Fitness advantage from nuptial gifts in female fireflies

Abstract 1. In many insects, males provide nuptial gifts to females in the form of spermatophores, sperm-containing structures produced by male accessory glands.
2. The work reported here examined the influence of both spermatophore number and spermatophore size on female reproductive output in two related firefly beetles, Photinus ignitus and Ellychnia corrusca (Coleoptera: Lampyridae). Based on differences in adult diet, male spermatophores were predicted to increase female reproductive output to a greater extent in P. ignitus than in E. corrusca .
3. Female fecundity was significantly higher in triply mated females than in singly mated females in both species, with no difference between mating treatments in female lifespan or egg hatching success. No effects of second male spermatophore size on fecundity, lifespan, or egg hatching success were detected in either species.
4. These results suggest a direct fitness advantage from multiple mating for females in both species, although enhanced fecundity may be due either to allocation of spermatophore nutrients to eggs or to other substances transferred within the spermatophore acting as oviposition stimulants.     

Emerging issues in the evolution of animal nuptial gifts

Uniquely positioned at the intersection of sexual selection, nutritional ecology and life-history theory, nuptial gifts are widespread and diverse. Despite extensive empirical study, we still have only a rudimentary understanding of gift evolution because we lack a unified conceptual framework for considering these traits. In this opinion piece, we tackle several issues that we believe have substantively hindered progress in this area. Here, we: (i) present a comprehensive definition and classification scheme for nuptial gifts (including those transferred by simultaneous hermaphrodites), (ii) outline evolutionary predictions for different gift types, and (iii) highlight some research directions to help facilitate progress in this field.     

Sperm storage mediated by cryptic female choice for nuptial gifts

Polyandrous females are expected to discriminate among males through postcopulatory cryptic mate choice. Yet, there is surprisingly little unequivocal evidence for female-mediated cryptic sperm choice. In species in which nuptial gifts facilitate mating, females may gain indirect benefits through preferential storage of sperm from gift-giving males if the gift signals male quality. We tested this hypothesis in the spider Pisaura mirabilis by quantifying the number of sperm stored in response to copulation with males with or without a nuptial gift, while experimentally controlling copulation duration. We further assessed the effect of gift presence and copulation duration on egg-hatching success in matings with uninterrupted copulations with gift-giving males. We show that females mated to gift-giving males stored more sperm and experienced 17% higher egg-hatching success, compared with those mated to no-gift males, despite matched copulation durations. Uninterrupted copulations resulted in both increased sperm storage and egg-hatching success. Our study confirms the prediction that the nuptial gift as a male signal is under positive sexual selection by females through cryptic sperm storage. In addition, the gift facilitates longer copulations and increased sperm transfer providing two different types of advantage to gift-giving in males.     

Molecular dissection of nuptial gifts in divergent strains of Ostrinia moths

Seminal fluid proteins (SFPs) produced in the male accessory glands and ejaculatory duct are subject to strong sexual selection, often evolve rapidly and therefore may play a key role in reproductive isolation and species formation. However, little is known about reproductive proteins for species in which males transfer ejaculate to females using a spermatophore package. By combining RNA sequencing and proteomics, we characterize putative SFPs, identify proteins transferred in the male spermatophore and identify candidate genes contributing to a one‐way gametic incompatibility between Z and E strains of the European corn borer moth Ostrinia nubilalis. We find that the accessory glands and ejaculatory duct secrete over 200 highly expressed gene products, including peptidases, peptidase regulators and odourant‐binding proteins. A comparison between Ostrinia strains reveals that accessory gland and ejaculatory duct sequences with hormone degradation and peptidase activity are among the most extremely differentially expressed. However, most spermatophore peptides lack reproductive tissue bias or canonical secretory signal motifs and aproximately one‐quarter may be produced elsewhere before being sequestered by the male accessory glands during spermatophore production. In addition, most potential gene candidates for postmating reproductive isolation do not meet standard criteria for predicted SFPs and almost three‐quarters are novel, suggesting that both postmating sexual interactions and gametic isolation likely involve molecular products beyond traditionally recognized SFPs.     

Terminal investment in the gustatory appeal of nuptial food gifts in crickets

Investment in current versus future reproduction represents a prominent trade‐off in life‐history theory and is likely dependent on an individual's life expectancy. The terminal investment hypothesis posits that a reduction in residual reproductive value (i.e. potential for future offspring) will result in increased investment in current reproduction. We tested the hypothesis that male decorated crickets (Gryllodes sigillatus), when cued to their impending mortality, should increase their reproductive effort by altering the composition of their nuptial food gifts (i.e. spermatophylaxes) to increase their gustatory appeal to females. Using a repeated‐measures design, we analysed the amino acid composition of spermatophylaxes derived from males both before and after injection of either a saline control or a solution of heat‐killed bacteria. The latter, although nonpathogenic, represents an immune challenge that may signal an impending survival threat. One principal component explaining amino acid variation in spermatophylaxes, characterized by a high loading to histidine, was significantly lower in immune‐challenged versus control males. The relevance of this difference for the gustatory appeal of gifts to females was assessed by mapping spermatophylax composition onto a fitness surface derived in an earlier study identifying the amino acid composition of spermatophylaxes preferred by females. We found that immune‐challenged males maintained the level of attractiveness of their gifts post‐treatment, whereas control males produced significantly less attractive gifts post‐injection. These results are consistent with the hypothesis that cues of a survival‐threatening infection stimulate terminal investment in male decorated crickets with respect to the gustatory appeal of their nuptial food gifts.     

Flash signals, nuptial gifts and female preference in photinus fireflies

The evolution of male courtship signals such as the bioluminescentflashes of fireflies may be shaped, at least in part, by femalepreference for particular characteristics of the male signal.These female preferences for male courtship signals may ariseas a result of the benefits of choosing males with particulartraits. One possible benefit of mate choice occurs if femalescan use male courtship signals as an honest indicator of malenutritional contributions at mating, nuptial gifts. This paperreviews female preference for male flash characteristics inPhotinus fireflies (Coleoptera: Lampyridae), and the potentialfor females to use male flash characteristics to predict nuptialgift quality. In Photinus firefly species with single pulseflashes females preferentially respond to flashes of greaterintensity and duration. Male Photinus provide a nuptial giftto females at mating in the form of a spermatophore and flashduration serves as a good predictor of spermatophore mass formales collected early in the season. However, Photinus firefliesdo not feed as adults, so spermatophore mass decreases withsubsequent matings. In response, nutrient-limited females maystop preferentially responding to longer duration flashes, increasingtheir overall responsiveness later in the mating season as theyforage for spermatophores. Therefore, the evolution of malecourtship signals in Photinus fireflies is the product not onlyof female preference for male flash characteristics, but alsothe costs and benefits of female choice that shape these preferences.     

An invasion of cheats; the evolution of worthless nuptial gifts

Nuptial gifts are food items or inedible tokens that are transferred to females during courtship or copulation . Tokens are of no direct value to females, and it is unknown why females require such worthless gifts as a precondition of mating. One hypothesis is that token giving arose in species that gave nutritious gifts and males exploited female preferences for nutritional gifts by substituting more easily obtainable but worthless items. An invasion of such behavior would require that females accept the substitute gift and copulate for a period of time similar to that with genuine gifts. We show that both these prerequisites are met in the dance fly Rhamphomyia sulcata, in which females normally accept a nutritious gift. We removed the gift from copulating pairs and replaced it with either a large or small prey item or inedible token. We found that although pairs copulated longest with a large genuine gift, the tokens resulted in copula durations equivalent to those with a small genuine gift. We also observed that males that returned to the lek with tokens re-paired successfully. These findings suggest that female behavior in genuine gift-giving species is susceptible to the invasion of male cheating on reproductive investment.     

Mating stimulates female feeding: testing the implications for the evolution of nuptial gifts

Nutritional benefits from nuptial gifts have been difficult to detect in some species, raising the question: what maintains nuptial feeding when gifts do not benefit females? The sensory trap hypothesis proposes that nuptial feeding may be explained by pre‐existing sensory responses that predispose females to ingest gifts. Recent studies have shown that male seminal proteins can induce a nonspecific increase in female feeding after mating, which may represent a sensory trap for nuptial feeding if it results in increased intake of post‐mating gifts. I tested these ideas using female beetles that ingest a spermatophore after mating. I show that males stimulate strongly increased female feeding post‐mating. However, there was little evidence for dose dependence in the feeding response that could allow males to stimulate feeding beyond the female optimum. Moreover, the post‐mating feeding response could not explain nuptial feeding: despite feeding more in general, newly mated females were less likely than nonmated females to ingest spermatophore gifts.     

Sensory exploitation as an evolutionary origin to nuptial food gifts in insects

Nuptial food gifts given by males to females at mating are widespread in insects, but their evolutionary origin remains obscure. Such gifts may arise as a form of sensory trap that exploits the normal gustatory responses of females, favouring the selective retention of sperm of gift-giving males. I tested this hypothesis by offering foreign food gifts, synthesized by males of one cricket species, to females of three non-gift-giving species. Females provisioned with novel food gifts were 'fooled' into accepting more sperm than they otherwise would in the absence of a gift. These results support the hypothesis that nuptial food gifts and post-copulatory female mating preferences coevolve through a unique form of sensory exploitation.     

Multiple mating and lifetime reproductive output in female dobsonflies that receive nuptial gifts

In mating of the dobsonfly, Protohermes grandis (Thunberg), the male attaches the spermatophore externally to the female genitalia. The spermatophore includes a large gelatinous mass which the female detaches and feeds on after mating. While the female consumes this nuptial food gift, sperm is evacuated from the remaining portion of the spermatophore (sperm package) into her reproductive tract. Under laboratory conditions, mated females maintained receptivity throughout their lifetime, and they remated even on the day following copulation. A single insemination may supply enough sperm, as females mated only once deposited fertile eggs throughout life and, when dissected after death, all females had sperm in the spermatheca. There was a positive correlation between longevity and the number of matings. Lifetime fecundity also increased as mating multiplied. However, the size of eggs and hatchlings was not influenced by the number of matings. It seems that large spermatophore consumption by female P. grandis provides nutrients that increase fitness not in offspring quality, but in their quantity.     

父方投资与性角色逆转现象: 螽斯类昆虫的婚礼食物及对性选择方向的影响

大多数动物在繁殖过程中,雌性在繁殖过程中要比雄性付出更大投资,如相对于精子较大的卵子细胞,较长的育幼时间等,因而在交配过程中,雌性具有选择权,而雄性之间相互竞争以取得与雌性的交配权。然而自然世界中并不总是竞争的雄性（competitive male）-选择的雌性（selective female）这种婚配形式。在螽斯类昆虫中,雄性同样具有较大的父方投资。在繁殖期间,雄性螽斯争相鸣叫,求偶,且在交配后要给予雌性特殊的营养物质-精包,供雌性取食。因此在特定情况下,雌性之间将进行竞争以获取雄性配偶,雄性变得更具有选择性。影响这种性角色逆转的主要因素是可获得资源的紧缺。父方投资理论和性选择理论预测雄性显著地对后代投资时,雌性将表现出典型的雄性特征,她们竞争追求性活跃的雄性,而雄性将表现出典型的雌性特征,对配偶具有选择性。螽斯类昆虫中这种特殊的性角色逆转现象符合性选择理论和父方投资理论的预测。     

Food fight: sexual conflict over free amino acids in the nuptial gifts of male decorated crickets

In decorated crickets, Gryllodes sigillatus, the spermatophore that a male transfers at mating includes a gelatinous spermatophylax that the female consumes, delaying her removal of the sperm‐filled ampulla. Male fertilization success increases with the length of time females spend feeding on the spermatophylax, while females may benefit by prematurely discarding the spermatophylaxes of undesirable males. This sexual conflict should favour males that produce increasingly appealing spermatophylaxes, and females that resist this manipulation. To determine the genetic basis of female spermatophylax feeding behaviour, we fed spermatophylaxes to females of nine inbred lines and found that female genotype had a major influence on spermatophylax feeding duration. The amino acid composition of the spermatophylax was also significantly heritable. There was a positive genetic correlation between spermatophylax feeding duration and the gustatory appeal of the spermatophylax. This correlation suggests that genes expressed in males that produce more manipulative spermatophylaxes are positively linked to genes expressed in females that make them more vulnerable to manipulation. Outbred females spent less time feeding on spermatophylaxes than inbred females, and thus showed greater resistance to male manipulation. Further, in a nonspermatophylax producing cricket (Acheta domesticus), females were significantly more prone to feeding on spermatophylaxes than outbred female Gryllodes. Collectively, these results suggest a history of sexually antagonistic coevolution over the consumption of nuptial food gifts.     

Unpredictable environments,nuptial gifts and the evolution of sexual size dimorphism in insects: an experiment

Many insects have a mating system where males transfer nutrients to females at mating, which are often referred to as ''nuptial gifts''. Among butterflies, some of the characteristic features of these species are polyandry (females mate multiple times), and relatively large male ejaculates. When males produce part of the resources used for offspring, the value of body size might then increase for males and decrease for females. The male/female size ratio is also observed to increase when the degree of polyandry and gift size increase. Butterfly species where gift-giving occurs are generally more variable in body size, suggesting that food quality/quantity fluctuates during juvenile stages. This will cause some males to have much to provide and some females to be in great need, and could be conducive to the evolution of a gift-giving mating system. In such a system, growing male and female juveniles should react differently to food shortage. Females should react by maturing at a smaller size since their own lack of reproductive resources can partly be compensated for by male contributions. Males have to pay the full cost of decreased reproduction if they mature at a small size, making it more important for males to keep on growing, even when growth is costly. An earlier experiment with the polyandrous and gift-giving butterfly, Pieris napi, supported this prediction. The pattern is expected to be absent or reversed for species with small nuptial gifts, where females do not benefit from mating repeatedly, and will thus be dependent on acquiring resources for reproduction on their own. To test this prediction, we report here on an experiment with the speckled wood butterfly, Pararge aegeria. We find that growth response correlates with mating system in the two above species, and we conclude that differences in environmental conditions between species may act as an important factor in the evolution of the mating system and sexual size dimorphism.     

Paternity of offspring in multiply-mated,female crickets: the effect of nuptial food gifts and the advantage of mating first

The spermatophore transferred by male decorated crickets (Gryllodes sigillatus) includes a large gelatinous mass, the spermatophylax, that is consumed by the female after mating. This nuptial gift preoccupies the female while sperm are discharged from the remaining portion of the spermatophore, the sperm ampulla, into her reproductive tract. There is considerable variation in the mass of the spermatophylax, and about half of all males produce spermatophylaxes that are too small to ensure complete sperm transfer. We tested two hypotheses concerning the maintenance of this variation: (i) males trade-off investment in spermatophylaxes against copulation frequency; and (ii) males synthesize the largest spermatophylaxes of which they are physiologically capable. Males synthesizing large and small food gifts were permitted multiple mating opportunities with the same females, and allozyme markers were used to establish the paternity of offspring. There was a significant advantage to those males that mated first irrespective of gift size. This advantage probably arose, in part, because the sperm of first males would have had exclusive access to females'' eggs during the first 24 hours of oviposition, and underscores the benefits of matings with virgin females. The paternity of ''small-gift'' males increased with gift mass, but there was no such increase in ''large-gift'' males. This difference probably stems from the relationship between gift mass and sperm transfer: most of the gifts of the large-gift males would have been above the threshold needed to achieve complete inseminations, whereas those of small-gift males would have been below the threshold. Within mating-order positions, there was no significant difference in the paternity of large-gift and small-gift males, a result seemingly consistent with the ''trade-off'' hypothesis. However, there was no correlation between spermatophylax mass and male mating frequency, so that the mechanism by which small-gift males offset their fertilization disadvantage remains unknown.     

Infection increases the value of nuptial gifts, and hence male reproductive success, in the Hymenolepis diminuta-Tenebrio molitor association

During copulation, male insects pass accessory gland components to the female with the spermatophore. These gifts can affect female reproductive behaviour, ovulation and oviposition. Here, we show that female mealworm beetles, Tenebrio molitor, mated with males infected with metacestodes of the rat tapeworm, Hymenolepis diminuta, produced significantly more offspring than those mated with uninfected males. There is a significant positive relationship between parasite intensity in the male and reproductive output in the female. Infection results in a significant increase in bean-shaped accessory gland (BAG) size. We suggest that infected males pass superior nuptial gifts to females and discuss the confounding effects of infection in male and female beetles upon overall fitness costs of infection for the host and the likelihood that the parasite is manipulating host investment in reproduction.