A. Cordero Rivera

Evolution of female colour polymorphism in damselflies: testing the hypotheses

Abstract The existence of several female colour morphs is a conspicuous characteristic of many damselflies that show one male-like (androchrome) and several nonmale-like (gynochrome) morphs. We tested several adaptive hypotheses and the null model for the maintenance of female polychromatism (one androchrome and two gynochromes) in the damselfly Ceriagrion tenellum . We tested the null model by comparing the degree of genetic differentiation between the colour locus and a set of 19 neutral RAPD loci in five populations. Our results indicate that selection is acting to maintain similar frequencies between populations at the colour locus. Using mark–recapture techniques we found that mating success is not dependent on female coloration. We tested the mimicry hypothesis by presenting live and dead models to males. Dead models were highly attractive irrespective of coloration. In contrast, with live models males could not distinguish between androchromes and other males, and were more attracted to gynochrome females. Despite this, within populations morph frequencies remained constant over time and mating was at random with respect to female coloration. However, there was a positive relationship between male density and androchrome frequency in a comparative study of eight populations. We discuss our results in the framework of sexual conflict theory and suggest that andro- and gynochrome females are using different strategies to control their number of matings. The different morphs might be maintained in a balanced polymorphism by a combination of density- and frequency-dependent mechanisms. Copyright 2002 Published by Elsevier Science Ltd on behalf of The Association for the Study of Animal Behaviour

MOLECULAR EVIDENCE FOR SELECTION ON FEMALE COLOR POLYMORPHISM IN THE DAMSELFLY ISCHNURA GRAELLSII

The significance of female color polymorphism in Odonata remains controversial despite many field studies. The importance of random factors (founder effects, genetic drift and migration) versus selective forces for the maintenance of this polymorphism is still discussed. In this study, we specifically test whether the female color polymorphism of Ischnura graellsii (Odonata, Coenagrionidae) is under selection in the wild. We compared the degree of genetic differentiation based on RAPD markers (assumed to be neutral) with the degree of differentiation based on color alleles. Weir and Cockerhams θ values showed a significant degree of population differentiation for both sets of loci (RAPD and color alleles) but the estimated degree of population differentiation (θ) was significantly greater for the set of RAPD loci. This result shows that some sort of selection contributes to the maintenance of similar color morph frequencies across the studied populations. Our results combined with those of previous field studies suggest that at least in some I. graellsii populations, density‐dependent mechanisms might help to prevent the loss of this polymorphism but cannot explain the similarity in morph frequencies among populations.

Sperm competition in Odonata (Insecta): the evolution of female sperm storage and rivals' sperm displacement

Odonates (dragonflies) are well known for the ability of the males to displace sperm stored in the females sperm-storage organs during copulation. By this means, copulating males are able to increase their fertilization success. This ability has been used as an example to illustrate a conflict of interests between the sexes in which males have evolved sperm-displacement mechanisms whilst females have presumably evolved means to avoid sperm displacement. The present review has four aims: (1) to describe the copulatory mechanisms used during sperm displacement; (2) to analyse the causes of sperm usage patterns; (3) to discuss this information using current hypotheses on conflict between the sexes; and (4) to illuminate topics for further research. Four copulatory mechanisms are described: sperm removal (physical withdrawal of stored sperm), sperm repositioning (‘pushing’ of rival sperm to sites where its use will be least likely), female sensory stimulation to induce sperm ejection, and sperm flushing (displacement of sperm using the copulating males sperm). Sperm-precedence studies in Odonata are scarce and their values vary considerably between species. In those species in which sperm displacement is incomplete, the last copulating male obtains a high but variable short-term fertilization success which decreases with time. Some male and female factors affecting sperm precedence patterns are mentioned: (1) male variation in genital morphology; (2) duration of copulation influenced by the male (the longer the copulation, the more stored sperm displaced); (3) adaptations of the sperm-storage organs that allow the female to manipulate the sperm she has received (i.e. avoiding sperm displacement, re-distributing sperm masses, favouring sperm located in certain sites and ejecting sperm after copulation). We suggest that male and female odonates have co-evolved at the level of genital function with the control of stored sperm as the focus of the conflict. The benefits for males in this co-evolution lie in maximizing their fertilization success. However, it is not clear what females obtain from storing sperm and making it unreachable during sperm displacement. Two hypothetical benefits that females may obtain for which some evidence has been gathered are genetic diversity and viability genes. It is finally suggested that odonates can become excellent subjects of study for testing current ideas related to sexual conflict and speciation processes through sexual selection.

POSTMATING SEXUAL SELECTION: ALLOPATRIC EVOLUTION OF SPERM COMPETITION MECHANISMS AND GENITAL MORPHOLOGY IN CALOPTERYGID DAMSELFLIES (INSECTA: ODONATA)

Abstract Postmating sexual selection theory predicts that in allopatry reproductive traits diverge rapidly and that the resulting differentiation in these traits may lead to restrictions to gene flow between populations and, eventually, reproductive isolation. In this paper we explore the potential for this premise in a group of damselflies of the family Calopterygidae, in which postmating sexual mechanisms are especially well understood. Particularly, we tested if in allopatric populations the sperm competition mechanisms and genitalic traits involved in these mechanisms have indeed diverged as sexual selection theory predicts. We did so in two different steps. First, we compared the sperm competition mechanisms of two allopatric populations of Calopteryx haemorrhoidalis (one Italian population studied here and one Spanish population previously studied). Our results indicate that in both populations males are able to displace spermathecal sperm, but the mechanism used for sperm removal between both populations is strikingly different. In the Spanish population males seem to empty the spermathecae by stimulating females, whereas in the Italian population males physically remove sperm from the spermathecae. Both populations also exhibit differences in genital morphometry that explain the use of different mechanisms: the male lateral processes are narrower than the spermathecal ducts in the Italian population, which is the reverse in the Spanish population. The estimated degree of phenotypic differentiation between these populations based on the genitalic traits involved in sperm removal was much greater than the differentiation based on a set of other seven morphological variables, suggesting that strong directional postmating sexual selection is indeed the main evolutionary force behind the reproductive differentiation between the studied populations. In a second step, we examined if a similar pattern in genital morphometry emerge in allopatric populations of this and other three species of the same family (Calopteryx splendens, C. virgo and Hetaerina cruentata). Our results suggest that there is geographic variation in the sperm competition mechanisms in all four studied species. Furthermore, genitalic morphology was significantly divergent between populations within species even when different populations were using the same copulatory mechanism. These results can be explained by probable local coadaptation processes that have given rise to an ability or inability to reach and displace spermathecal sperm in different populations. This set of results provides the first direct evidence of intraspecific evolution of genitalic traits shaped by postmating sexual selection.

Thermal requirements and phenology of the Eucalyptus snout beetle Gonipterus scutellatus Gyllenhal

Abstract:  Laboratory experiments and field surveys were carried out to study the thermal requirements and phenology of the Eucalyptus snout beetle Gonipterus scutellatus (Curculionidae) and its parasitoid, Anaphes nitens (Mymaridae). Developmental times were recorded for G. scutellatus life stages: egg to first instar larva, first instar to pre‐pupal larva, prepupae to adults and the complete life cycle. Experiments were performed in temperature‐controlled chambers maintained at 10, 15, 20, 25 and 30°C with a photoperiod of 11 : 13 h of light : darkness and 50–60% RH. To calculate the minimum threshold temperature of the parasitoid, parasitized egg capsules were kept under similar conditions. During 1998 and 1999 we studied the phenology and the day‐degree (DD) accumulation of G. scutellatus and its parasitoid in plots of Eucalyptus globulus at six different sites in NW Spain. Every 2 weeks, the numbers of snout beetle adults and egg capsules were counted in each plot. The rate of parasitism was estimated by collecting 90 egg capsules from each plot on each sampling date. We recorded the temperatures in each plot to test whether differences in temperature alone could account for the phenology of this snout beetle. To complete a full life cycle from egg to adult, the weevil required a mean of 1119.83 ± 20.59 DD above a base temperature of 6.11°C. The parasitoid had a base temperature of 5.09°C and needed 318.16 DD to complete a life cycle. Our model indicated that three generations of snout beetle could develop each year, corresponding to peaks of snout beetle numbers in the field in March–April, June–July and November. In some years only one generation of G. scutellatus was recorded due probably to the effectiveness of the parasitoid. Differences in numbers of adults and egg capsule were recorded between neighbouring ‘coastal plots’ and between neighbouring ‘inland plots’. Hence, climate alone does not appear to explain the phenology of G. scutellatus.

Are parthenogenetic and sexual Ischnura hastata damselflies equally fertile? Testing sexual conflict theories

Recent theories of sexual selection stress the importance of conflicts over reproduction in shaping the reproductive traits of males and females. Except when the reproductive interests of both sexes coincide, which only occurs under strict monogamy, there is a conflict of interests between the sexes over the number of matings and reproductive decisions. It has been suggested that males are selected to “harm” females if this increases male reproductive success, even at the expenses of female fitness. One prediction of such an hypothesis is that sperm is selected to maximize the probability of fertilization, and this sometimes can cause a decrease in fertility due to multispermy, genetic incompatibility, toxic seminal products that harm females, etc. We have tested this hypothesis by comparing the fertility rates of parthenogenetic and sexual Ischnura hastata populations. Our results show that sexual females are less fertile than parthenogenetic ones, which is in agreement with the sexual conflict predictions tested in this study.Recent theories of sexual selection stress the importance of conflicts over reproduction in shaping the reproductive traits of males and females. Except when the reproductive interests of both sexes coincide, which only occurs under strict monogamy, there is a conflict of interests between the sexes over the number of matings and reproductive decisions. It has been suggested that males are selected to “harm” females if this increases male reproductive success, even at the expenses of female fitness. One prediction of such an hypothesis is that sperm is selected to maximize the probability of fertilization, and this sometimes can cause a decrease in fertility due to multispermy, genetic incompatibility, toxic seminal products that harm females, etc. We have tested this hypothesis by comparing the fertility rates of parthenogenetic and sexual Ischnura hastata populations. Our results show that sexual females are less fertile than parthenogenetic ones, which is in agreement with the sexual conflict predictio...

Fertility and paternity in the Eucalyptus snout-beetle Gonipterus scutellatus: females might benefit from sperm mixing

Mating in Gonipterus scutellatus is very long and lasts on average 6.98 ± 0.49 hr. This could be interpreted as a mechanism of male guarding, but males neither increase copulation duration nor ejaculate volume when sperm competition risk increases. We studied paternity patterns using RAPDs, and the influence of multiple matings on fertility, fecundity and sperm supply. Using twicemated females, we compared the percentage of polymorphic bands shared between first male/offspring, second male/offspring, mother/offspring, unrelated adults (males). In agreement with a previous study on paternity with the sterilemale technique, we found that sperm mixing seems the prevalent sperm competition mechanism. In a second experiment, adults were randomly assigned to three treatments: females mated only once; females allowed to mate ad libitum with one male; and females that could mate with seven different males. Females of the last group showed a significant increase in fertility. Additionally, we compared the stored s...Mating in Gonipterus scutellatus is very long and lasts on average 6.98 ± 0.49 hr. This could be interpreted as a mechanism of male guarding, but males neither increase copulation duration nor ejaculate volume when sperm competition risk increases. We studied paternity patterns using RAPDs, and the influence of multiple matings on fertility, fecundity and sperm supply. Using twicemated females, we compared the percentage of polymorphic bands shared between first male/offspring, second male/offspring, mother/offspring, unrelated adults (males). In agreement with a previous study on paternity with the sterilemale technique, we found that sperm mixing seems the prevalent sperm competition mechanism. In a second experiment, adults were randomly assigned to three treatments: females mated only once; females allowed to mate ad libitum with one male; and females that could mate with seven different males. Females of the last group showed a significant increase in fertility. Additionally, we compared the stored sperm volumes among four groups: female whose copulation was interrupted after 15 min, those that mated once, twice or three times. This experiment indicates that sperm supply increases with the number of matings, excluding the possibility of sperm removal. These facts and the occurrence of sperm mixing might explain the lack of male response to increased male density in previous studies.