation. We did not detect a particular pattern of sperm allocation in the left or right spermathecae. Sperm adopted a granular or filamentous aggregation state in the female uterus and spermathecae, respectively.

Discussion

No evidence for asymmetric sperm deposition was identified that could be associated with asymmetric genital morphology or twisted complexing of genitalia. Male genital asymmetry may potentially have evolved as a consequence of a complex internal alignment of reproductive organs during copulation in order to optimize low sperm transfer rates.

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

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

Male genital lobe morphology affects the chance to copulate in Drosophila pachea

INTRODUCTION

Male genitalia are thought to ensure transfer of sperm through direct physical contact with female during copulation. However, little attention has been given to their pre-copulatory role with respect to sexual selection and sexual conflict. Males of the fruitfly Drosophila pachea have a pair of asymmetric external genital lobes, which are primary sexual structures and stabilize the copulatory complex of female and male genitalia. We wondered if genital lobes in D. pachea may have a role before or at the onset of copulation, before genitalia contacts are made.

RESULTS

We tested this hypothesis with a D. pachea stock where males have variable lobe lengths. In 92 mate competition trials with a single female and two males, females preferentially engaged into a first copulation with males that had a longer left lobe and that displayed increased courtship vigor. In 53 additional trials with both males having partially amputated left lobes of different lengths, we observed a weaker and non-significant effect of left lobe length on copulation success. Courtship durations significantly increased with female age and when two males courted the female simultaneously, compared to trials with only one courting male. In addition, lobe length did not affect sperm transfer once copulation was established.

CONCLUSION

Left lobe length affects the chance of a male to engage into copulation. The morphology of this primary sexual trait may affect reproductive success by mediating courtship signals or by facilitating the establishment of genital contacts at the onset of copulation.

Sexual selection on the genital lobes of male Drosophila simulans

Sexual selection is thought to be responsible for the rapid divergent evolution of male genitalia with several studies detecting multivariate sexual selection on genital form. However, in most cases, selection is only estimated during a single episode of selection, which provides an incomplete view of net selection on genital traits. Here, we estimate the strength and form of multivariate selection on the genitalia arch of Drosophila simulans when mating occurs in the absence of a competitor and during sperm competition, in both sperm defence and offense roles (i.e., when mating first and last). We found that the strength of sexual selection on the genital arch was strongest during noncompetitive mating and weakest during sperm offense. However, the direction of selection was similar across selection episodes with no evidence for antagonistic selection. Overall, selection was not particularly strong despite genitals clearly evolving rapidly in this species.

Repeated evolution of asymmetric genitalia and right-sided mating behavior in the Drosophila nannoptera species group

BACKGROUND

Male genitals have repeatedly evolved left-right asymmetries, and the causes of such evolution remain unclear. The Drosophila nannoptera group contains four species, among which three exhibit left-right asymmetries of distinct genital organs. In the most studied species, Drosophila pachea, males display asymmetric genital lobes and they mate right-sided on top of the female. Copulation position of the other species is unknown.

RESULTS

To assess whether the evolution of genital asymmetry could be linked to the evolution of one-sided mating, we examined phallus morphology and copulation position in D. pachea and closely related species. The phallus was found to be symmetric in all investigated species except D. pachea, which displays an asymmetric phallus with a right-sided gonopore, and D. acanthoptera, which harbors an asymmetrically bent phallus. In all examined species, males were found to position themselves symmetrically on top of the female, except in D. pachea and D. nannoptera, where males mated right-sided, in distinctive, species-specific positions. In addition, the copulation duration was found to be increased in the nannoptera group species compared to closely related outgroup species.

CONCLUSION

Our study shows that gains, and possibly losses, of asymmetry in genital morphology and mating position have evolved repeatedly in the nannoptera group. Current data does not allow us to conclude whether genital asymmetry has evolved in response to changes in mating position, or vice versa.

Fitness advantages of the biased use of paired laterally symmetrical penises in an insect

The evolution of laterality, that is the biased use of laterally paired, morphologically symmetrical organs, has attracted the interest of researchers from a variety of disciplines. It is, however, difficult to quantify the fitness benefits of laterality because many organs, such as human hands, possess multimodal functions. Males of the earwig Labidura riparia (Insecta: Dermaptera: Labiduridae) have morphologically similar laterally paired penises, only one of which is used for inseminating the female during a single copulation bout, and thus provide a rare opportunity to address how selection pressure may shape the evolution of population-level laterality. Our population studies revealed that in 10 populations, located at 2.23-43.3° north, the right penis is predominantly used for copulating (88.6%). A damaged penis was found in 23% of rare left-handers, suggesting that the left penis can function as a spare when the right one is damaged. By pairing L. riparia females with surgically manipulated males, we found that males forced to use the right penis outperformed left-handed males in copulation (the probability of establishing genital coupling during the 1-hr observation period: odds ratio [OR] of 3.50) and insemination (probability of transferring a detectable amount of sperm: OR of 2.94). This right-handed advantage may be due to the coiled morphology of the sperm storage organ with a right-facing opening. Thus, female genital morphology may play a significant role in the evolution of handedness and may have acted as a driving force to reduce penis number in related taxa.

The evolution of asymmetric genitalia in Coleoptera

The evolution of asymmetry in male genitalia is a pervasive and recurrent phenomenon across almost the entire animal kingdom. Although in some taxa the asymmetry may be a response to the evolution of one-sided, male-above copulation from a more ancestral female-above condition, in other taxa, such as Mammalia and Coleoptera, this explanation appears insufficient. We carried out an informal assessment of genital asymmetry across the Coleoptera and found that male genital asymmetry is present in 43% of all beetle families, and at all within-family taxonomic levels. In the most diverse group, Cucujiformia, however, genital asymmetry is comparatively rare. We also reconstructed the phylogeny of the leiodid tribe Cholevini, and mapped aspects of genital asymmetry on the tree, revealing that endophallus sclerites, endophallus, median lobe and parameres are, in a nested fashion, increasingly unlikely to have evolved asymmetry. We interpret these results in the light of cryptic female choice versus sexually antagonistic coevolution and advocate further ways in which the phenomenon may be better understood.

This article is part of the themed issue ‘Provocative questions in left–right asymmetry’.

Investigating the functional morphology of genitalia during copulation in the grasshopper Melanoplus rotundipennis (Scudder, 1878) via correlative microscopy

We investigated probable functions of the interacting genitalic components of a male and a female of the flightless grasshopper species Melanoplus rotundipennis (Scudder, 1878) (frozen rapidly during copulation) via correlative microscopy; in this case, by synergizing micro-computed tomography (micro-CT) with digital single lens reflex camera photography with focal stacking, and scanning electron microscopy. To assign probable functions, we combined imaging results with observations of live and museum specimens, and function hypotheses from previous studies, the majority of which focused on museum specimens with few investigating hypotheses in a physical framework of copulation. For both sexes, detailed descriptions are given for each of the observed genitalic and other reproductive system components, the majority of which are involved in copulation, and we assigned probable functions to these latter components. The correlative microscopy approach is effective for examining functional morphology in grasshoppers, so we suggest its use for other animals as well, especially when investigating body regions or events that are difficult to access and understand otherwise, as shown here with genitalia and copulation. J. Morphol. 278:334-359, 2017. © 2017 Wiley Periodicals, Inc.