Gonadal Degeneration Is Mediated by Apoptotic Processes in the Semelparous Gray Side-Gilled Sea Slug Pleurobranchaea maculata

AbstractSpecies undergoing postreproductive death experience great changes in their reproductive organs, which are driven by numerous physiological processes. To assess whether apoptotic processes are involved in the dynamics of the reproductive organs of Pleurobranchaea maculata, the gonadal structure of this semelparous side-gilled sea slug was studied using light and scanning electron microscopy. Apoptotic cells at different gonadal developmental stages were detected by in situ TUNEL assay. Apoptosis was primarily focused on spermatogonia during gonadal cell proliferation, probably as a regulatory mechanism that maintains homeostasis in reproductive cells. Visible gonadal degeneration at the end of the reproductive period is accompanied by apoptosis of the basal lamina cells of the acini, suggesting that apoptotic processes are involved in the gonadal degeneration observed in P. maculata.

The new genus Aglaona: the first abyssal aglajid (Heterobranchia: Cephalaspidea: Aglajidae) with a description of two new species from the north-western Pacific Ocean

In this paper we describe the new genus Aglaona, the first abyssal genus of the family Aglajidae, comprising two new species: Aglaona rudmani sp. nov. from the Sea of Okhotsk (inhabiting a depth of 3206 m) and Aglaona valdesi sp. nov. from the Pacific slope of the Kuril Islands (at a depth of 3374–3580 m). For species descriptions and inference of relationships, we have followed an integrative approach, including molecular phylogenetic analyses based on four markers (COI, 16S, H3 and 28S) and a morphological analysis based on traditional anatomical dissections and scanning electron microscopy. The new genus is characterized by a well-developed radula with marginal teeth (2:1:0:1:2) and an internal bulloid shell with a wing-like parietal callus. External and internal morphology of Aglaonagen. nov. species is similar to that of the philinoid genus Laona (family Laonidae). Phylogenetic analyses support the inclusion of the new genus in the family Aglajidae, but its sister-relationships are unresolved. Our results suggest that Aglaonagen. nov. possesses several plesiomorphic characters, and that the reduction of shell and radula in Aglajidae occurred in parallel in different lineages.

Frequent origins of traumatic insemination involve convergent shifts in sperm and genital morphology

Traumatic insemination is a mating behavior during which the (sperm) donor uses a traumatic intromittent organ to inject an ejaculate through the epidermis of the (sperm) recipient, thereby frequently circumventing the female genitalia. Traumatic insemination occurs widely across animals, but the frequency of its evolution, the intermediate stages via which it originates, and the morphological changes that such shifts involve remain poorly understood. Based on observations in 145 species of the free-living flatworm genus Macrostomum, we identify at least nine independent evolutionary origins of traumatic insemination from reciprocal copulation, but no clear indication of reversals. These origins involve convergent shifts in multivariate morphospace of male and female reproductive traits, suggesting that traumatic insemination has a canalizing effect on morphology. We also observed sperm in both the sperm receiving organ and within the body tissue of two species. These species had intermediate trait values indicating that traumatic insemination evolves through initial internal wounding during copulation. Finally, signatures of male-female coevolution of genitalia across the genus indicate that sexual selection and sexual conflict drive the evolution of traumatic insemination, because it allows donors to bypass postcopulatory control mechanisms of recipients.

No hybrid snowcocks in the Altai-Hyper-variable markers can be problematic for phylogenetic inference

A recent article in Ecology and Evolution featured the discovery of hybrid snowcocks (Tetraogallus) and speculated on the hybrid origin of an extant species (T. altaicus). Comprehensive re-analyses of original data from the latter paper reliably refute the phylogenetic hypothesis taken as firm evidence of a past hybridization event in these birds. The new re-analyses showed that there is no evidence of hybridization in these snowcocks from the data available so far.

Experimentally evolved and phenotypically plastic responses to enforced monogamy in a hermaphroditic flatworm

Sexual selection is considered a potent evolutionary force in all sexually reproducing organisms, but direct tests in terms of experimental evolution of sexual traits are still lacking for simultaneously hermaphroditic animals. Here, we tested how evolution under enforced monogamy affected a suite of reproductive traits (including testis area, sex allocation, genital morphology, sperm morphology and mating behaviour) in the outcrossing hermaphroditic flatworm Macrostomum lignano, using an assay that also allowed the assessment of phenotypically plastic responses to group size. The experiment comprised 32 independent selection lines that evolved under either monogamy or polygamy for 20 generations. While we did not observe an evolutionary shift in sex allocation, we detected effects of the selection regime for two male morphological traits. Specifically, worms evolving under enforced monogamy had a distinct shape of the male copulatory organ and produced sperm with shorter appendages. Many traits that did not evolve under enforced monogamy showed phenotypic plasticity in response to group size. Notably, individuals that grew up in larger groups had a more male-biased sex allocation and produced slightly longer sperm than individuals raised in pairs. We conclude that, in this flatworm, enforced monogamy induced moderate evolutionary but substantial phenotypically plastic responses.

Sexual conflict in hermaphrodites

Hermaphrodites combine the male and female sex functions into a single individual, either sequentially or simultaneously. This simple fact means that they exhibit both similarities and differences in the way in which they experience, and respond to, sexual conflict compared to separate-sexed organisms. Here, we focus on clarifying how sexual conflict concepts can be adapted to apply to all anisogamous sexual systems and review unique (or especially important) aspects of sexual conflict in hermaphroditic animals. These include conflicts over the timing of sex change in sequential hermaphrodites, and in simultaneous hermaphrodites, over both sex roles and the postmating manipulation of the sperm recipient by the sperm donor. Extending and applying sexual conflict thinking to hermaphrodites can identify general evolutionary principles and help explain some of the unique reproductive diversity found among animals exhibiting this widespread but to date understudied sexual system.

Molecular and morphological systematics of neustonic nudibranchs (Mollusca : Gastropoda : Glaucidae : Glaucus), with descriptions of three new cryptic species

A recent molecular phylogenetic study on Glaucus, a genus of neustonic aeolid nudibranchs, revealed undescribed cryptic diversity. Glaucus atlanticus is sister to the traditional species Glaucus marginatus, which is a complex of four genetically distinct cryptic species (Informal clade ‘Marginatus’). The present paper revises the systematics of Glaucus and provides formal descriptions for three new species in the informal clade ‘Marginatus’ substantiated by species delimitation analyses. Molecular and morphological evidence confirms that the type species of Glaucus, Glaucus atlanticus, has a cosmopolitan subtropical distribution and is characterised by having a uniseriate ceratal arrangement, a penial spine and a longitudinal, medial silver stripe on the sole of the foot. Examination of type material indicates that the name G. marginatus should be retained for the most widespread of these species, found in the Indian and Pacific Oceans. This species is characterised by molecular diagnostic characters as well as the presence of a bursa copulatrix. Glaucus marginatus is sister to the undescribed species Glaucus bennettae, sp. nov., which is found in the South Pacific Ocean and lacks a bursa copulatrix. The other two undescribed species, Glaucus thompsoni, sp. nov. and Glaucus mcfarlanei, sp. nov. are only known from the North Pacific Ocean, and are characterised by molecular diagnostic characters as well as possessing and lacking a bursa copulatrix, respectively. Because sister species of Glaucus differ in their reproductive anatomy, we hypothesise that mating behaviour has played a role in cladogenesis in this group. ZooBank Publication code: http://zoobank.org/References/E352E264-A440-4AF1-8565-B57B7EEE25BC

Phylogeny of the family Aglajidae (Pilsbry, 1895) (Heterobranchia: Cephalaspidea) inferred from mtDNA and nDNA

The family Aglajidae includes several species of benthic, carnivorous cephalaspidean sea slugs, which generally lack a radula, have an internal shell, a posterior shield with short to moderate caudal lobes, and sensory cilia present on the head. The present study reports a phylogenetic analysis of the Aglajidae based on the mitochondrial genes 16S and CO1 and the nuclear gene H3, including 160 specimens of 54 species, that confirms the monophyly of Aglajidae as well as most taxonomically established genera, with some exceptions. Although support values are low for some clades, the analysis recovered the following clades within the Aglajidae: Odontoglaja, Nakamigawaia, and Melanochlamys. Chelidonura appears to be paraphyletic and the monophyly of a Chelidonura-Navanax-Aglaja clade is strongly supported in the Bayesian analysis, plus three of the four individual gene trees (COI, COI without 3rd codon positions, 16S and H3). However, the relatively low levels of support in the maximum likelihood analyses prevent us from proposing the synonymization of Navanax and Aglaja with Chelidonura. Melanochlamys is the sister clade of Chelidonura+Aglaja+Navanax. Odontoglaja is basal to the rest of the Aglajidae, confirming previous hypotheses on the loss of the radula in Aglajidae. Nakamigawaia and Melanochlamys are monophyletic, and should be maintained as valid. The monophyly of Philinopsis is strongly supported in the Bayesian analysis and in three of the four individual gene trees. Further research on this group is necessary to further affirm the monophyly of Chelidonura+Aglaja+Navanax and Philinopsis. Based on the results of this phylogenetic analysis, a reclassification of the taxonomy of Aglajidae is probably necessary. Additional genes should provide more information and probably fully resolve this situation. The present molecular study (including ABGD species delineation analyses) suggests the existence of previously undetected species complexes that require additional study to determine the extent of undocumented biodiversity.

Reproductive strategies in hermaphroditic gastropods: conceptual and empirical approaches

An individual optimizes its reproductive success by adopting a particular reproductive strategy. Studying the details of a reproductive strategy leads to an understanding of how sexual selection acts, as the former is the process via which the individual reproduces successfully. Hermaphroditic gastropods display a bewildering diversity of reproductive strategies, which may be due to their mode of gender expression, when compared with well-studied separate-sexed species. Extensive theoretical, observational, and experimental research has been conducted on this topic. However, despite our knowledge about the reproductive system of hermaphroditic gastropods, we still need to fill the gap between pre- and post-copulatory processes and reproductive success. Here, we review and propose conceptual and empirical approaches aimed at understanding reproductive strategies of hermaphroditic gastropods. In sum, our suggestions are (i) to focus on sex-biased traits, (ii) to take biologically reliable measurements at both the pre- and post-copulatory level that relate to reproductive success, and (iii) to examine the fitness consequences of biased sex allocation.

Color pattern variation in a shallow-water species of opisthobranch mollusc

The color pattern of benthic opisthobranch sea slugs (Mollusca: Gastropoda) appears to play an important defensive role, and numerous species seem to have aposematic (warning) colorations. Color pattern is an important trait for opisthobranch identification-this conclusion is based on the assumption that most species have limited color variation. For those species in which color variation is recognized, the reasons for the variation remain unknown. In this paper we study Philinopsis pusa, a benthic putative species of opisthobranch sea slug with a broad range of color pattern. Lighter individuals appear to be camouflaged on the white sand environment in which the animals are typically found, whereas darker individuals appear conspicuously different from their background. Because of its broad color variation, P. pusa has been subdivided into different species. Animals were collected and observed in the Bahamas during a 6-year span. The color pattern of the specimens was subjectively classified into five phenotypic classes. Two mitochondrial genes (16S, CO1) were sequenced from 41 specimens. The association between color pattern, body length, burrowing escaping behavior, and the genetic structure of the population was investigated. We found two genetically distinct groups in the target population but no significant association between color pattern and genetic structure. Additionally, there was no significant association between color pattern and ontogeny or defensive behavior in these organisms. The present paper suggests that general assumptions on the biological and evolutionary role of color in opisthobranchs need to be carefully evaluated.

Functions, diversity, and evolution of traumatic mating

Copulation can involve the wounding of the mating partner by specialised devices. This type of mating, which we term traumatic mating, has been regarded as exceptional. Its prevalence, however, has not been compared across taxa, nor have its functions and putative evolutionary pathways. A categorisation has been lacking to date. We here show that traumatic mating is a widespread and diverse phenomenon that likely evolved via several pathways. Its putative functions include: (i) anchorage during mating; (ii) stimulation of short-term female reproductive investment; (iii) male paternity advantages; and (iv) enhanced fertilisation efficiency in transitions to internal fertilisation. Both natural and sexual selection have likely contributed to the parallel evolution of traumatic intromittent organs in phylogenetically distant taxa. These organs are sometimes remarkably similar in shape and often, but not always, inject sperm. The target sites of trauma infliction and the nature of secretions delivered alongside sperm are thus far poorly studied, but data on both are needed to elucidate the function of traumatic mating. The few existing studies that explicitly quantify fitness impacts of traumatic mating indicate that this strategy may often be costly to the party being wounded. However, a comprehensive approach to assess overall investments and returns for both sexes is a major target for future work. Finally, for the first time, we corroborate quantitatively the hypothesis that traumatic mating evolved relatively more often among hermaphroditic than among gonochoric taxa.

The evolution of reproductive isolation in a simultaneous hermaphrodite, the freshwater snail Physa

BACKGROUND

The cosmopolitan freshwater snail Physa acuta has recently found widespread use as a model organism for the study of mating systems and reproductive allocation. Mitochondrial DNA phylogenies suggest that Physa carolinae, recently described from the American southeast, is a sister species of P. acuta. The divergence of the acuta/carolinae ancestor from the more widespread P. pomilia appears to be somewhat older, and the split between a hypothetical acuta/carolinae/pomilia ancestor and P. gyrina appears older still.

RESULTS

Here we report the results of no-choice mating experiments yielding no evidence of hybridization between gyrina and any of four other populations (pomilia, carolinae, Philadelphia acuta, or Charleston acuta), nor between pomilia and carolinae. Crosses between pomilia and both acuta populations yielded sterile F1 progeny with reduced viability, while crosses between carolinae and both acuta populations yielded sterile F1 hybrids of normal viability. A set of mate-choice tests also revealed significant sexual isolation between gyrina and all four of our other Physa populations, between pomilia and carolinae, and between pomilia and Charleston acuta, but not between pomilia and the acuta population from Philadelphia, nor between carolinae and either acuta population. These observations are consistent with the origin of hybrid sterility prior to hybrid inviability, and a hypothesis that speciation between pomilia and acuta may have been reinforced by selection for prezygotic reproductive isolation in sympatry.

CONCLUSIONS

We propose a two-factor model for the evolution of postzygotic reproductive incompatibility in this set of five Physa populations consistent with the Dobzhansky-Muller model of speciation, and a second two-factor model for the evolution of sexual incompatibility. Under these models, species trees may be said to correspond with gene trees in American populations of the freshwater snail, Physa.

Sexual conflict in Gerris gillettei (Insecta: Hemiptera): intraspecific intersexual correlated morphology and experimental assessment of behaviour and fitness

The contemporary dynamics of sexually antagonistic coevolution caused by sexual conflicts have seldom been investigated at the intraspecific level. We characterized natural populations of Gerris gillettei and documented significant intersexual correlations for morphological traits previously related to sexual conflict in water striders. These results strongly indicate that sexually antagonistic coevolution contributed to population differentiation and resulted in different balances of armaments between the sexes within natural populations of this species. No-choice mating experiments further revealed that both male and male-female relative arms levels influence copulation duration. However, there were no asymmetries in reproductive behaviour and fitness between sympatric and allopatric mating pairs, suggesting that differentiation by sexual conflict was not sufficient to influence the outcome of mating interactions. Altogether, these results question the relative importance of female connexival spines vs. genitalia traits in mediating pre- and post-copulatory conflict in Gerris.

Bateman gradients in hermaphrodites: an extended approach to quantify sexual selection

Sexual selection is often quantified using Bateman gradients, which represent sex-specific regression slopes of reproductive success on mating success and thus describe the expected fitness returns from mating more often. Although the analytical framework for Bateman gradients aimed at covering all sexual systems, empirical studies are biased toward separate-sex organisms, probably because important characteristics of other systems remain incompletely treated. Our synthesis complements the existing Bateman gradient approach with three essential reproductive features of simultaneous hermaphrodites. First, mating in one sex may affect fitness via the opposite sex, for example, through energetic trade-offs. We integrate cross-sex selection effects and show how they help characterizing sexually mutualistic versus antagonistic selection. Second, male and female mating successes may be correlated, complicating the interpretation of Bateman gradients. We show how to quantify the impact of this correlation on sexual selection and propose a principal component analysis on male and female mating success to facilitate interpretation. Third, self-fertilization is accounted for by adding selfed progeny as a separate category of reproductive success to analyses of Bateman gradients. Finally, using a worked example from the snail Biomphalaria glabrata, we illustrate how the extended analytical framework can enhance our understanding of sexual selection in hermaphroditic animals and plants.

Cutaneous application of an accessory-gland secretion after sperm exchange in a terrestrial slug (Mollusca: Pulmonata)

Competition for fertilisation in hermaphroditic animals seems to have led to many odd behaviours and complex morphologies involved in the transfer of accessory-gland products to the partner. Terrestrial slugs of the genus Deroceras show remarkably elaborate and interspecifically diverse penis morphologies and mating behaviours. Most species have an appending penial gland, which in Deroceras panormitanum consists of a few long fingers that are everted after sperm exchange and laid onto the partner's back. To investigate whether this gland transfers a secretion onto the partner's skin, we killed slugs at different mating stages and studied their penial glands and skin histologically. Two types of secretion granules appeared at a very early stage of courtship, and the penial gland was already filled 15min into the courtship. At copulation, the gland everted this secretion onto the partner's body, where it remained for at least 50min. No lysis of skin tissue or other effects on the skin were observed. The slugs tried to lick the received secretion off their own body, and some droplets were observed to be shed with the body mucus. Our results indicate the external application of a glandular substance that could function as either a pheromone or allohormone. The behaviours of the recipients suggest sexual conflict, although mutual interest cannot be ruled out.

A phylogenetic test of the size-advantage model: evolutionary changes in mating behavior influence the loss of sex change in a fish lineage

The size-advantage model asserts that mating behavior influences the incidence and direction of sex change in animals. Selection for protogyny (female to male sex change) occurs in mating systems in which large males monopolize and pair spawn with females; however, gonochorism (no sex change) is favored when adults spawn in groups and sperm competition is present. Despite widespread empirical and theoretical support for the model, these predictions have not been tested within a phylogenetic context. Here we show that the loss of sex change within a lineage of reef fishes is influenced by evolutionary changes in two traits related to their mating behavior: mating group structure and sperm competition intensity. Phylogenetic reconstructions of the reproductive evolution of groupers (Epinephelidae) indicate that protogyny and paired spawning are the ancestral conditions for the lineage; both gonochorism and group spawning evolved independently at least four times in three different genera. Evolutionary transformations from protogyny to gonochorism (loss of sex change) are associated with equivalent transformations in mating group structure from paired to group spawning, and sperm competition is considerably higher in gonochoric species than in protogynous species. These results provide explicit phylogenetic support for predictions of the size-advantage model, demonstrating that selection for protogynous sex change decreases as mating group size and sperm competition intensity increase.

Sexual selection is involved in speciation in a land snail radiation on crete

We investigated the importance of sexual selection in facilitating speciation in a land snail radiation on Crete. We used differences in the genitalia of the Cretan Xerocrassa species as potential indices of sexual selection. First, we rejected the hypothesis that differences in the genitalia of the Xerocrassa species can be explained by genetic drift using coalescent simulations based on a mitochondrial gene tree. Second, we showed that there is no evidence for the hypothesis that the differences in the genitalia can be explained by natural selection against hybrids under the assumption that this is more likely in geographically overlapping species pairs and clades. Third, we showed that there is a positive scaling between male spermatophore-producing organs and female spermatophore-receiving organs indicating sexual coevolution. The spermatophore enables the sperm to escape from the female gametolytic organ. Thus, the coevolution might be a consequence of sexual conflict or cryptic female choice. Finally, we showed that the evolution of differences in the length of the flagellum that forms the tail of the spermatophore is concentrated toward the tips of the tree indicating that it is involved in speciation. If speciation is facilitated by sexual selection, niches may remain conserved and nonadaptive radiation may result.

Evidence for costs of mating and self-fertilization in a simultaneous hermaphrodite with hypodermic insemination, the Opisthobranch Alderia willowi

Simultaneous hermaphrodites offer the chance to study antagonistic coevolution between the sexes when individuals function in both roles. Traumatic mating by hypodermic insemination has repeatedly evolved in hermaphroditic taxa, but evidence for the fitness costs of such male-advantage traits is lacking. When reared in isolation, specimens of the sea slug Alderia willowi (Opisthobranchia: Sacoglossa) initially laid clutches of unfertilized eggs but 4 days later began self-fertilizing; this is only the third report of selfing in an opisthobranch. Hypodermic insemination may allow selfing in Alderia if penetration of the body wall bypasses internal mechanisms that promote outcrossing. Selfing specimens and slugs reared in pairs had reduced fecundity compared to isolated slugs laying unfertilized clutches, suggesting that hypodermic insemination imposes a cost of mating. Egg production increased for field-caught slugs separated after mating compared to slugs held in pairs, a further indication that accessibility to mates imposes a fitness cost to the female function. Such antagonism can confer a competitive advantage to slugs mating in the male role but diminish reproduction in the female role among hermaphrodites capable of long-term sperm storage. Alderia willowi is also a rare case of poecilogony, with adults producing either planktotrophic or lecithotrophic larvae. Our rearing studies revealed that most slugs switched between expressed development modes at some point; such reproductive flexibility within individuals is unprecedented, even among poecilogonous species.

Sex allocation and sexual conflict in simultaneously hermaphroditic animals

Links between sex allocation (SA) and sexual conflict in simultaneous hermaphrodites have been evident since Charnov's landmark paper published 30 years ago. We discuss two links, namely the potential for sexual conflict over SA between sperm donor and recipient, and the importance of post-copulatory sexual selection and the resulting sexual conflict for the evolution of SA. We cover the little empirical and theoretical work exploring these links, and present an experimental test of one theoretical prediction. The link between SA and sexual conflict is an interesting field for future empirical and theoretical research.

Intraspecific divergence in sperm morphology of the green sea urchin, Strongylocentrotus droebachiensis: implications for selection in broadcast spawners

BACKGROUND

Sperm morphology can be highly variable among species, but less is known about patterns of population differentiation within species. Most studies of sperm morphometric variation are done in species with internal fertilization, where sexual selection can be mediated by complex mating behavior and the environment of the female reproductive tract. Far less is known about patterns of sperm evolution in broadcast spawners, where reproductive dynamics are largely carried out at the gametic level. We investigated variation in sperm morphology of a broadcast spawner, the green sea urchin (Strongylocentrotus droebachiensis), within and among spawnings of an individual, among individuals within a population, and among populations. We also examined population-level variation between two reproductive seasons for one population. We then compared among-population quantitative genetic divergence (QST) for sperm characters to divergence at neutral microsatellite markers (FST).

RESULTS

All sperm traits except total length showed strong patterns of high diversity among populations, as did overall sperm morphology quantified using multivariate analysis. We also found significant differences in almost all traits among individuals in all populations. Head length, axoneme length, and total length had high within-male repeatability across multiple spawnings. Only sperm head width had significant within-population variation across two reproductive seasons. We found signatures of directional selection on head length and head width, with strong selection possibly acting on head length between the Pacific and West Atlantic populations. We also discuss the strengths and limitations of the QST-FST comparison.

CONCLUSION

Sperm morphology in S. droebachiensis is highly variable, both among populations and among individuals within populations, and has low variation within an individual across multiple spawnings. Selective pressures acting among populations may differ from those acting within, with directional selection implicated in driving divergence among populations and balancing selection as a possible mechanism for producing variability among males. Sexual selection in broadcast spawners may be mediated by different processes from those acting on internal fertilizers. Selective divergence in sperm head length among populations is associated with ecological differences among populations that may play a large role in mediating sexual selection in this broadcast spawner.