Life history and species composition of the damselfly assemblage along the urban tract of a river in central Italy

The species composition of the damselfly assemblage and the life history patterns of two Coenagrionidae (Ischnura elegans and Cercion lindeni) were investigated along the urban tract of a river characterized by increasing organic pollution. The assemblage was dominated by generalist species, usually recorded in lentic habitats, rather than by typical riverine species and the proportion of the latter decrease at the most polluted sites. At the end of Winter, the mean size and in star distribution were different between the sampling sites showing that the life history of both species examined were influenced by a degradation of the environmental quality. A longer reproductive period, absence of diapause, and tolerance of low oxygen concentration appear to be key factors that allow generalist species I. elegans and C. lindeni to predominate at the polluted sites. This revised version was published online in August 2006 with corrections to the Cover Date.     

Resource limitation,predation risk and compensatory growth in a damselfly

Periods of poor nutrition during early development may have negative fitness consequences in subsequent periods of ontogeny. In insects, suppression of growth and developmental rate during the larval stage are likely to affect size and timing of maturity, which in turn may lead to reduced reproductive success or survivorship. In light of these costs, individuals may achieve compensatory growth via behavioural or physiological mechanisms following food limitation. In this study, we examined the effects of a temporary period of food restriction on subsequent growth and age and size at maturity in the larval damselfly Ischnura verticalis (Odonata: Coenagrionidae). We also asked whether this temporary period of reduced nutrition affected subsequent foraging behaviour under predation risk. I. verticalis larvae exposed to a temporary food shortage suffered from a reduced growth rate during this period relative to a control group that was fed ad libitum. However, increased growth rates later in development ensured that adult body size measurements (head and pronotum widths) did not differ between the treatments upon emergence. In contrast, adult dry mass did not catch up to that of the controls, indicating that the increased growth rates for size dimensions occur at the cost of similar gains in mass. Predators reduced foraging effort of larvae, but this reduction did not differ between control larvae and those previously exposed to poor nutrition.     

Hybridization and the inheritance of female colour polymorphism in two ischnurid damselflies (Odonata: Coenagrionidae)

Female‐limited polychromatism is frequent in many species of Odonata. Ischnura elegans has three colour morphs: one male‐like coloured (androchrome) and two additional gynochrome brown morphs (infuscans and rufescens‐obsoleta morphs). A total of 19 progenies obtained from once‐mated females were reared in the laboratory in three generations. Results indicate that the colour morphs are controlled by the same genetic system as previously described for I. graellsii, i.e. an autosomal locus with female‐limited expression and with three alleles with a hierarchy of dominance (p^a > pⁱ > p°). Five interspecific crossings between female I. graellsii and male I. elegans, five crossings between hybrid females and male I. elegans and one crossing between female I. graellsii and a hybrid male further confirmed that the genetic system is the same in both species. A survey of morph frequencies in north‐west Spain revealed that I. elegans shows high variability in androchrome frequency (4–91%) between nearby populations, whereas in I. graellsii androchromes never are the majority morph (5–40%). The highest androchrome frequency in I. graellsii was found in populations closest to a locality where both species have hybridized, and that now has the highest androchrome frequency of I. elegans. We hypothesize that I. elegans genes have been incorporated into the genome of I. graellsii resulting in increased androchrome frequency in the latter species. Low androchrome frequency in I. elegans seems also related to the influence of I. graellsii genes. Therefore, we suggest that hybridization between both taxa is contributing to the temporal maintenance of contrasting androchrome frequencies in nearby populations. © 2005 The Linnean Society of London, Biological Journal of the Linnean Society, 2005, 85 , 471–481.     

Molecular and ecological signatures of an expanding hybrid zone

Many species are currently changing their distributions and subsequently form sympatric zones with hybridization between formerly allopatric species as one possible consequence. The damselfly Ischnura elegans has recently expanded south into the range of its ecologically and morphologically similar sister species Ischnura graellsii. Molecular work shows ongoing introgression between these species, but the extent to which this species mixing is modulated by ecological niche use is not known. Here, we (1) conduct a detailed population genetic analysis based on molecular markers and (2) model the ecological niche use of both species in allopatric and sympatric regions. Population genetic analyses showed chronic introgression between I. elegans and I. graellsii across a wide part of Spain, and admixture analysis corroborated this, showing that the majority of I. elegans from the sympatric zone could not be assigned to either the I. elegans or I. graellsii species cluster. Niche modeling demonstrated that I. elegans has modified its environmental niche following hybridization and genetic introgression with I. graellsii, making niche space of introgressed I. elegans populations more similar to I. graellsii. Taken together, this corroborates the view that adaptive introgression has moved genes from I. graellsii into I. elegans and that this process is enabling Spanish I. elegans to occupy a novel niche, further facilitating its expansion. Our results add to the growing evidence that hybridization can play an important and creative role in the adaptive evolution of animals.     

Function of abdomen wave behavior in larvalischnura verticalis (Odonata: Coenagrionidae)

Odonate larvae are important predators in the littoral zone of freshwater systems, and in an effort understand their population dynamics, researchers have described and analyzed many of their behaviors. The behavior we call abdomen wave has been considered to be both an aid to respiration and an aggressive display to conspecifics. We found that, under laboratory conditions, abdomen wave was not increased when the oxygen concentration was low or when larvae were in the presence of conspecifics. These results suggest the behavior is not involved in oxygen uptake or in agonistic interactions. The behavior was increased when larvae had food in their guts and was most dramatically increased in the hours preceding a molt. Abdomen wave may function to loosen the exoskeleton prior to molting and/or release of metabolites.     

NEGATIVE FREQUENCY‐DEPENDENT SELECTION IN FEMALE COLOR POLYMORPHISM OF A DAMSELFLY

Negative frequency‐dependent selection (NFDS) is one of the most powerful selective forces maintaining genetic polymorphisms in nature. Recently many prospective cases of polymorphisms by NFDS have been reported. Some of them are very complicated, although strongly supportive of the NFDS. Here we investigate NFDS in wild populations of the dimorphic damselfly Ischnura senegalensis, in which females occur as andromorphs and gynomorphs. Specifically, we (1) test fitness responses to morph frequencies, (2) built a simple population genetic model, and (3) compare the observed and predicted morph‐frequency dynamics. Fitnesses of the two morphs are an inverse function of its own frequency in a population, and are about equal when their frequencies are similar. Thus the conditions necessary for NFDS are satisfied. The long‐term field surveys show that the morph frequencies oscillate with a period of two generations. Morph frequencies in a small population undergo large oscillations whereas those in a large population do small oscillations. The demographic properties of the observed dynamics agree well with those of our model. This example is one of the simplest confirmed cases of NFDS maintaining genetic polymorphisms in nature.     

Population genetic structure of sexual and parthenogenetic damselflies inferred from mitochondrial and nuclear markers

It has been postulated that obligate asexual lineages may persist in the long term if they escape from negative interactions with either sexual lineages or biological enemies; and thus, parthenogenetic populations will be more likely to occur in places that are difficult for sexuals to colonize, or those in which biological interactions are rare, such as islands or island-like habitats. Ischnura hastata is the only known example of natural parthenogenesis within the insect order Odonata, and it represents also a typical example of geographic parthenogenesis, as sexual populations are widely distributed in North America, whereas parthenogenetic populations of this species have only been found at the Azores archipelago. In order to gain insight in the origin and distribution of parthenogenetic I. hastata lineages, we have used microsatellites, mitochondrial and nuclear DNA sequence data, to examine the population genetic structure of this species over a wide geographic area. Our results suggest that sexual populations of I. hastata in North America conform to a large subdivided population that has gone through a recent spatial expansion. A recent single long distance dispersal event, followed by a demographic expansion, is the most parsimonious hypothesis explaining the origin of the parthenogenetic population of this species in the Azores islands.     

Nuptial feeding by male bushcrickets: an indicator of male quality?

Male bushcrickets transfer a spermatophore at mating that consistsof a sperm-containing ampulla and a product of the accessoryglands, the spermatophylax, that is consumed by the female duringinsemination. Male Requena verticalis produce functionally differentspermatophores depending on the availability of sexually receptivefemales. They will maintain high mating frequency by providinga gift sufficient to ensure sperm transfer, or will invest parentallyin females when their mating frequency is low. We examined therelationship between male quality and nuptial feeding underconditions where males invest in ejaculate protection or inparental investment. When investing in ejaculate protection,males reduced the quality of the spermatophylax meal by reducingboth the concentration of protein and the absolute amount ofprotein it contained. There was no relationship between malephenotype and gift size or quality. Moreover, we could findno evidence for the recently advanced hypothesis that femalescan exercise mate choice by interfering with insemination. However,when males were investing parentally, we found a positive associationbetween spermatophylax size and male size, but no relationshipbetween protein content and male size. Males with high levels offluctuating asymmetry invested more heavily in the nutritionalcontent of their spermatophylaxes than did symmetrical males.Thus, male quality does influence nuptial feeding, but in amanner predicted by a model of indirect fitness benefits frommate choice.     

Morphological specializations of dorsal rim ommatidia in the compound eye of dragonflies and damselfies (Odonata)

We have examined the fine structure of dorsal rim ommatidia in the compound eye of the three odonate species Sympetrum striolatum, Aeshna cyanea and Ischnura elegans. These ommatidia exhibit several specializations: (1) the rhabdoms are very short, (2) there is no rhabdomeric twist, and (3) the rhabdoms contain only two, orthogonally-arranged microvillar orientations. The dorsal rim ommatidia of several other insect species are known to be anatomically specialized in a similar way and to be responsible for polarization vision. We suggest that the dorsal rim area of the odonate compound eye plays a similar role in polarization vision. Since the Odonata are a primitive group of insects, the use of polarized skylight for navigation may have developed early in insect phylogeny.