Mimicry and the evolution of premating isolation in Heliconius melpomene Linnaeus

Ecological divergence can cause speciation if adaptive traits have pleiotropic effects on mate choice. In Heliconius butterflies, mimetic patterns play a role in mate detection between sister species, as well as signalling to predators. Here we show that male butterflies from four recently diverged parapatric populations of Heliconius melpomene are more likely to approach and court their own colour patterns as compared with those of other races. A few exceptions, where males were more attracted to patterns other than their own, suggest that some mimetic patterns are sub-optimal in mate choice. Genotype frequencies in hybrid zones between races of H. melpomene suggest that mating is random, so reinforcement is unlikely to have played a role in intra-specific divergence. In summary, co-evolved divergence of colour pattern and mate preference occurs rapidly and is likely the first step in Heliconius speciation.     

Inbreeding affects sexual signalling in males but not females of Tenebrio molitor

In many species of animals, individuals advertise their quality with sexual signals to obtain mates. Chemical signals such as volatile pheromones are species specific, and their primary purpose is to influence mate choice by carrying information about the phenotypic and genetic quality of the sender. The deleterious effects of consanguineous mating on individual quality are generally known, whereas the effect of inbreeding on sexual signalling is poorly understood. Here, we tested whether inbreeding reduces the attractiveness of sexual signalling in the mealworm beetle, Tenebrio molitor, by testing the preferences for odours of inbred and outbred (control) individuals of the opposite sex. Females were more attracted to the odours produced by outbred males than the odours produced by inbred males, suggesting that inbreeding reduces the attractiveness of male sexual signalling. However, we did not find any difference between the attractiveness of inbred and outbred female odours, which may indicate that the quality of females is either irrelevant for T. molitor males or quality is not revealed through female odours.     

Male size does not affect the strength of male mate choice for high-quality females in Drosophila melanogaster

Theory predicts that the strength of male mate choice should vary depending on male quality when higher-quality males receive greater fitness benefits from being choosy. This pattern extends to differences in male body size, with larger males often having stronger pre- and post-copulatory preferences than smaller males. We sought to determine whether large males and small males differ in the strength (or direction) of their preference for large, high-fecundity females using the fruit fly, Drosophila melanogaster. We measured male courtship preferences and mating duration to show that male body size had no impact on the strength of male mate choice; all males, regardless of their size, had equally strong preferences for large females. To understand the selective pressures shaping male mate choice in males of different sizes, we also measured the fitness benefits associated with preferring large females for both large and small males. Male body size did not affect the benefits that males received: large and small males were equally successful at mating with large females, received the same direct fitness benefits from mating with large females, and showed similar competitive fertilization success with large females. These findings provide insight into why the strength of male mate choice was not affected by male body size in this system. Our study highlights the importance of evaluating the benefits and costs of male mate choice across multiple males to predict when differences in male mate choice should occur.     

Direct fitness benefits explain mate preference,but not choice,for similarity in heterozygosity levels

Under sexual selection, mate preferences can evolve for traits advertising fitness benefits. Observed mating patterns (mate choice) are often assumed to represent preference, even though they result from the interaction between preference, sampling strategy and environmental factors. Correlating fitness with mate choice instead of preference will therefore lead to confounded conclusions about the role of preference in sexual selection. Here we show that direct fitness benefits underlie mate preferences for genetic characteristics in a unique experiment on wild great tits. In repeated mate preference tests, both sexes preferred mates that had similar heterozygosity levels to themselves, and not those with which they would optimise offspring heterozygosity. In a subsequent field experiment where we cross fostered offspring, foster parents with more similar heterozygosity levels had higher reproductive success, despite the absence of assortative mating patterns. These results support the idea that selection for preference persists despite constraints on mate choice.     

Parasite levels in blue-black grassquits correlate with male displays but not female mate preference

The handicap principle proposes that male sexual ornaments anddisplays provide honest indicators of quality. Female preferencefor high-quality males, however, may be driven not only by geneticbenefits but also by indirect benefits. We investigated theimpact of parasitism on morphological, ornamental, and behavioralcharacteristics of male and female blue-black grassquits (Volatiniajacarina) in captivity. First, we tested whether male displaysand morphology were influenced by parasitism. Second, we assessedif females were attentive to variation in male morphology anddisplays linked to parasitism. Third, we tested whether parasitismin females influenced health and mate preferences. We maintained2 groups of birds in captivity: nonmedicated birds developedhigh levels of coccidian parasitism, whereas medicated birdswere free of parasitism. Parasitized males developed, relativeto nonparasitized males, lower weight/tarsus indices and mass.They also showed relative deficiencies in their displays, withless persistence and lower rates. Despite the negative effectsof parasitism on males, females did not prefer nonparasitizedmales. This held for both parasitized and nonparasitized females.Our data suggest that coccidian parasitism has adverse effectson morphological condition and expression of displays. Theseeffects, however, appear not to be attended to by females; moreover,female mate preferences appear not to be impacted by the threatof parasitism. It thus seems that female mate preferences maynot depend only on sexual characters affected by parasitismin this species.     

Female preference and larval performance of sunflower moth,Homoeosoma electellum,on sunflower pre‐breeding lines

The preference–performance hypothesis for insect herbivores predicts that adult females should preferentially choose hosts on which their offspring perform better. We tested this hypothesis for the sunflower moth, Homoeosoma electellum (Hulst) (Lepidoptera: Pyralidae), using 16 sunflower (pre‐breeding) lines, derived from a number of wild species of Helianthus, including Helianthus annuus L., Helianthus deserticola Heiser, Helianthus paradoxus Heiser, Helianthus praecox Engelm. & Gray ssp. hirtus (Heiser) Heiser, Helianthus praecox Engelm. & Gray ssp. runyonii (Heiser) Heiser, Helianthus petiolaris Nutt., Helianthus resinosus Small, and Helianthus tuberosus L. (Asteraceae), that are suitable for introducing wild sunflower germplasm into commercial cultivars. Female moths showed a range of ovipositional preference measures to the various lines. Combined data for three Helianthus species represented by multiple lines showed significant differences in female preference with respect to the parental species. Larval performance, determined by proportion of infested neonate larvae reaching the pupal stage, or mean pupal weight, varied across the lines and, as for the female preference data, also showed significant differences among the three parental Helianthus species represented by multiple lines. These data suggest that the characteristics in the pre‐breeding lines influencing female sunflower moth preference and larval performance likely originate from the parental species and may be consistently transferred to the derived pre‐breeding lines. Of particular note with regard to potential plant resistance mechanisms, lines derived from H. tuberosus showed consistent low preference–performance measures. Female preference and larval performance (for both measures) were strongly correlated, indicating that females preferred plants and lines on which larvae performed better, in support of the preference–performance hypothesis.     

Seasonal variation in larval host plants and consequences for Heliconius erato (Lepidoptera: Nymphalidae) adult body size

Abstract Adult body size, a key life history component, varies strongly within and between Heliconius erato phyllis (Lepidoptera: Nymphalidae) populations. In the present study, we determined whether seasonal variation in adult body size is temperature related and/or determined by seasonal changes of host plants (Passifloraceae) used by the larval stage. A population of H. erato phyllis located in a Eucalyptus plantation (Barba Negra Forest, Barra do Ribeiro County, Rio Grande do Sul State, Brazil) was sampled every 45 days from March 1997 to October 1998 to quantify seasonal variation in adult body size and use of larval host plants. In the laboratory, the effects of the following factors on adult body size were quantified: (i) host plant species (Passiflora misera or Passiflora suberosa); (ii) food quantity consumed by larvae (experimentally manipulated for each passion vine species); (iii) winter and summer temperatures (15 and 25°C, respectively); and (iv) the interaction between host plant species and temperature. Adults emerging during summer were larger than those emerging in other seasons. Female butterflies oviposited selectively on P. misera even when the dominant passion vine was P. suberosa. They only switched from using P. misera to P. suberosa during later autumn and winter, when P. misera vines were completely defoliated. The laboratory feeding trials with both passion vines showed a strong positive association between food quantity consumed by larvae and adult size. They also confirmed that adults are larger when their larvae are reared on P. misera than on P. suberosa. Temperature during larval development had no effect on H. erato phyllis adult size. Thus, seasonal variation of H. erato phyllis adult size in a given place is primarily determined by the availability and quality of host plant species used by the larval stage.     

Heightened condition dependent expression of structural coloration in the faces,but not wings,of male and female flies

Structurally colored sexual signals are a conspicuous and widespread class of ornament used in mate choice, though the extent to which they encode information on the quality of their bearers is not fully resolved. Theory predicts that signaling traits under strong sexual selection as honest indicators should evolve to be more developmentally integrated and exaggerated than nonsexual traits, thereby leading to heightened condition dependence. Here, we test this prediction through examination of the sexually dimorphic faces and wings of the cursorial fly Lispe cana. Males and females possess structural UV-white and golden faces, respectively, and males present their faces and wings to females during close-range, ground-based courtship displays, thereby creating the opportunity for mutual inspection. Across a field-collected sample of individuals, we found that the appearance of the faces of both sexes scaled positively with individual condition, though along separate axes. Males in better condition expressed brighter faces as modeled according to conspecific flies, whereas condition scaled with facial saturation in females. We found no such relationships for their wing interference pattern nor abdomens, with the latter included as a nonsexual control. Our results suggest that the structurally colored faces, but not the iridescent wings, of male and female L. cana are reliable guides to individual quality and support the broader potential for structural colors as honest signals. They also highlight the potential for mutual mate choice in this system, while arguing for 1 of several alternate signaling roles for wing interferences patterns among the myriad taxa which bear them.     

Mating status correlates with dorsal brightness in some but not all poison frog populations

Sexual signals are important for intraspecific communication and mate selection, but their evolution may be driven by both natural and sexual selection, and stochastic processes. Strawberry poison frogs (Oophaga pumilio) show strong color divergence among populations, but coloration also varies among individuals of the same population. The importance of coloration for female mate choice has been studied intensely, and sexual selection seems to affect color divergence in strawberry poison frogs. However, the effect of coloration on mating success under field conditions has received very little attention. Furthermore, few studies examined how phenotypic variation among individuals of the same color morph affects mate selection under natural conditions. We measured the spectral reflectance of courting and noncourting individuals and their background substrates in three geographically separated populations. In one population (Sarapiquí, Costa Rica), we found that naturally occurring courting pairs of males and females had significantly brighter dorsal coloration than individual males and females not engaged in courtship interactions. Our field observations suggest that, in the wild, females prefer brighter males while the reason for the higher courtship activity of brighter females remains unclear. Overall our results imply that brightness differences among individuals of the same color morph may actually affect reproductive success in some populations of strawberry poison frogs.     

Oviposition preference and larval performance of North American monarch butterflies on four Asclepias species

Monarch butterflies, Danaus plexippus L. (Lepidoptera: Nymphalidae), occur world‐wide and are specialist herbivores of plants in the milkweed family (Asclepiadaceae). In North America, two monarch populations breed east and west of the continental divide in areas populated by different host plant species. To examine the population variation in monarch responses to different Asclepias species, we measured oviposition preference and larval performance among captive progeny reared from adult butterflies collected in eastern and western North America. Host plant use was evaluated using two milkweed species widely distributed in eastern North America (A. incarnata and A. syriaca), and two species common to western North America (A. fascicularis and A. speciosa). We predicted that exposure to different host plant species in their respective breeding ranges could select for divergent host use traits, so that monarchs should preferentially lay more eggs on, and larvae should perform better on, milkweed species common to their native habitats. Results showed that across all adult female butterflies, oviposition preferences were highest for A. incarnata and lowest for A. fascicularis, but mean preferences did not differ significantly between eastern and western monarch populations. Larvae from both populations experienced the highest survival and growth rates on A. incarnata and A. fascicularis, and we again found no significant interactions between monarch source population and milkweed species. Moreover, the average rank order of larval performance did not correspond directly to mean female oviposition preferences, suggesting that additional factors beyond larval performance influence monarch oviposition behavior. Finally, significant family level variation was observed for both preference and performance responses within populations, suggesting an underlying genetic variation or maternal effects governing these traits.     

Male discrimination of female mucous trails permits assortative mating in a marine snail species

Recent research has shown the potential for nonallopatric speciation, but we lack an adequate understanding of the mechanisms of prezygotic barriers and how these evolve in the presence of gene flow. The marine snail Littorina saxatilis has distinct ecotypes in different shore microhabitats. Ecotypes hybridize in contact zones, but gene flow is impeded by assortative mating. Earlier studies have shown that males and females of the same ecotype copulate for longer than mates of different ecotype. Here we report a new mechanism that further contributes to reproductive isolation between ecotypes in the presence of gene flow. This mechanism is linked to the ability of males to track potential partners by following their mucous trail. We show that cliff ecotype males follow the trails of females of the same ecotype for longer than females of the alternate (boulder) ecotype. In addition, cliff males are more likely to follow the mucous trail in the correct direction if the trail is laid by a cliff-female. The capacity to discriminate the ecotype of female mucous trails combined with differential copulation times creates a strong prezygotic reproductive barrier between ecotypes of L. saxatilis that reduces gene flow from cliff to boulder ecotypes by >/=80%.     

Pollen donor performance can be consistent acrossmaternal plants in wild radish (Raphanus sativus,Brassicaceae): a necessary condition for the action of sexualselection

The possibility that sexual selection has played a role in theevolution of plant reproductive characters remains interesting, butcontroversial. One reason is that clear demonstration of the necessaryconditions for sexual selection is lacking. For sexual selection tooccur, differences in pollen donor performance must be consistent acrossmaternal plants and not be due to mechanisms such asself-incompatibility that produce interactions between maternal plantsand pollen donors. Here, I performed two experiments with wild radish totest whether differences in pollen donor performance are consistentacross maternal plants and not due to subtle effects of theincompatibility system. In the first, all maternal and paternal lineageshad different S-alleles. There were 16 maternal plants, four in each offour lineages and four pollen donors, one in each of four lineages. Rankorder of pollen donor performance, in terms of number of seeds siredafter mixed pollination, was highly consistent across maternal plantsand maternal lineages. In addition, maternal stress treatment had aneffect on mating success of pollen donors, but the effect was subtle anddid not affect the rank order of seeds sired by the four pollen donors.In the second experiment, pollinations were performed on both mature andimmature stigmas. Immature stigmas allowed some self seed set, so theincompatibility system was compromised. There was some nonrandom seedpaternity on both mature and immature stigmas. However, the amount ofnonrandom mating was less on immature stigmas. Taken together, theseexperiments show that the kind of consistent nonrandom mating necessaryfor sexual selection occurs in wild radish, but that the incompatibilitysystem and the mechanisms for sorting among compatible mates may overlapin time of development or in somepathways.     

Female choice for male cuticular hydrocarbon profile in decorated crickets is not based on similarity to their own profile

Indirect genetic benefits derived from female mate choice comprise additive (good genes) and nonadditive genetic benefits (genetic compatibility). Although good genes can be revealed by condition‐dependent display traits, the mechanism by which compatibility alleles are detected is unclear because evaluation of the genetic similarity of a prospective mate requires the female to assess the genotype of the male and compare it to her own. Cuticular hydrocarbons (CHCs), lipids coating the exoskeleton of most insects, influence female mate choice in a number of species and offer a way for females to assess genetic similarity of prospective mates. Here, we determine whether female mate choice in decorated crickets is based on male CHCs and whether it is influenced by females' own CHC profiles. We used multivariate selection analysis to estimate the strength and form of selection acting on male CHCs through female mate choice, and employed different measures of multivariate dissimilarity to determine whether a female's preference for male CHCs is based on similarity to her own CHC profile. Female mating preferences were significantly influenced by CHC profiles of males. Male CHC attractiveness was not, however, contingent on the CHC profile of the choosing female, as certain male CHC phenotypes were equally attractive to most females, evidenced by significant linear and stabilizing selection gradients. These results suggest that additive genetic benefits, rather than nonadditive genetic benefits, accrue to female mate choice, in support of earlier work showing that CHC expression of males, but not females, is condition dependent.     

Variation in male effects on female fecundity in Drosophila melanogaster

In many species, males have the capacity to directly influence (either positively or negatively) the fitness of their mates and offspring, not only via parental care contributions and/or precopulatory resource provisioning, but also via the post‐copulatory activity of those substances passed on to their mates in their ejaculates. Here, we examine how an individual male's identity may be related to phenotypic variation in short‐term female fecundity in the model species, Drosophila melanogaster. The effect of male identity on short‐term fecundity stimulation of females was repeatable across time and accounted for over a fifth of the total observed phenotypic variation in fecundity in two independent populations. The functional explanations for these results and the implications for our understanding of the factors that contribute to the adaptive significance of mating preferences and/or sexual conflict are discussed.     

Oviposition preference but not adult feeding preference matches with offspring performance in the bronze bug Thaumastocoris peregrinus

Optimal foraging and optimal oviposition are two major forces leading to plant selection by insect females, but the contribution of these forces to the host‐selection process has been little studied for sucking herbivores. We studied feeding and oviposition behavior of a global pest, the bronze bug, Thaumastocoris peregrinus Carpintero & Dellapé (Heteroptera: Thaumastocoridae), using dual‐choice bioassays to evaluate the preference of females between host species, developmental leaf stage, or prior plant exposure to conspecifics. We assessed the link between these preferences and the performance of the offspring, by comparing survival and developmental time of nymphs reared on the various treatments. Finally, we compared the composition of the leaf wax of healthy and damaged leaves, and tested the effects of leaf wax on female preference behavior. Using healthy adult leaves of Eucalyptus tereticornis Sm. (Myrtaceae) as a reference, we found that females prefer to feed on Eucalyptus grandis W. Hill ex Maiden and E. tereticornis adult leaves that had been previously damaged by female conspecifics, whereas they reject juvenile leaves of E. tereticornis as food. Females also prefer to oviposit on leaves previously damaged by conspecifics but they rejected E. grandis as oviposition substrate. Nymphal performance varied among leaf treatments, suggesting a correlation with oviposition preference (but not feeding preference). Epicuticular wax extracts from damaged leaves contained higher concentrations of long‐chain, saturated linear alkanes, aldehydes, and alcohols than extracts from undamaged leaves. However, a choice assay failed to demonstrate an oviposition preference based on leaf surface wax chemistry. We discuss these findings in the context of the preference‐performance relationship.