José A. Andrés

Genetic divergence of the seminal signal—receptor system in houseflies: the footprints of sexually antagonistic coevolution?

To understand fully the significance of cryptic female choice, we need to focus on each of those postmating processes in females which create variance in fitness among males. Earlier studies have focused almost exclusively on the proportion of a females eggs fertilized by different males (sperm precedence). Yet, variance in male postmating reproductive success may also arise from differences in ability to stimulate female oviposition and to delay female remating. Here, we present a series of reciprocal mating experiments among genetically differentiated wild-type strains of the housefly Musca domestica. We compared the effects of male and female genotype on oviposition and remating by females. The genotype of each sex affected both female oviposition and remating rates, demonstrating that the signal-receptor system involved has indeed diverged among these strains. Further, there was a significant interaction between the effects of male and female genotype on oviposition rate. We discuss ways in which the pattern of such interactions provides insights into the coevolutionary mechanism involved. Females in our experiments generally exhibited the weakest, rather than the strongest, response to males with which they are coevolved. These results support the hypothesis that coevolution of male seminal signals and female receptors is sexually antagonistic.

Copulation duration and fertilization success in a damselfly: an example of cryptic female choice?

Copulation duration is highly variable (0.5-3 h) in the damselfly, Ceriagrion tenellum (Coenagrionidae). Using laboratory experiments, we tested four adaptive hypotheses to explain this variation: the effect of time constraints, in-copula mate guarding, sperm displacement and cryptic female choice. Copulation duration was negatively correlated with time of day, as predicted by the first two hypotheses, and positively correlated with male density, as predicted by the mate-guarding hypothesis. Males prolonged copulation in response to the volume of sperm stored by females, suggesting they were able to detect and quantify the amount of sperm stored. This behaviour is not explained by mate guarding or time constraint effects. Males removed all the sperm from the bursa copulatrix in just 10 min. Our results also suggest that, because the duct is too narrow to allow male genitalia to enter, males do not remove spermathecal sperm. Therefore, direct sperm removal could not explain long copulations. Prolonged copulations could also have evolved as a result of cryptic female choice if they increase male fertilization success by female-mediated processes. Our results support this idea: male fertilization success was greater after long copulations. Apparently, male copulatory behaviour elicits female responses that increase male fertilization success. Copyright 2000 The Association for the Study of Animal Behaviour.

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

Male coercion and convenience polyandry in a calopterygid damselfly

Abstract Copulation in odonates requires female cooperation because females must raise their abdomen to allow intromission. Nevertheless in Calopteryx haemorrhoidalis haemorrhoidalis (Odonata) males commonly grasp ovipositing females and apparently force copulations. This has been interpreted as a consequence of extreme population density and male-male competition. We studied this behavior at two sites on a river that had different densities over three years. As predicted, at high densities most matings were forced (i.e. not preceded by courtship), but at low density most were preceded by courtship. Courtship matings were shorter at high density, but density did not affect the duration of forced matings. Females cooperated in forced matings even if they had very few mature eggs. Furthermore, females mated more times if they experienced higher male harassment during oviposition, and at low density second and subsequent matings were more likely to be forced. We interpret these results to mean that females engage in “convenience polyandry”, because they gain more by accepting copulation than by resisting males. The results also suggest that females might trade copulations for male protection, because under extreme population density harassment by males is so intense that they can impede oviposition.

Searching for candidate speciation genes using a proteomic approach: seminal proteins in field crickets

In many animals, male seminal proteins influence gamete interactions and fertilization ability and are probably involved in barriers to gene flow between diverging lineages. Here we use a proteomic approach to identify seminal proteins that are transferred to females during copulation and that may be involved in fertilization barriers between two hybridizing field crickets (Gryllus firmus and Gryllus pennsylvanicus). Analyses of patterns of divergence suggest that much of the field cricket genome has remained undifferentiated following the evolution of reproductive isolation. By contrast, seminal protein genes are highly differentiated. Tests of selection reveal that positive selection is likely to be responsible for patterns of differentiation. Together, our observations suggest that some of the loci encoding seminal proteins may indeed play a role in fertilization barriers in field crickets.

GENEALOGICAL DISCORDANCE AND PATTERNS OF INTROGRESSION AND SELECTION ACROSS A CRICKET HYBRID ZONE

In recently diverged species, ancestral polymorphism and introgression can cause incongruence between gene and species trees. In the face of hybridization, few genomic regions may exhibit reciprocal monophyly, and these regions, usually evolving rapidly under selection, may be important for the maintenance of species boundaries. In animals with internal fertilization, genes encoding seminal protein are candidate barrier genes. Recently diverged hybridizing species such as the field crickets Gryllus firmus and G. pennsylvanicus, offer excellent opportunities to investigate the origins of barriers to gene exchange. These recently diverged species form a well-characterized hybrid zone, and share ancestral polymorphisms across the genome. We analyzed DNA sequence divergence for seminal protein loci, housekeeping loci, and mtDNA, using a combination of analytical approaches and extensive sampling across both species and the hybrid zone. We report discordant genealogical patterns and differential introgression rates across the genome. The most dramatic outliers, showing near-zero introgression and more structured species trees, are also the only two seminal protein loci under selection. These are candidate barrier genes with possible reproductive functions. We also use genealogical data to examine the demographic history of the field crickets and the current structure of the hybrid zone.

The inheritance of female colour morphs in the damselfly Ceriagrion tenellum (Odonata, Coenagrionidae)

Female-limited polychromatism is found in many species of Odonata. In Ceriagrion tenellum (Coenagrionidae) one of the morphs is red-coloured, like the conspecific male (androchrome, erythrogastrum morph), whereas most females are red and black (typica morph) or black (melanogastrum morph). Virgin females of this species were mated in the laboratory and their progeny reared (13 crosses). Results of these crosses indicate that colour morphs are controlled by one autosomal locus with female-limited expression. A second laboratory generation (two crosses) confirmed this inheritance system. This locus has three alleles (one per phenotype) and a hierarchy of dominance: typica > melanogastrum > erythrogastrum. The dominance relationships of andro/gynochrome alleles in polymorphic damselflies so far studied are discussed. The frequencies of female morphs in natural populations are highly variable, but in all cases typica females are the commonest.

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.

MC1R-dependent, melanin-based colour polymorphism is associated with cell-mediated response in the Eleonora’s falcon

Colour polymorphism in vertebrates is usually under genetic control and may be associated with variation in physiological traits. The melanocortin 1 receptor (Mc1r) has been involved repeatedly in melanin‐based pigmentation but it was thought to have few other physiological effects. However, recent pharmacological studies suggest that MC1R could regulate the aspects of immunity. We investigated whether variation at Mc1r underpins plumage colouration in the Eleonora’s falcon. We also examined whether nestlings of the different morphs differed in their inflammatory response induced by phytohemagglutinin (PHA). Variation in colouration was due to a deletion of four amino acids at the Mc1r gene. Cellular immune response was morph specific. In males, but not in females, dark nestling mounted a lower PHA response than pale ones. Although correlative, our results raise the neglected possibility that MC1R has pleiotropic effects, suggesting a potential role of immune capacity and pathogen pressure on the maintenance of colour polymorphism in this species.

Life cycle and biological control of the Eucalyptus snout beetle (Coleoptera, Curculionidae) by Anaphes nitens (Hymenoptera, Mymaridae) in north-west Spain

1 Gonipterus scutellatus is a weevil that has become a pest in most Eucalyptus plantations in Africa, America and Europe. The egg parasitoid Anaphes nitens has been introduced into many countries as a biological control agent. Even if control has been successful in most countries no detailed study of the interactions between both species has been published.