Sexual dimorphism and sexual conflict in the diving beetle Agabus uliginosus (L.) (Coleoptera: Dytiscidae)

Sexual conflict can drive intersexual arms races, with female resistance and male persistence traits coevolving antagonistically. Such arms races are well documented in some diving beetles, although the extent of sexual conflict in this family remains unclear. The European dytiscid Agabus uliginosus has a strikingly dimorphic female; individuals from most regions are smooth and male‐like, whereas those from some populations have a strongly roughened dorsum, a trait that has attracted the name dispar. We demonstrate that rough and smooth females differ consistently in the development of dorsal surface microreticulation, and that these females are associated with males that differ in the development of their persistence traits. These findings extend the occurrence of pre‐insemination sexual conflict and associated intrasexual dimorphism in Dytiscidae, and suggest that such mating systems are relatively widespread in these beetles.     

Speciation and mitochondrial DNA diversification of the diving beetles Agabus bipustulatus and A. wollastoni (Coleoptera, Dytiscidae) within Macaronesia

Diversification of populations of two morphologically similar diving beetles within the Agabus tristis group, A. wollastoni and A. bipustulatus, was investigated, with partial mtDNA cytochrome b (Cyt b) sequences, allozymes and landmark‐based morphometrics. The Madeiran endemic A. wollastoni was collected from 11 localities. Population genetic and morphological variation was compared to Scandinavian localities of the widespread west Palearctic A. bipustulatus, recorded also from the Azores. Agabus wollastoni and European A. bipustulatus specimens representing eight and 13 localities respectively, were used in evaluating their phylogenetic relationship. Maximum parsimony analysis of the Cyt b sequences showed that both the A. bipustulatus and A. wollastoni specimens form well‐supported monophyletic groups. Three lines of evidence suggest that Agabus wollastoni has speciated through a few founders: (1) a well‐supported mtDNA line; (2) the mean heterozygosity of A. wollastoni is lower when compared to A. bipustulatus on the mainland; and (3) several uncommon alleles of A. bipustulatus are missing in A. wollastoni. The Azorean A. bipustulatus population was drastically affected by the colonization event, since several loci have become fixed with a resulting lower mean heterozygosity. The colonization was relatively recent, as the mtDNA lineage represented in the Azores is deeply nested within the A. bipustulatus clade. Population structure shows moderate inbreeding of A. wollastoni, and extensive substructuring at all localities with moderate gene‐flow between them. Morphological variation in A. wollastoni showed significant differentiation among several populations. Island colonizations, population structure of A. wollastoni, and an observed pattern of variation of the α‐glycerophosphate dehydrogenase locus are discussed. © 2003 The Linnean Society of London, Biological Journal of the Linnean Society, 2003, 79, 653–666.     

Thermal tolerance and geographical range size in the Agabus brunneus group of European diving beetles (Coleoptera: Dytiscidae)

Aim Within clades, most taxa are rare, whilst few are common, a general pattern for which the causes remain poorly understood. Here we investigate the relationship between thermal performance (tolerance and acclimation ability) and the size of a species’ geographical range for an assemblage of four ecologically similar European diving beetles (the Agabus brunneus group) to examine whether thermal physiology relates to latitudinal range extent, and whether Brown’s hypothesis and the environmental variability hypothesis apply to these taxa. Location Europe. Methods In order to determine the species tolerances to either low or high temperatures we measured the lethal thermal limits of adults, previously acclimated at one of two temperatures, by means of thermal ramping experiments (± 1°C min^?1). These measures of upper and lower thermal tolerances (UTT and LTT respectively) were then used to estimate each species’ thermal tolerance range, as total thermal tolerance polygons and marginal UTT and LTT thermal polygons. Results Overall, widespread species have higher UTTs and lower LTTs than restricted ones. Mean upper lethal limits of the Agabus brunneus group (43 to 46°C), are similar to those of insects living at similar latitudes, whilst mean lower lethal limits (?6 to ?9°C) are relatively high, suggesting that this group is not particularly cold‐hardy compared with other mid‐temperate‐latitude insects. Widespread species possess the largest thermal tolerance ranges and have a relatively symmetrical tolerance to both high and low temperatures, when compared with range‐restricted relatives. Over the temperature range employed, adults did not acclimate to either high or low temperatures, contrasting with many insect groups, and suggesting that physiological plasticity has a limited role in shaping distribution. Main conclusions Absolute thermal niche appears to be a good predictor of latitudinal range, supporting both Brown’s hypothesis and the environmental variability hypothesis. Restricted‐range species may be more susceptible to the direct effect of climate change than widespread species, notwithstanding the possibility that even ‘thermally‐hardy’, widespread species may be influenced by the indirect effects of climate change such as reduction in habitat availability in Mediterranean areas.     

Fine structure of the compound eye of the fungus beetle Neotriplax lewisi (Coleoptera, Cucujiformia, Erotylidae)

Abstract. In many ways, the apposition eye of the erotylid fungus beetle Neotriplax lewisi resembles that of chrysomelids: its 400–500 mostly hexagonal ommatidia are of the acone type and possess "open rhabdoms," a tapetum is not present, and axons penetrate the basement membrane in distinct bundles of eight. The eye also shows some unusual features that, at present, defy clear functional interpretation. Firstly, the cuticle of the interfacetal areas stains differently from that of the corneal lenses and, secondly, the two rhabdom systems in each ommatidium (central and peripheral) both possess microvilli that are oriented in such a way as to permit e-vector discrimination. On the basis of comparisons with other open rhabdom eyes, it is postulated that vision in N. lewisi involves neither high resolving power nor superior absolute sensitivity. However, this beetle can distinguish illuminated from shaded areas, and seems specialized to make use of sky polarization (probably in the UV and green regions of the spectrum) and/or the position of the sun as a course-stabilizing function during flights.     

Speciation of Iberian diving beetles in Pleistocene refugia (Coleoptera, Dytiscidae)

The Mediterranean basin is an area of high diversity and endemicity, but the age and origin of its fauna are still largely unknown. Here we use species-level phylogenies based on approximately 1300 base pairs of the genes 16S rRNA and cytochrome oxidase I to establish the relationships of 27 of the 34 endemic Iberian species of diving beetles in the family Dytiscidae, and to investigate their level of divergence. Using a molecular clock approach, 18-19 of these species were estimated to be of Pleistocene origin, with four to six of them from the Late Pleistocene ( approximately 100 000 years). A second, lower speciation frequency peak was assigned to Late Miocene or Early Pliocene. Analysis of the distributional ranges showed that endemic species placed in the tip nodes of the trees are significantly more likely to be allopatric with their sisters than endemic species at lower node levels. Allopatric sister species are also significantly younger than sympatric clades, in agreement with an allopatric mode of speciation and limited subsequent range movement. These results strongly suggest that for some taxa Iberian populations were isolated during the Pleistocene long enough to speciate, and apparently did not expand their ranges to recolonize areas north of the Pyrenees. This is in contradiction to observations from fossil beetles in areas further north, which document large range movements associated with the Pleistocene glacial cycles hypothesized to suppress population isolation and allopatric speciation.     

Evolution of salinity tolerance in the diving beetle tribe Hygrotini (Coleoptera,Dytiscidae)

Some species of the diving beetle tribe Hygrotini (subfamily Hydroporinae) are among the few insects able to tolerate saline concentrations more than twice that of seawater. However, the phylogenetic relationships of the species of Hygrotini, and the origin and evolution of tolerance to salinity in this lineage, are unknown. In this work, we aim to reconstruct how many times salinity tolerance did evolve in Hygrotini, whether this evolution was gradual or if tolerance to hypersalinity could evolve directly from strictly freshwater (FW) species, and to estimate the probabilities of transition between habitats. We build a phylogeny with ca. 45% of the 137 species of Hygrotini, including all major lineages and almost all of the known halophile or tolerant species. We used sequence data of four mitochondrial (COI‐5′, COI‐3′, 16S + tRNA and NADH1) and three nuclear (28S, 18S and H3) gene fragments, plus ecological data to reconstruct the history of the salinity tolerance using Bayesian inference. Our results demonstrate multiple origins of the tolerance to salinity, although most saline and hypersaline species were concentrated in two lineages. The evolution of salinity was gradual, with no direct transitions from FW to hypersaline habitats, but with some reversals from tolerant to FW species. The oldest transition to saline tolerance, at the base of the clade with the highest number of saline species, was dated in the late Eocene‐early Oligocene, a period with decreasing temperature and precipitation. This temporal coincidence suggests a link between increased aridity and the development of tolerance to saline waters, in agreement with recent research in other groups of aquatic Coleoptera.     

Development of microsatellite markers for an extremely limited distributed rare diving beetle species,Acilius kishii,and a widely distributed species,Acilius japonicus (Coleoptera: Dytiscidae)

Acilius kishii Nakane, 1963 (Coleoptera: Dytiscidae) is an endangered diving beetle species distributed in only one location, Lake Yashaga‐Ike, Honshu Island, Japan. Acilius japonicus, which is related to A. kishii, is distributed widely in northern Honshu Island and Hokkaido Island in Japan. In this study, we identified 14 microsatellite loci for A. kishii and A. japonicus, including both polymorphic and monomorphic loci, using the next‐generation sequencing method. We observed that 5 and 10 loci showed polymorphisms in 31 and 32 individuals of A. kishii and A. japonicus, respectively. The observed and expected heterozygosities were 0.00–1.00 and 0.00–0.74, respectively. These microsatellite loci could be useful for future conservation genetic studies, including monitoring of genetic diversity and extinction risk of A. kishii.     

Evolution of the Juan Fernández diving beetle,Rhantus selkirki (Coleoptera,Dytiscidae)

Here we provide evidence that confinement in Robinson Crusoe Island (located about 660 km west of continental Chile) over evolutionary time leads to strong morphological modifications in diving beetle (Dytiscidae) larvae. We analysed a large set of morphological larval characters for all currently recognised genera of Colymbetinae as a framework, to infer phylogenetic relationships within the large genus Rhantus Dejean, 1833 and, in particular, of the charismatic Juan Fernández diving beetle, Rhantus selkirki Jäch, Balke & Michat, 2015, comparing our results with a recent phylogeny of the Colymbetinae based on DNA sequence data. We suggest that adaptation to the island's particular habitats resulted in the reversal of certain characters of R. selkirki back to the plesiomorphic states. This may cause the species to be erroneously interpreted as more ‘primitive’ if only morphological characters are analysed. Confinement in the particular, shallow and barely vegetated aquatic habitats of Robinson Crusoe Island for a long time seems to have led to this divergent morphology, particularly in characters related to swimming ability such as several leg and urogomphal setae. In this way, R. selkirki larvae secondarily resemble those of some earlier diverging dytiscid lineages such as Agabinae and Copelatinae, which typically creep on the bottom of water bodies and do not swim well.     

Range expansion and ancestral niche reconstruction in the Mediterranean diving beetle genus Meladema (Coleoptera,Dytiscidae)

Species of the genus Meladema (Dytiscidae, Colymbetinae) are some of the largest macroinvertebrates in the western Palearctic region, being top predators in fishless streams. Two of the three described species, Meladema imbricata (Wollaston, 1871) and Meladema lanio (Fabricius, 1775) are Macaronesian endemics from the Canary Islands and Madeira, respectively, while the third, Meladema coriacea Laporte, 1835, is widely distributed from Morocco and the Iberian Peninsula to Turkey, including the Canary Islands. Previous phylogenetic analysis using only mitochondrial markers revealed the existence of two cryptic lineages within M. coriacea, one restricted to Corsica and the other including the rest of sampled populations. We reconstruct here the evolutionary history of the species of Meladema using a more comprehensive sampling covering its whole geographical range, adding nuclear markers and Bayesian molecular dating. Using environmental niche modelling, we test for possible differences in climatic preferences among lineages and reconstruct their ancestral climatic niche. Our results strongly supported the existence of four monophyletic lineages represented by the three recognized species plus a fourth cryptic lineage with populations of M. coriacea from the Tyrrhenian islands (Corsica, Sardinia and Montecristo). This pattern is not likely to be the result of mitochondrial artefacts due to Wolbachia infection, as all 11 tested individuals were negative for this parasite. Dating analysis placed the origin of Meladema in the Middle Miocene although diversification among extant Meladema lineages started in the early Pleistocene and took place in a relatively short time period. Phylogeographic analysis inferred a continental origin of Meladema, with an independent colonization of the Macaronesian and Mediterranean islands. From the south‐western Mediterranean region, the continental M. coriacea expanded its range up to Bulgaria and Turkey in the northern basin and to Tunisia in the southern. Results of niche modelling showed that seasonality is the critical factor in shaping the current distribution of Meladema. Island lineages (M. imbricata, M. lanio and the Tyrrhenian lineage of M. coriacea) occur in sites with low seasonality, within the range of the reconstructed ancestral climatic niche of the genus. On the contrary, continental M. coriacea expanded its range to localities outside the ancestral climatic range of the genus, with a higher seasonality and aridity.     

Inter- and intrasexual dimorphism in the diving beetle Hydroporus memnonius Nicolai (Coleoptera: Dytiscidae)

Sexual conflict can drive rapid intersexual arms races, and lead to pronounced sexual dimorphism. Such dimorphism is frequent in diving beetles, where males typically possess expanded front and middle tarsi, supplied with adhesive setae to grasp females during mating, and females often have rough dorsal surfaces which hinder male attachment. In a number of species, females are dimorphic, being either smooth and male-like, or heavily sculptured dorsally. Smooth and sculptured females often have distinct biogeographies, and may be expected to be associated with specific counter-adaptations in males. The European diving beetle, Hydroporus memnonius Nicolai, includes a smooth male-like female, and a matt morph, var. castaneus Aubé, which are largely allopatric in distribution. We show that the two morphs differ in the density and intensity of their surface microreticulation, and that matt females are associated with morphologically distinct males, which have developed specific countermeasures on their tarsi, including a greater number of large adhesive setae, individually larger in area. Such males are expected to be more successful in pairing with both matt and shining females, and it is suggested that a process of population replacement, partly driven by sexual interactions, may occur where the two forms overlap in range.  © 2008 The Linnean Society of London, Biological Journal of the Linnean Society , 2008, 94 , 685–697.     

Eye and optic lobe metamorphosis in the sunburst diving beetle, Thermonectus marmoratus (Coleoptera: Dytiscidae)

Nearly nothing is known about the transition that visual brain regions undergo during metamorphosis, except for Drosophila in which larval eyes and the underlying neural structure are strongly reduced. We have studied the larvae of the sunburst diving beetle, Thermonectus marmoratus (Coleoptera: Dytiscidae), which are sophisticated visually oriented predators characterized by six elaborate stemmata on each side of the head and an associated large optic lobe. We used general neurohistological staining and 3D reconstruction to determine how the eyes and optic lobe of T. marmoratus change morphologically during metamorphosis. We find that in third (last) instar larvae, the adult neuropils are already forming de novo dorsally and slightly anteriorly to the larval neuropils, while the latter rapidly degenerate. Larval eyes are eventually reduced to distinct areas with dark pigmentation. This complete reorganization, which may be an evolutionarily conserved trait in holometabolous insects, occurs despite the considerable costs that must apply to such a visually complex animal. Our findings are consistent with the concept that stemmata are homologous to the most posterior ommatidia of hemimetabolous insects, an idea also recently supported by molecular data.     

Preference of oviposition plant and hatchability of the diving beetle,Dytiscus sharpi (Coleoptera: Dytiscidae) in the laboratory

For the conservation of the diving beetle, Dytiscus sharpi (Wehncke) (Coleoptera: Dytiscidae), which is included in the Red List of Japan, it is critical to understand its ecological background. In the present study, oviposition preference and hatchability were investigated under laboratory conditions. Possible candidates of oviposition plants were observed in the natural breeding habitats of D. sharpi. Eight aquatic plants were identified, and Oenanthe javanica (Blume) was the major species present in March, when first instar larvae appeared. The number of eggs laid was investigated under the following conditions: (i) eight field plants were authentically recreated at a similar covering rate to that in the natural habitat; (ii) each of the eight plants was set at the same covering rate; (iii) a single plant taken from the field was placed separately. Significant oviposition preference for O. javanica was found when these eight aquatic plants were present in the same aquaria. In addition, hatchability in O. javanica was significantly higher than that in any of the other plants. These results suggest that O. javanica is an important aquatic plant for oviposition by D. sharpi.     

Retinal ultrastructure may mediate polarization sensitivity in larvae of the Sunburst diving beetle, Thermonectus marmoratus (Coleoptera: Dytiscidae)

A number of invertebrates are known to be sensitive to the polarization of light and use this trait in orientation, communication, or prey detection. In these animals polarization sensitivity tends to originate in rhabdomeric photoreceptors that are more or less uniformly straight and parallel. Typically, polarization sensitivity is based on paired sets of photoreceptors with orthogonal orientation of their rhabdomeres. Sunburst diving beetle larvae are active swimmers and highly visual hunters which could potentially profit from polarization sensitivity. These larvae, like those of most Dytiscids, have a cluster of six lens eyes or stemmata (designated E1 through E6) on each side of the head capsule. We examined the ultrastructure of the photoreceptor cells of the principal eyes (E1 and E2) of first instar larvae to determine whether their rhabdomeric organization could support polarization sensitivity. A detailed electron microscopical study shows that the proximal retinas of E1 and E2 are in fact composed of photoreceptors with predominantly parallel microvilli and that neighboring rhabdomeres are oriented approximately perpendicularly to one another. A similar organization is observed in the medial retina of E1, but not in the distal retinas of E1&2. Our findings suggest that T. marmoratus larvae might be able to analyze polarized light. If so, this could be used by freshly hatched larvae to find water or within the water to break the camouflage of common prey items such as mosquito larvae. Physiological and behavioral tests are planned to determine whether larvae of T. marmoratus can actually detect and exploit polarization signals.     

The species delimitation problem applied to the Agabus bipustulatus complex (Coleoptera, Dytiscidae) in north Scandinavia

Agabus bipustulatus (Linnaeus) is one of the most common aquatic beetles in Europe. Two species have been traditionally recognized within the Palaearctic Agabus bipustulatus complex (Coleoptera, Dytiscidae) in Scandinavia: a lowland form A. bipustulatus and a high-altitude form A. solieri Aube. The specific status of solieri has been debated for more than a century but no quantitative investigation has been made to evaluate the status of this taxon. In this study we show that there is no clear-cut delimitation between the two forms, either morphological or genetic, across an altitudinal gradient in north Scandinavia. Morphological differences between 22 populations were analysed separately for each sex with both thin-plate splines relative warp analysis and 'classical-length' morphometries. Genetic variation at five polymorphic enzyme loci was analysed among seven populations. The morphological studies showed gradual variation correlated with altitude, in particular in the character that is traditionally used to separate solieri and bipustulatus , and in both the beetles' morphometric size and the lateral width of the metasternal plate, which is connected to flight capacity. The genetic study indicates that the a-Gpdh enzyme locus, which is involved in the transfer of energy to the flight muscles, is evidently subject to directional selection. Only minor population differences were observed without this system. Subdivision was found in some populations and was probably caused by migration from outside or within the local population. The overall conclusion is that there is no clear-cut species delimitation between A. bipustulatus and A. solieri in north Scandinavia. This indicates that A. solieri is a cold-adapted altitudinal form of the variable A. bipustulatus; additional support for this is the finding of solieri 'look alikes' in cold springs in areas normally inhabited by bipustulatus.     

Twisted rhabdomeres in the compound eye of a tipulid fly (Diptera)

Summary The individual rhabdomeres of the outer retinular cells (R1–6) in the tipulid fly, Ptilogyna, twist about their long axes. Proximally, the rhabdoms become partitioned off by processes from the retinular cells, so that the basal region of each rhabdomere is enclosed in a pocket formed by its own cell (Fig. 2). This organisation of the rhabdom enables each rhabdomere to twist while supported within its own retinular cell, and while the cell itself maintains its orientation with respect to the entire ommatidium. Theory predicts that the rhabdomeral twisting should significantly reduce the polarisation sensitivity of R1–6, but have little effect on the efficiency with which unpolarised light is absorbed.     

Paroster extraordinarius sp. nov., a new groundwater diving beetle from the Flinders Ranges, with notes on other diving beetles from gravels in South Australia (Coleoptera: Dytiscidae)

The first groundwater (stygobitic) diving beetle is reported from South Australia. Paroster extraordinarius sp. nov. (Dytiscidae: Hydroporini) is described and figured. Its morphology and mitochondrial DNA sequences place it in the hydroporine genus Paroster Sharp. Phylogenetic analysis shows that the new species is imbedded in a clade of stygobitic species from the Yilgarn area of Western Australia. The evolution of this species is discussed and compared with observations on the behaviour and distribution of other dytiscid beetles found in subterranean habitats in South Australia.     

Field observation of predation on a horsehair worm (Gordioida: Chordodidae) by a diving beetle larva Cybister brevis Aubé (Coleoptera: Dytiscidae)

Diving beetles (Coleoptera: Dytiscidae) are carnivorous in both the larval and adult stages; larvae are exclusively predatory, whereas adults also scavenge for food. They are known to prey on zooplankton, insects, gastropods, fish, amphibians and reptiles. However, there have been no previous reports detailing direct predation on adult Gordioida by Dytiscidae in the field. This study represents the first observation of a diving beetle larva, Cybister brevis Aubé (Coleoptera: Dytiscidae), predating on an adult horsehair worm (Gordioida: Chordodidae). This might be the first report of predation on horsehair worms by insects.     

Review of the genus Fontidessus Miller & Spangler, 2008 (Coleoptera,Dytiscidae, Hydroporinae,Bidessini) with description of four new species

The genus Fontidessus Miller & Spangler, 2008 (Coleoptera: Dytiscidae: Hydroporinae: Bidessini) is reviewed. The genus now includes seven species with three previously described, and four new species described here: F. microphthalmus Miller & Montano, sp. n.; F. bettae Miller & Montano, sp. n.; F. christineae Miller & Montano, sp. n., and F. aquarupe Miller & Montano, sp. n. Each species is diagnosed and described, including the previously known species, based on new specimens and new information. Habitus, male genitalia and other diagnostic features are illustrated for each species. A key to the seven species is provided. Fontidessus species are unique to hygropetric habitats in the Guiana Shield craton of northern South American.     

A novel,unusual (at least for beetles) mode of Kenyon cell production in the diving beetle Cybister lateralimarginalis Deg. (Coleoptera: Dytiscidae)

Kenyon cell production in the mushroom bodies of Cybister lateralimarginalis is a peculiar process. It has been found that each proliferative center contains one giant neuroblast, which divides unequally, and its smaller daughter cell becomes the 2nd order neuroblast dividing unequally as well. The smaller daughter cell of this neuroblast becomes a ganglion mother cell. The latter, as usual, divides equally producing two Kenyon cells.     

Comparative osteology of the penguin‐like mid‐Cenozoic Plotopteridae and the earliest true fossil penguins,with comments on the origins of wing‐propelled diving

We compared the osteology of the late Eocene to early Miocene penguin‐like Plotopteridae from the North Pacific Basin with that of Paleocene stem group representatives of the Sphenisciformes and identified previously unrecognized similarities and differences. New data on the osteology of plotopterids, like the shape of the caudal end of the mandible, support a position of plotopterids outside the Suloidea, the clade formed by Sulidae, Phalacrocoracidae, and Anhingidae. However, as assumed by previous authors, the diving adaptations of plotopterids and sphenisciforms are likely to have evolved independently, and the resemblances in different parts of the postcranial skeleton therefore constitute one of the more striking examples of parallelism among tetrapods. We note that close relatives of both plotopterids and penguins forage by plunge diving. Whereas underwater locomotion of diving birds with a swimming ancestor is usually driven by the feet, we hypothesize that plotopterids and other wing‐propelled divers are more likely to have had volant ancestors that initiated diving by shallow plunges into the sea.