SPERM COMPETITION SELECTS FOR NUPTIAL FEEDING IN A BUSHCRICKET

Correlated sexual selection on male genitalia, copulatory performance and nuptial gifts in a bushcricket (Orthoptera: Tettigoniidae) indicated by allometric scaling

We adopt an allometric framework of scaling relationships for comparison between mating-related traits in the middle European bushcricket Roeseliana roeselii (Hagenbach, 1822). Eight characters, covering ontogenetic fitness (size traits; fixed at final moult), male condition (mass traits) and mating motivation (reproductive behaviours), were analysed in unrestricted matings and in matings involving genital manipulation. Shortening the male titillators had no effect on mating-related traits in males. However, titillators, known to be under sexual selection, scale hyperallometrically, with larger males possessing proportionally longer titillators, performing more titillator movements and exhibiting a reduced duration of copulation. Scaling was also hyperallometric for spermatophore mass, with larger males being heavier and transferring heavier nuptial gifts. Both titillator length and spermatophore mass might be condition-dependent indicators, because their variances were nearly twice as large those of body size or body mass. Mass traits were also dynamic, increasing by 11% for male body mass and 17% for spermatophore mass between the first and second matings. Sexual selection by female choice seems to favour larger trait size in the bushcricket R. roeselii, acting in concert on titillator length, intensity of titillator movements and spermatophore mass.

Size-dependent mating pattern in a nuptial gift-giving insect

The reproductive interests of females and males often diverge in terms of the number of mating partners, an individual's phenotype, origin, genes, and parental investment. This conflict may lead to a variety of sex-specific adaptations and also affect mate choice in both sexes. We conducted an experiment with the bush-cricket Pholidoptera griseoaptera (Orthoptera, Tettigoniidae), a species in which females receive direct nutritional benefits during mating. Mated individuals could be assigned due to the genotype of male spermatodoses, which are stored in the female's spermatheca. After 3 weeks of possible copulations in established mating groups which were random replications with four females and males we did not find consistent assortative mating preference regarding to body size of mates. However, our results showed that the frequency of within-pair copulations (192 analyzed mating events in 128 possible pairwise combinations) was positively associated with the body size of both mated individuals with significant interaction between sexes (having one mate very large, association between body size and the number of copulations has weaken). Larger individuals also showed a higher degree of polygamy. This suggests that body size of this nuptial gift-giving insect species is an important sexual trait according to which both sexes choose their optimal mating partner.

FEMALE CONTROL OF SPERM TRANSFER AND INTRASPECIFIC VARIATION IN SPERM PRECEDENCE: ANTECEDENTS TO THE EVOLUTION OF A COURTSHIP FOOD GIFT

Manipulation of ejaculates is believed to be an important avenue of female choice throughout the animal kingdom, but evidence of its importance to sexual selection remains scarce. In crickets, such manipulation is manifest in the premature removal of the externally attached spermatophore, which may afford females an important means of postcopulatory mate choice. We tested the hypothesis that premature spermatophore removal contributes significantly to intraspecific variation in sperm precedence by (1) experimentally manipulating spermatophore attachment durations of competing male Gryllodes sigillatus and (2) employing protein electrophoresis to determine the paternity of doubly mated females. The relative spermatophore attachment durations of competing males had a significant influence on male paternity, but the pattern of sperm precedence deviated significantly from the predictions of an ideal lottery. Instead, paternity data and morphological evidence accorded best with a model of partial sperm displacement derived here. Our model is similar to a displacement model of Parker et al. in that sperm of the second male mixes instantaneously with that of the first throughout the displacement process, but the novel feature of our model is that the number of sperm displaced is only a fraction of the number of sperm transferred by the second male. Regardless of the underlying mechanism, female G. sigillatus can clearly alter the paternity of their offspring through their spermatophore-removal behavior, and employ such cryptic choice in favoring larger males and those providing larger courtship food gifts. We discuss how female control of sperm transfer and intraspecific variation in sperm precedence may be important precursors to the evolution of gift giving in insects.

Weighing costs and benefits of mating in bushcrickets (Insecta: Orthoptera: Tettigoniidae), with an emphasis on nuptial gifts, protandry and mate density

Sexual selection is a major force driving evolution and is intertwined with ecological factors. Differential allocation of limited resources has a central role in the cost of reproduction. In this paper, I review the costs and benefits of mating in tettigoniids, focussing on nuptial gifts, their trade-off with male calling songs, protandry and how mate density influences mate choice. Tettigoniids have been widely used as model systems for studies of mating costs and benefits; they can provide useful general insights. The production and exchange of large nuptial gifts by males for mating is an important reproductive strategy in tettigoniids. As predicted by sexual selection theory spermatophylax size is condition dependent and is constrained by the need to invest in calling to attract mates also. Under some circumstances, females benefit directly from the nuptial gifts by an increase in reproductive output. However, compounds in the nuptial gift can also benefit the male by prolonging the period before the female remates. There is also a trade-off between adult male maturation and mating success. Where males mature before females (protandry) the level of protandry varies in the direction predicted by sperm competition theory; namely, early male maturation is correlated with a high level of first inseminations being reproductively successful. Lastly, mate density in bushcrickets is an important environmental factor influencing the behavioural decisions of individuals. Where mates are abundant, individuals are more choosey of mates; when they are scarce, individuals are less choosey. This review reinforces the view that tettigoniids provide excellent models to test and understand the economics of matings in both sexes.

Understanding reversals in the relative strength of sexual selection on males and females: a role for sperm competition?

Sperm competition affects sexual selection intensity on males, but models suggest it cannot affect the relative intensity of sexual selection on males compared to females. However, if sperm competition depresses the payoff for male multiple mating, it could affect the relative intensity of sexual selection and even cause sexual selection to be more intense on females than males (reversal of typical pattern). To evaluate how sperm competition, energy availability, and parental investment affect the intensity of sexual selection on each sex, I constructed a simulation model using the relationship between fecundity and number of mates to estimate sexual selection gradients. Unlike earlier models, I include a trade-off between paternal investment and sperm competition ability. The amount of energy available for reproduction affects the sexual selection gradient for each sex. Reversals in the sex experiencing stronger sexual selection do occur when additional paternal investment reduces a male's ability to compete for fertilizations within females. The shape of the distribution of mates for each sex (determined by mate competition) is also important. Output from the model is qualitatively similar to empirical data from insects with paternal investment. This model challenges previous thinking about the role of sperm competition in sex-role reversal.

Structure of spermatodoses in shield-back bushcrickets (Tettigoniidae, Tettigoniinae)

Many aspects of the reproductive anatomy and physiology of tettigoniids have been studied extensively. These include the large, externally visible spermatophores and the bundles of sperm, known as spermatodesms. However, spermatodoses, spermatophore-like structures found within the spermatheca, seem to have been almost completely overlooked: their structure has not been described since 1913 and they have subsequently received only passing mention in the literature. Each time the female mates, a separate spermatodose is formed. Here I use photographs, from light-microscopy, of whole and sectioned spermatodoses to describe the external and internal structure of spermatodoses of nine different genera within the subfamily Tettigoniinae. The structure of the spermatodoses is very similar for the different genera. Each spermatodose is pear- or onion-shaped and consists of a thin outer layer, enclosing a thick, gelatinous inner layer. A large sperm mass occupies the bulbous end of the spermatodose, while a thin sperm-tube leads from the sperm mass, along the center of the elongated neck of the spermatodose, and appears to exit at the pointed-tip of the spermatodose. Feather-like bundles of sperm (spermatodesms) were clearly visible within the sperm mass and also appeared to be present within the sperm-tube. The wall of the sperm tube appeared to be composed of material similar to that of the outer layer of the spermatodose. Within the spermatheca, spermatodoses appeared to be stratified in that only one of them ever occupied the position nearest to the spermathecal duct. The possible function of spermatodoses is discussed: it is proposed that they have evolved as a result of sexual conflict and function to protect the sperm from being destroyed by the female while they are in storage.

The enigmatic mating behaviour and reproduction of a simultaneous hermaphrodite, the nudibranchAeolidiella glauca(Gastropoda, Opisthobranchia)

Hermaphroditic mating behaviour is poorly investigated and understood, especially in internally fertilized species. This study describes the mating behaviour and reproduction of the simultaneous hermaphrodite Aeolidiella glauca. Unlike other nudibranchs, A. glauca was found to transfer sperm via external spermatophores that were attached to the partner's back. Despite an elaborate courtship, the actual spermatophore transfer, which always involved two animals only, was of short duration. In most matings (88%) spermatophores were reciprocally exchanged. Shortly after transfer the ventral spermatophore cover dissolved and some sperm gathered on the epidermis. A few even penetrated epidermal cells, occasionally causing considerable damage. Most sperm, however, migrated along the body surface towards the gonopore. Migrating sperm needed 4–5 h to reach and enter the gonopore, but sperm consumption by the recipient sometimes delayed this process. Almost 30% of spermatophores fell off soon after transfer, but spermatophores from reciprocal matings and those placed in certain positions stayed on for longer. Slugs mated readily with different partners and egg production was very high. However, sperm from a single mating may not suffice to fertilize all eggs. Aeolidiella glauca's unique mating behaviour may offer ample opportunities for both sperm competition and alternative sperm utilization.

Ejaculate expenditure by malebush crickets decreases with sperm competition intensity

Male bushcrickets transfer a spermatophore at mating that consists of a sperm-containing ampulla and a sperm-free mass, the spermatophylax, that is consumed by the female during insemination. The costs of spermatophore production for males and benefits of consumption for females result in reversals in courtship roles in nutrient limited populations that increase both the risk and intensity of sperm competition. Here we show that under conditions characteristic of courtship role reversal, male expenditure on the spermatophore is dependent on female size. When mating with small females, males increase the amount of spermatophylax material and sperm, as expected from the increased sperm competition risk associated with courtship role reversal. However, males reduce the amount of spermatophylax material and sperm transferred to larger females. Since larger females have a higher mating success when competing for nurturant males, the intensity of sperm competition covaries with female size. Reduced ejaculate expenditure under increased sperm competition intensity is in accord with theoretical expectation.

Unpredictable food and sexual size dimorphism in insects

The evolution of sexual size dimorphism is likely to be affected by the amount of resources each sex invests in offspring. Male nuptial gifts, occurring in many insect species, might reduce the value of large size in females and increase the value for males. For large nuptial gifts and an accompanying shift in dimorphism to evolve, males with larger gifts should be rewarded, in effect trading the larger gift for more offspring. We suggest that food variability, causing some males to have much to provide and some females to be in great need, would be conducive to the evolution of such a mating system, and we present comparative data on butterflies supporting the suggestion. In a gift-giving mating system, growing male and female juveniles should react differently to food shortage. A female maturing at small size can to some extent buffer her disadvantage through nuptial gifts, whereas a male maturing at small size will suffer from his inability to provide substantial gifts. Thus, males benefit more than females from continued growth in the face of food shortage, leading to a shift in size dimorphism. Here we confirm this prediction in a butterfly, Pieris napi, with large nuptial gifts.