Ischnura Species Research Articles

Diversity of UV Reflection Patterns in Odonata

Dragonflies and damselflies (Odonata) are large-eyed diurnal insects that exhibit a variety of color patterns on their wings and/or bodies. Because Odonata can perceive light with wavelengths extending from ultraviolet (UV) to red, the color patterns with UV reflection can be visually recognized by each other. The UV reflection patterns of Odonata have been investigated by using a UV camera and optically measuring the reflectance of body and wing surfaces. The UV reflection of Odonata can be classified into three classes, namely iridescent UV reflection, wax-based UV reflection, and UV reflection by epidermal granules. In this review, I present UV images of these three classes and representative examples of reflectance. Among the Odonata with colorful iridescent wings consisting of multilayer structures, some species (e.g., Chalcopteryx scintillans) predominantly reflect UV light, whereas other species (e.g., Calopteryx japonica) mostly reflect light at wavelengths above 400 nm. Whitish wax (called pruinescence) on the body and/or wing surface strongly reflect light including UV, due to the light-scattering fine structures produced by the wax. The chemical composition of the dragonfly’s abdominal UV-reflective wax differs from previously identified waxes of other organisms. The blue-tailed damselflies Ischnura species exhibit immature-adult-specific UV reflection by the pteridine pigment-based selective light scattering and absorption. Iridescent coloration and wax-based color changes are generally important for mate recognition and male-male competition in Odonata, although the ecological importance of UV reflection remains largely unknown except for few examples in damselflies.

Interspecific crossing between blue‐tailed damselflies Ischnura elegans and I. senegalensis in the laboratory

AbstractIschnura species (Odonata) are among the most common damselflies in the world, which often exhibit female color polymorphisms. One morph, called androchrome, is similar to males in its color pattern, whereas the other morphs, generally referred to as gynochromes, exhibit female‐specific colors. In several Ischnura species, the female polymorphism is heritable, although molecular and genetic mechanisms remain largely unknown. The dominant‐recessive patterns of the female color morphs may differ between species. For example, androchromic females are dominant to gynochromic females in Ischnura elegans, whereas androchromic females are recessive in Ischnura senegalensis. Here we report a case of interspecific hybridization between a gynochrome female of I. elegans and a male of I. senegalensis in the laboratory. We obtained 61 hybrid adult offspring, of which all 31 females were of gynochrome morph. DNA analyses of the hybrids confirmed that nuclear DNA sequences were derived from both parent species, whereas mitochondrial DNA sequences were maternally inherited. In the hybrids, the postocular spots of female heads, the shape of male appendages, and the color of female's cerci resembled those of I. elegans, whereas the size of abdominal blue spots was similar to that of I. senegalensis. The shape of prothorax and basal abdominal markings were intermediate in females. The larval developmental traits and the morphological changes in the final larval instar of the hybrids were similar to those of I. senegalensis. To our knowledge, this is the first report of hybrids between two damselfly species with different dominant‐recessive patterns of female color morphs.

Macroevolutionary Origin and Adaptive Function of a Polymorphic Female Signal Involved in Sexual Conflict

Intersexual signals that reveal developmental or mating status in females have evolved repeatedly in many animal lineages. Such signals have functions in sexual conflict over mating and can therefore influence sexually antagonistic coevolution. However, we know little about how female signal development modifies male mating harassment and thereby sexual conflict. Here, we combine phylogenetic comparative analyses of a color polymorphic damselfly genus (Ischnura) with behavioral experiments in one target species to investigate the evolutionary origin and current adaptive function of a developmental female color signal. Many Ischnura species have multiple female color morphs, which include a male-colored morph (male mimics) and one or two female morphs that differ markedly from males (heterochrome females). In Ischnura elegans, males and male-mimicking females express a blue abdominal patch throughout postemergence life. Using phenotypic manipulations, we show that the developmental expression of this signaling trait in heterochrome females reduces premating harassment prior to sexual maturity. Across species this signal evolved repeatedly, but in heterochrome females its origin is contingent on the signal expressed by co-occurring male-mimicking females. Our results suggest that the co-option of a male-like trait to a novel female antiharassment function plays a key role in sexual conflict driven by premating interactions.

The complete mitochondrial genome of Enallagma cyathigerum (Odonata: Coenagrionidae) and phylogenetic analysis

To better understand the diversity and evolution of Odonata, we sequenced and annotated the complete mitochondrial genome (mitogenome) of Enallagma cyathigerum. This mitogenome was 16,661 bp in size and encoded the typical 37 genes, i.e. 13 protein-coding genes (PCGs), 22 transfer RNA genes (tRNAs) and two ribosomal RNA genes. The nucleotide composition of the E. cyathigerum mitogenome was significantly biased toward A and T, with an A + T content of 74.2%. Eleven PCGs started with a typical ATN codon, whereas the remaining two PCGs (nad1 and nad3) used TTG as the initial codon. All the 22 tRNAs had a typical secondary cloverleaf structure. The Bayesian phylogenetic analysis based on the concatenated nucleotide sequences of 13 PCGs strongly supported the sister relationship of E. cyathigerum and two Ischnura species from the same family Coenagrionidae. The phylogenetic tree strongly supported the monophyly of each of the two suborders (Zygoptera and Anisoptera) and recovered a phylogeny of Zygoptera + (Anisoptera + Anisozygoptera).

Female color polymorphism ofIschnura capreolusHagen, 1861 (Odonata: Coenagrionidae) with notes on behavior and ontogenetic color changes

Polymorphism in coenagrionids is widely known, mainly for Ischnura. Here, we present the case of I. capreolus, a Neotropical species of which, until now, little information concerning color polymorphism and ontogenetic color changes was known. We used a marking and recapture method to evidence ontogenetic coloration changes in females. We show that, in addition to the gynochrome morph, I. capreolus females also exhibit at least one additional morph: an androchrome color pattern. Gynochrome females (97.1% of marked females) are green-yellow when young and change to a brownish pattern with age, within a week. Androchrome females (2.9% of marked females) exhibit blue coloration, similarly to males. Our study provides the first evidence of ontogenetic color change in I. capreolus. Furthermore, field observations show that I. capreolus is similar to other Ischnura species regarding sexual harassment behavior.

Genetic divergence predicts reproductive isolation in damselflies

Reproductive isolation is the defining characteristic of a biological species, and a common, but often untested prediction is a positive correlation between reproductive isolation and genetic divergence. Here, we test for this correlation in odonates, an order characterized by strong sexual selection. First, we measure reproductive isolation and genetic divergence in eight damselfly genera (30 species pairs) and test for a positive correlation. Second, we estimate the genetic threshold preventing hybrid formation and empirically test this threshold using wild populations of species within the Ischnura genus. Our results indicate a positive and strong correlation between reproductive isolation and genetic distance using both mitochondrial and nuclear genes cytochrome oxidase II (COII: r=0.781 and 18S-28S: r=0.658). Hybridization thresholds range from -0.43 to 1.78% for COII and -0.052-0.71% for 18S-28S, and both F1 -hybrids and backcrosses were detected in wild populations of two pairs of Ischnura species with overlapping thresholds. Our study suggests that threshold values are suitable to identify species prone to hybridization and that positive isolation-divergence relationships are taxonomically widespread.

The influence of stochastic and selective forces in the population divergence of female colour polymorphism in damselflies of the genus Ischnura

Disentangling the relative importance and potential interactions of selection and genetic drift in driving phenotypic divergence of species is a classical research topic in population genetics and evolutionary biology. Here, we evaluate the role of stochastic and selective forces on population divergence of a colour polymorphism in seven damselfly species of the genus Ischnura, with a particular focus on I. elegans and I. graellsii. Colour-morph frequencies in Spanish I. elegans populations varied greatly, even at a local scale, whereas more similar frequencies were found among populations in eastern Europe. In contrast, I. graellsii and the other five Ischnura species showed little variation in colour-morph frequencies between populations. F(ST)-outlier analyses revealed that the colour locus deviated strongly from neutral expectations in Spanish populations of I. elegans, contrasting the pattern found in eastern European populations, and in I. graellsii, where no such discrepancy between morph divergence and neutral divergence could be detected. This suggests that divergent selection has been operating on the colour locus in Spanish populations of I. elegans, whereas processes such as genetic drift, possibly in combination with other forms of selection (such as negative frequency-dependent selection), appear to have been present in other regions, such as eastern Europe. Overall, the results indicate that both selective and stochastic processes operate on these colour polymorphisms, and suggest that the relative importance of factors varies between geographical regions.

The growth/predation risk trade-off: so what is the mechanism?

Among damselflies in natural lakes, Ischnura species grow faster than coexisting Enallagma species, but Enallagma species have higher survival under predation than Ischnura species. This growth/predation risk trade-off apparently allows these taxa to coexist in ponds and lakes across the Holarctic. However, laboratory studies presented here show that the mechanism assumed by most theoretical and empirical studies to mediate this trade-off, namely activity simultaneously modulating foraging returns and predation risk, does not operate in this system. Ischnura verticalis larvae were more active than larvae of Enallagma species in a short-term behavioral experiment, which explains why Ischnura experiences greater mortality from predation. However, this greater activity did not translate into higher feeding rates. Ischnura verticalis and Enallagma species ate comparable amounts of food in both the short-term behavioral experiment and a longer feeding and digestion experiment. In spite of no difference in the amount of food ingested or assimilated, I. verticalis larvae grew faster than Enallagma larvae because they were better able to physiologically convert assimilated food into their own biomass in the presence of mortality threats. From these studies we understand the phenotypic mechanisms determining the antagonistic patterns of relative growth and survival between these two genera, but why these patterns exist remains unclear.

PHYSIOLOGICAL AND BEHAVIORAL RESPONSES TO PREDATORS SHAPE THE GROWTH/PREDATION RISK TRADE-OFF IN DAMSELFLIES

Most organisms must simultaneously find enough food for themselves while trying not to become food for some other organism. Previous field experiments have shown that larvae of Enallagma and Ischnura species are able to coexist in the littoral zones of lakes because they resolve this growth/predation risk trade-off differently: Ischnura species grow more quickly than Enallagma species, but Ischnura species suffer higher mortality rates than Enallagma. We performed a series of laboratory studies to explore the mechanistic basis for the difference in growth between the genera. When held in complete isolation and with unlimited food, larvae of a number of Enallagma species that coexist with fish accumulated mass at much faster rates than Ischnura species. This difference in isolation was due to the fish-lake Enallagma simply ingesting more food. In contrast, when held in the presence of other damselflies or a fish predator, Ischnura had significantly higher growth rates than Enallagma species from fish lakes. All species decreased the amount of food they ingested in the presence of the fish predator as compared to when fish were absent, which resulted in decreased growth in the presence of the predator for all species. However, the interspecific differences in growth rate were due primarily to differences in the abilities of the species to convert ingested food into their own biomass; in the presence of fish, comparably sized larvae ingested nearly identical amounts of food, but Ischnura larvae grew faster because they converted significantly more ingested food into their own biomass than did larvae of Enallagma species from fish lakes. This difference in conversion efficiency between the genera was not apparent when larvae were raised in complete isolation. These results indicate that Enallagma and Ischnura species differ in physiological stress responses to the presence of predators, and this difference may facilitate the coexistence of Enallagma and Ischnura species in the field.

THE CONSEQUENCES OF CHANGING THE TOP PREDATOR IN A FOOD WEB: A COMPARATIVE EXPERIMENTAL APPROACH

Changing the top predator in a food web often results in dramatic changes in species composition at lower trophic levels; many species are extirpated and replaced by new species in the presence of the new top predator. These shifts in species composition also often result in substantial alterations in the strengths of species interactions. However, some species appear to be little affected by these changes that cause species turnover at other positions in the food web. An example of such a difference in species responses is apparent in the distributions of coenagrionid damselflies (Odonata: Zygoptera) among permanent water bodies with and without fish as top predators. Enallagma species segregate between ponds and lakes that do and do not support fish populations, with each lake type having a characteristic Enallagma assemblage. In contrast, species of Ischnura, the sister genus to Enallagma, are common to both fish and fishless ponds and lakes. Previous research has shown that Enallagma species segregate because they are differentially vulnerable to the top predators in each lake type: dragonflies in fishless lakes and fish in fish lakes. This paper reports the results of a series of laboratory and field experiments quantifying the mortality and growth effects of interactions in the food webs surrounding Enallagma and Ischnura species in both lake types. These results are compared to determine how features of the food web change to force segregation of Enallagma species between the lake types but permit Ischnura species to inhabit both. The results of experiments conducted in a fishless lake show that damselflies are not food limited in this lake type, but that they do strongly compete via interference mechanisms. Interference effects between the genera are symmetrical. Ischnura species have substantially higher growth rates than Enallagma species under all conditions in fishless lakes. Although both Enallagma and Ischnura experience substantial mortality from predation by dragonflies (Anax and Aeshna species, the top predators in fishless lakes), these dragonflies display a significant bias towards feeding on Ischnura. Mortality rates due to dragonfly predation are not density dependent. The results of experiments done in a fish lake indicate that damselflies are food limited and thus compete for resources in fish lakes. Ischnura growth rates are also substantially higher than Enallagma species in the fish-lake system. Dragonfly species that coexist with fish (Basiaeschna and Epitheca species) do not impose significant mortality on coexisting damselflies, but they do compete for resources with the damselflies, and they may also generate feeding interference in the damselflies. Fish impose significantly higher mortality on Ischnura species than on coexisting Enallagma species, and this mortality is negatively density dependent. The coexistence of Enallagma and Ischnura species is fostered in both lake types by trade-offs in their abilities to avoid predators and to utilize resources. Native Enallagma species are better at avoiding coexisting predators in each lake type, but these abilities come at the expense of the ability to utilize resources effectively and to avoid the predator found in the other lake type. In contrast, Ischnura are better at utilizing resources in both lake types, but these abilities come at the expense of effectively avoiding both fish and dragonflies. Understanding the trade-offs faced by species at similar trophic positions within a food web is critical to predicting changes in food webs following major environmental perturbations such as changing the top predator.

Sperm transfer, displacement and precedence in Ischnura graellsii (Odonata : Coenagrionidae)

Copulation in Ischnura graellsii may be divided into three stages, according to the movements and position of the male's abdomen. We measured sperm volumes in males and females interrupted at different phases of copulation in laboratory-reared and field specimens. The results showed that males remove sperm from the female during stage I, and do not transfer sperm until stage Il of the copulation. In the field females interrupted during stage I of copulation had less sperm than postcopula females, and the volume of sperm in laboratory females mated once or twice was similar. These results suggest that males can remove most of the sperm during stage I of copulation. Preparations of in-copula specimens showed the horns of the penis (used to remove sperm) inside the bursa copulatoox and the spermatheca. Therefore males can remove sperm from both organs, in contrast to the other Ischnura species so far studied, where males can empty only the bursa. The length of these horns is positively correlated with male body length, and there are significant differences in length between the left and right horns of individual males. This suggests great variability in the male's ability to remove sperm. On the other hand, ejaculate volume is positively related to male and female size, and negatively to male age. Males are likely to be able to detect the presence of sperm in females: if the effect of population density and time of start of copulation are taken into account, copulations are longer with mated than with virgin females. Using genetic markers, sperm precedence was studied by rearing the female offspring of 6 females mated with two males of different genotype. In 5 out of 6 crosses, the second male fertilized all the eggs laid by the female in her first clutch. On average, the following clutches were progressively more fertilized by the first male, but there were striking differences between crosses. These differences are probably due to the variability in the amount of sperm transferred and/or removed.

The adaptive significance of the prolonged copulations of the damselfly, Ischnura graellsii (Odonata: Coenagrionidae)

Copulation in the damselfly Ischnura graellsii lasted 1–5 h, and took place in the afternoon and evening at the study site in north-west Spain. Copula duration was measured in the laboratory under controlled temperature, humidity, photoperiod and density. At high density (15–20 males/insectary), copulations that started early in the day were always long, while at low density (two males/insectary) they could be short or long. At both densities, copulations with previously mated females were longer than with unmated females. The duration of stage II of copulation, when the male transfers sperm to the female, was constant and independent of density and time of day. A study of egg production in females mated only once indicated that copula duration is not correlated with the proportion of fertile eggs laid, and that females start running out of sperm after about 15 days. These results indicate that prolonged copulations of I. graellsii have a guarding function, but the existence of more sperm displacement in long copulations cannot be rejected. Guarding takes place during stage I of copulation, before the males invest sperm in the female, which is unusual. The relation between copula duration and postcopulatory behaviour in Ischnura species so far studied is discussed.