Influence of female reproductive anatomy on the outcome of sperm competition in Drosophila melanogaster

Females as well as males can influence the outcome of sperm competition, and may do so through the anatomy of their reproductive tracts. Female Drosophila melanogaster store sperm in two morphologically distinct organs: a single seminal receptacle and, normally, two spermathecae. These organs have different temporal roles in sperm storage. To examine the association between sperm storage organ morphology and sperm competition, we used a mutant type of female with three spermathecae. Although the common measure of sperm competition, P(2), did not differ between females with two and three spermathecae, the pattern of sperm use over time indicated that female morphology did affect male reproductive success. The rate of offspring production by females with three spermathecae rose and fell more rapidly than by females with two spermathecae. If females remate or die before using up second male sperm, then second male reproductive success will be higher when they mate with females with three spermathecae. The results indicate that temporal patterns of sperm use as well as P(2) should be taken into account when measuring the outcome of sperm competition.     

A cost of cryptic female choice in the yellow dung fly

Female dung flies Scathophaga stercoraria (L.) store sperm from several males in three or four spermathecae. Selection on the number of spermathecae was successful and the morphological intermediate stages in the evolution from three to four spermathecae are illustrated. The genetic quality of a male from a female's perspective depends on an interaction between their genotypes and the microhabitat in which the offspring will grow. Females influence the paternity pattern of their offspring, and do this differently in different microhabitats. Females with four spermathecae are better able to influence paternity than are those with three spermathecae. However, there must be a cost to building and maintaining an extra spermatheca. We estimate, using the animal model on pedigree data, that this cost is approximately five eggs per clutch, i.e. around 8% of the mean clutch size. This is a substantial cost and such costs should not be ignored in discussions of the benefits to females of assessing the genetic qualities of their mating partners. We suggest that the number of spermathecae in the study population is stable because the relative benefits in quality of offspring through cryptic female choice is balanced by the costs in total numbers of offspring.     

A Possible Explanation for Cryptic Female Choice in the Yellow Dung Fly,Scathophaga stercoraria (L.)

Cryptic, or post-copulatory, female choice could markedly affect the outcome of sperm competition, i.e. a female could differentially manipulate ejaculates within her own body, affecting the fertilization successes of her mating partners. Female yellow dung flies, Scathophaga (Scatophaga) stercoraria, have three spermathecae, the sperm-storage organs, and can to some extent store the sperm of different males in different places. I show that a female's body size, as well as those of her mates, influences the process of sperm storage. Furthermore, females lay eggs of different genotypes under different environmental conditions. Females use both cues correlated with single locus variation (at the locus for the enzyme phosphoglucomutase, PGM) and quantitative trait variation (in body size and development time) when using sperm to fertilize their eggs. It is proposed that this allows a female to match the genotypes of her offspring to the conditions in which the larvae must grow, thus increasing their subsequent fitness. I describe an experiment where larvae of different PGM genotypes were raised in different environments and the most successful genotype was different in different environments. The complexity of the female reproductive system may therefore have evolved because the best father for a female's offspring, from the female's viewpoint, is different under different environmental conditions. The effect interacts with the established male-determined effects to influence the outcome of sperm competition.     

Cryptic female choice: frogs reduce clutch size when amplexed by undesired males

In species with internal fertilization, females can 'cryptically' choose (e.g. through sperm selection) which individuals sire their offspring, even when their overt preferences for copulatory partners are overrun by male-male competition and sexual coercion. The experiment presented here reveals that control of paternity after copulation has begun is also possible in species with external fertilization. Females of the hybridogenetic Rana essonae-Rana esculenta (LL-LR) waterfrog complex adjust their clutch size in response to mate type: they release fewer eggs when amplexed by hybrid LR males who--jeopardize successful reproduction--than when amplexed by parental LL males. This reduction in the number of eggs laid can increase a female's residual reproductive value through a second mating in the same breeding season or a larger clutch size in the next year. We argue that cryptic female choice through clutch size adjustment (i) may have evolved more often than previously assumed, and (ii) can arise even where females mate only once during a reproductive period.     

Alternative Female Choice Tactics in the Scorpionfly Hylobittacus apicalis(Mecoptera) and Their Implications

Females of the scorpionfly Hylobittacus apicalis choose mateson the basis of material benefits (nuptial arthropod prey size)and probably on the basis of genetic benefits males deliverat mating. Females feed on the male's prey throughout copulation.They prefer males with large prey as mates and often refusemales who present small prey. That females may value male geneticquality is suggested by differences in ability of males to obtainlarge prey, which if inherited would influence offspring fitness,and by females often terminating mating with males with smallprey before they transfer any sperm or a complete ejaculate.Females hunt only when males with prey are not available becausehunting exposes individuals to predators. Female Hylobittacusapicalis exhibit alternative mate choice tactics, which arecondition-dependent in expression and probably comprise a conditionalstrategy. Body size, recent feeding history, and male availabilitydetermine how discriminating an individual female actually is,and these conditions may determine the value of material andgenetic benefits in mate choice decisions. The results suggestthat female choice controls male behavior. When females becomechoosy, males are forced to obtain rare large prey despite theincreased risks to males associated with this behavior. The implications of the findings on H. apicalis are discussedin relation to condition dependent female choice patterns inother species and the evolutionary maintenance of female choice.     

Female choice and the relatedness of mates in the guppy (<Emphasis Type="Italic">Poecilia reticulata</Emphasis>)

Several studies suggest that females may offset the costs of genetic incompatibility by exercising pre-copulatory or post-copulatory mate choice to bias paternity toward more compatible males. One source of genetic incompatibility is the degree of relatedness among mates; unrelated males are expected to be genetically more compatible with a female than her relatives. To address this idea, we investigated the potential for inbreeding depression and paternity biasing mechanisms (pre- and post-copulatory) of inbreeding avoidance in the guppy, Poecilia reticulata. Inbreeding resulted in a reduction in offspring number and quality. Females mated to siblings gave birth to significantly fewer offspring compared to females mated to non-siblings and inbred male offspring took longer to reach sexual maturity. There was no evidence of inbreeding avoidance in pre-copulatory behaviors of females or males. Sexual responsiveness of females to courting males and the number of sexual behaviors males directed at females did not decrease as a function of the relatedness of the two individuals. We also tested whether female guppies can use post-copulatory mechanisms to bias sperm usage toward unrelated males by comparing the number of offspring produced by females mated to two of their siblings (SS), two males unrelated to the female (NN), or to one unrelated male and a sibling male (NS). We found that NS females produced a number of offspring not significantly different than what would be expected if fertilization success were halfway between completely outbreeding (NN) and completely inbreeding (SS) females. This suggests that there is no significant improvement in the number of offspring produced by females mating to both related and unrelated males, relative to that which would be expected if sperm from both males were used equally. Our results suggest that female guppies do not discriminate against closely related males or their sperm.     

Sexual conflicts in spotted hyenas: male and female mating tactics and their reproductive outcome with respect to age,social status and tenure

We investigated the reproductive outcomes of male and female mating tactics in the spotted hyena, Crocuta crocuta, a female-dominated social carnivore with high maternal investment, an absence of paternal care and female control over copulation. Paternity was determined using microsatellite profiling of 236 offspring in 171 litters from three clans. We found little evidence that male tactics that sought to coerce or monopolize females were successful. Polyandry and sperm competition appeared to counter effectively pre-copulatory male tactics, such as harassment, monopolization and other tactics, such as infanticide, that were against the evolutionary interests of females, and may have contributed to the stability of the male dominance hierarchy, which operated as a social queue. At least 39% of 54 females mated multiply, and 35% of 75 twin litters were fathered by two sires. Polyandry may also serve to ensure fertilization, compensate for an initial poor-quality mate or ensure fertilization by genetically compatible mates. Female mate choice matched observed patterns of affiliative male-female behaviour, indicating that affiliative behaviour is a successful male mating tactic, and was consistent with the idea that male tenure may serve as an index of male quality, although male fertility may decline with extreme old age.     

Sexual selection for male dominance reduces opportunities for female mate choice in the European bitterling (Rhodeus sericeus)

Sexual selection involves two main mechanisms: intrasexual competition for mates and intersexual mate choice. We experimentally separated intrasexual (male-male interference competition) and intersexual (female choice) components of sexual selection in a freshwater fish, the European bitterling (Rhodeus sericeus). We compared the roles of multiple morphological and behavioural traits in male success in both components of sexual competition, and their relation to male reproductive success, measured as paternity of offspring. Body size was important for both female choice and male-male competition, though females also preferred males that courted more vigorously. However, dominant males often monopolized females regardless of female preference. Subordinate males were not excluded from reproduction and sired some offspring, possibly through sneaked ejaculations. Male dominance and a greater intensity of carotenoid-based red colouration in their iris were the best predictors of male reproductive success. The extent of red iris colouration and parasite load did not have significant effects on female choice, male dominance or male reproductive success. No effect of parasite load on the expression of red eye colouration was detected, though this may have been due to low parasite prevalence in males overall. In conclusion, we showed that even though larger body size was favoured in both intersexual and intrasexual selection, male-male interference competition reduced opportunities for female choice. Females, despite being choosy, had limited control over the paternity of their offspring. Our study highlights the need for reliable measures of male reproductive success in studies of sexual selection.     

Access to multiple mates increases fecundity but does not affect per‐offspring maternal investment in a marine gastropod

Females of many organisms mate more than once and with more than one male, suggesting that polyandry confers some advantage to the female or her offspring. However, variation in maternal investment in response to mate choice and mate number can confound efforts to determine if there are benefits of polyandry. Access to multiple mates could increase maternal investment in offspring via a number of different mechanisms. Few studies have determined if investment is influenced by mate choice and number, and data are particularly lacking for marine invertebrates. This study was designed to determine if maternal investment and offspring size increase with access to increasing numbers of mates in the protandrous intertidal slipper snail Crepidula cf. marginalis. Virgin female slipper limpets were exposed to one, three, or five potential mates and their fecundity, egg size, and hatchling size were measured for multiple clutches. Treatment had a significant effect on fecundity, with fecundity increasing with the number of potential mates. Treatment did not have an effect on the size of eggs or hatchlings, on the variation in egg size or hatchling size within broods, or on the frequency of oviposition. Treatment did alter the variation in average offspring size among females, but not in the way predicted by theory. The main result, that access to multiple mates does not have an effect on per offspring maternal investment, makes C. cf. marginalis an ideal candidate to study the effects of polyandry on offspring fitness without having to take into account confounding effects of variation in maternal investment.     

Female control of sperm ejection and retention in the cornsilk fly Euxesta eluta (Diptera: Ulidiidae)

Promiscuous mating systems provide the opportunity for females to bias fertilization toward particular males. However, distinguishing between male sperm competition and active female sperm choice is difficult for species with internal fertilization. Nevertheless, species that store and use sperm of different males in different storing structures and species where females are able to expel all or part of the ejaculates after copulation may be able to bias fertilization. We report a series of experiments aimed at providing evidence of female sperm choice in Euxesta eluta (Hendel), a species of ulidiid fly that expels and consumes ejaculates after copulation. We found no evidence of greater reproductive success for females mated singly, multiply with the same male, or mated multiply with different males. Female E. eluta possesses two spherical spermathecae and a bursa copulatrix for sperm storage, with a ventral receptacle. There was no significant difference in storing more sperm in spermathecae 24 h after copulation than immediately after copulation. Females mated with protein-fed males had greater reproductive success than similar females mated to protein-deprived males. Protein-fed females prevented to consume the ejaculate, retained more sperm when mated to protein-fed males than when mated to protein-deprived males. Our results suggest that female E. eluta can exert control of sperm retention of higher quality males through ejaculate ejection.     

Why do females mate multiply? A review of the genetic benefits

The aim of this review is to consider the potential benefits that females may gain from mating more than once in a single reproductive cycle. The relationship between non-genetic and genetic benefits is briefly explored. We suggest that multiple mating for purely non-genetic benefits is unlikely as it invariably leads to the possibility of genetic benefits as well. We begin by briefly reviewing the main models for genetic benefits to mate choice, and the supporting evidence that choice can increase offspring performance and the sexual attractiveness of sons. We then explain how multiple mating can elevate offspring fitness by increasing the number of potential sires that compete, when this occurs in conjunction with mechanisms of paternity biasing that function in copula or post-copulation. We begin by identifying cases where females use pre-copulatory cues to identify mates prior to remating. In the simplest case, females remate because they identify a superior mate and 'trade up' genetically. The main evidence for this process comes from extra-pair copulation in birds. Second, we note other cases where pre-copulatory cues may be less reliable and females mate with several males to promote post-copulatory mechanisms that bias paternity. Although a distinction is drawn between sperm competition and cryptic female choice, we point out that the genetic benefits to polyandry in terms of producing more viable or sexually attractive offspring do not depend on the exact mechanism that leads to biased paternity. Post-copulatory mechanisms of paternity biasing may: (1) reduce genetic incompatibility between male and female genetic contributions to offspring; (2) increase offspring viability if there is a positive correlation between traits favoured post-copulation and those that improve performance under natural selection; (3) increase the ability of sons to gain paternity when they mate with polyandrous females. A third possibility is that genetic diversity among offspring is directly favoured. This can be due to bet-hedging (due to mate assessment errors or temporal fluctuations in the environment), beneficial interactions between less related siblings or the opportunity to preferentially fertilise eggs with sperm of a specific genotype drawn from a range of stored sperm depending on prevailing environmental conditions. We use case studies from the social insects to provide some concrete examples of the role of genetic diversity among progeny in elevating fitness. We conclude that post-copulatory mechanisms provide a more reliable way of selecting a genetically compatible mate than pre-copulatory mate choice. Some of the best evidence for cryptic female choice by sperm selection is due to selection of more compatible sperm. Two future areas of research seem likely to be profitable. First, more experimental evidence is needed demonstrating that multiple mating increases offspring fitness via genetic gains. Second, the role of multiple mating in promoting assortative fertilization and increasing reproductive isolation between populations may help us to understand sympatric speciation.     

Microsatellite evidence for sperm storage and multiple paternity in the marine gastropod Crepidula coquimbensis

In gregarious species with copulation and internal fertilization, male-male competition and female cryptic choice may affect reproductive success of both sexes. We carried out a molecular analysis to study paternity and sperm use by females in the protandrous marine brooding gastropod Crepidula coquimbensis. In the field, a single female inhabits an empty hosting shell with up to six males. This gregarious behavior may promote intra-brood multiple paternity if females can store sperm from several consecutive copulations with the surrounding males. To study female sperm usage, the males sharing shelters with five different adult females were collected and preserved for paternity analysis. Females were transported alive to the laboratory and isolated for six months. After that, an additional male was offered to each of the five study females. Once the females had laid capsules, a total of 528 embryos from the five females were assigned paternity based on five microsatellite loci. Paternity analysis showed that every male sharing the empty hosting shell of a female as well as the additional male were assigned fatherhood of embryos laid by this specific female. Females can thus use sperms from multiple males including sperms stored for at least six months. In addition, in four out of the five offspring arrays, a similar contribution of each male to the brood was observed, a pattern associated with the close relationship between the number of fathers observed and the effective paternity index calculated. These results contrast with those of paternity analyses carried out in another species of the same genus, C. fornicata which is characterized by a stacking behavior in which the closest male to the female achieves the highest reproductive success. Male reproductive success may be largely influenced by the aggregation pattern and male mating opportunities in the Crepidula complex, a hypothesis to be examined further by studying other species exhibiting different grouping behavior.     

Postcopulatory female choice increases the fertilization success of novel males in the field cricket, Gryllus vocalis

Although recent studies have demonstrated that female crickets prefer novel males to previous mates, the relative contribution of pre- and postcopulatory behaviors to this advantage remain unknown, as do the reproductive consequences to males. I paired females either with previous or novel mates, and recorded the latency to mating and the time after mating at which the female removed the male's spermatophore, terminating sperm transfer. Females that mated with familiar males removed their spermatophores sooner than females that mated with novel males. Females paired with novel males also mated more quickly than females paired with familiar males, but this difference was not statistically significant. A molecular-based paternity analysis was used to determine whether the postcopulatory preference of females for novel males influences a male's fertilization success. Females were assigned to either mate three times with the same male and then once with a novel male, or four times with four different males. The paternity of the last male was higher when the female previously had mated repeatedly with the same male than when she had mated previously with different males. These results suggest that female spermatophore removal behavior influences male paternity such that novel males receive a fertility benefit.     

The breeding ecology of Cancer gracilis (Crustacea: Decapoda: Cancridae) and the mating systems of cancrid crabs

Brachyuran crabs in the family Cancridae are found in a variety of habitats, ranging from 'open' sandy or muddy grounds to 'structured' rocky bottoms rich in refuge space. Cancer gracilis Dana inhabits shallow open habitats in the north-east Pacific Ocean. Females in all reproductive conditions aggregate in relatively small areas, where mating activity appears most intense. Maximum mating activity was observed towards the end of the spawning/hatching season (August). Females play an active role in mating, often initiating interactions with the prospective male partner. Mating in the field was non-assortative with respect to size. There was no indication of direct mate choice by either males or females; resolution of male-male competitive interactions was size-based. Males below minimum mating size (77 mm carapace width) had spermatophores in the vas deferens but did not react to receptive females.
Sperm competition is likely because sperm can be retained across moults and multiple copulations are possible within a receptive period. Female spermathecae are of the 'ventral' type (contrary to what has been indicated for cancrids), and during intercourse are beyond the reach of male gonopods. Mechanisms to exclude competing sperm include male-produced sperm plugs and post-copulatory mate guarding.
Results are discussed in the context of the 'structured-to-open' habitat gradient, which may underlay much of the diversity in cancrid reproductive ecology. We argue that, by comparison with C. magister (a larger size sympatric species also found in open habitats), (1) reproductive asynchrony and female gregariousness in C. gracilis increase the environmental potential for polygyny, resulting in a female defence polygynous mating system, and (2) reproductive asynchrony inflates the operational sex ratio, leading to stronger sexual selection.     

Pre- and Post-Copulatory Mate Selection Mechanisms in an African Dung Beetle, <Emphasis Type="Italic">Circellium bacchus</Emphasis> (Coleoptera: Scarabaeidae)

Females of most taxa mate selectively. Mate selection may be: (1) pre-copulatory, involving active female choice and male-male competition, and (2) post-copulatory, with cryptic female choice and sperm competition. Because female dung beetles (Circellium bacchus) invest heavily in parental care by ball-rolling and remaining with developing larva they are, therefore, expected to be highly selective when mating. Mate choice in this species was investigated via behavioral observations and investigations of genital allometry of both sexes, leading to conclusions about the mechanisms of, and male characteristics important in, female choice. Male–male competition seems to be crucial in mate selection of C. bacchus, although the females appeared to show no active mate choice. There is a negative allometric relationship between genital size and body size of males as predicted by the ‘one size fits all’ hypothesis (where males have genitalia that fit average-sized females). For the females, no relationship was found between genital size and body size. This might be as a result of the non-sclerotized nature of female genitalia, which may allow for greater morphological plasticity.     

Effects of tapeworm infection on male reproductive success and mating vigor in the red flour beetle,Tribolium castaneum

Parasites may exert negative effects on host survivorship and reproductive success. The effects of parasites on female host fitness have been well documented; however, the effects of parasites on the reproductive success of male hosts and particularly the underlying mechanisms that alter male fitness are not well understood. Previous studies demonstrated that infection by rat tapeworm (Hymenolepis diminuta) reduced the fitness of male red flour beetles (Tribolium castaneum) in an environment of female mate choice and strong male-male competition. The present study determined the role of female mate choice and male insemination capacity on observed fitness reduction of male beetles by the tapeworm parasites. We found that infected males showed reduced mating vigor and consequently inseminated fewer females than did uninfected males. Specifically, tapeworm infection reduced the number of offspring sired by a male by 14-22% even when male-male competition and female mate choice were absent. Further, the insemination capacity of males diminished by 30% because of infection. Female beetles did not discriminate against infected males in precopulatory mate choice experiments. Copulatory courtship, a determinant of postcopulatory female choice, was not significantly different between infected and uninfected males. Hence, we concluded that female beetles did not show either pre- or postcopulatory choice against tapeworm-infected males. Therefore, tapeworm-induced reduction in the reproductive success of male beetles possibly results from altered reproductive biology, such as lower mating vigor and decreased sperm quantity or quality.     

Female and Male Moths Display Different Reproductive Behavior when Facing New versus Previous Mates

Multiple mating allows females to obtain material (more sperm and nutrient) and/or genetic benefits. The genetic benefit models require sperm from different males to fertilize eggs competitively or the offspring be fathered by multiple males. To maximize genetic benefits from multiple mating, females have evolved strategies to prefer novel versus previous mates in their subsequent matings. However, the reproductive behavior during mate encounter, mate choice and egg laying in relation to discrimination and preference between sexes has been largely neglected. In the present study, we used novel and previous mate treatments and studied male and female behavior and reproductive output in Spodoptera litura. The results of this study do not support the sperm and nutrient replenishment hypotheses because neither the number of mates nor the number of copulations achieved by females significantly increased female fecundity, fertility and longevity. However, females showed different oviposition patterns when facing new versus previous mates by slowing down oviposition, which allows the last male has opportunities to fertilize her eggs and the female to promote offspring diversity. Moreover, females that have novel males present called earlier and more than females that have their previous mates present, whereas no significant differences were found on male courtship between treatments. These results suggest that S. litura females can distinguish novel from previous mates and prefer the former, whereas males generally remate regardless of whether the female is a previous mate or not. In S. litura, eggs are laid in large clusters and offspring competition, inbreeding and disease transfer risks are thus increased. Therefore, offspring diversity should be valuable for S. litura, and genetic benefits should be the main force behind the evolution of female behavioral strategies found in the present study.     

Females avoid manipulative males and live longer

Abstract Female mate choice has been demonstrated in a wide variety of species and is now accepted as an important factor in sexual selection. One of the remaining questions, however, is why females prefer specific males. Do females or their offspring benefit from their choice? Or do females choose mates to minimize costs of mating? Here we show that, in the ovoviviparous cockroach Nauphoeta cinerea, where sexual selection has been well documented, females chose mates to avoid costly male manipulation. Females were partnered with preferred or nonpreferred mates, and fitness of the females measured. We found that females lived longer when they mated with preferred males. Female lifespan depended on the rate at which offspring developed from egg to parturition: slower development led to longer life. We manipulated the male pheromone and showed that the component of the pheromone blend that makes males attractive to females also delayed parturition. Thus, like other aspects of sexual conflict in this species, offspring development and thereby the mother's lifespan depended on exposure of females to specific components of the male pheromone. Males benefit from manipulating offspring development because females with accelerated parturition remained unreceptive whereas females with slower developing offspring readily remated after giving birth to their offspring. Our results suggest a hormone‐like role for the male pheromone in N. cinerea and provide the first direct evidence of mate choice to avoid male manipulation. This study shows that dominant males may not be preferred males if they are manipulating females, why multiple components with contrasting effects can exist in a sexual signal, and emphasizes the complex fitness relationships that can arise in species with sexual conflict.     

Males from populations with higher competitive mating success produce sons with lower fitness

Female mate choice can result in direct benefits to the female or indirect benefits through her offspring. Females can increase their fitness by mating with males whose genes encode increased survivorship and reproductive output. Alternatively, male investment in enhanced mating success may come at the cost of reduced investment in offspring fitness. Here, we measure male mating success in a mating arena that allows for male–male, male–female and female–female interactions in Drosophila melanogaster. We then use isofemale line population measurements to correlate male mating success with sperm competitive ability, the number of offspring produced and the indirect benefits of the number of offspring produced by daughters and sons. We find that males from populations that gain more copulations do not increase female fitness through increased offspring production, nor do these males fare better in sperm competition. Instead, we find that these populations have a reduced reproductive output of sons, indicating a potential reproductive trade‐off between male mating success and offspring quality.     

Remating in Drosophila melanogaster: an examination of the trading-up and intrinsic male-quality hypotheses

Female Drosophila melanogaster remate more frequently than necessary to ensure fertilization. We tested whether polyandrous females gain genetic benefits for their offspring by (1) selecting secondary sires of higher genetic-quality than original partners or (2) because post-copulatory mechanisms bias fertilizations towards genetically superior males. We screened 119 hemiclones of males for lifetime fitness then selected eight hemiclones (four of extreme high fitness and four of extreme low fitness) and mated them to virgin females. Females were then given the opportunity to remate with males of benchmark-genetic quality and their propensity to remate (fidelity) and sperm displacement scored. A female's fidelity and her level of sperm displacement varied depending on which hemiclone she mated first, but not on male-genetic quality. These findings indicate that female remating and sperm displacement are strongly influenced by male genotype, but provide no evidence that these traits contribute to adaptive female choice to obtain superior genes for offspring.