Sperm Precedence Patterns Research Articles

Snail sperm production characteristics vary with sperm competition risk

Sperm competition is widespread and influences both male investment in spermatogenic tissue and ejaculate characteristics. Sperm competition models assume trade-offs between sperm size and number, although such trade-offs may be difficult to detect. This study examines the effects of sperm competition risk on the sperm production characteristics of the freshwater snail Viviparus ater. In this prosobranch, females mate frequently and store sperm, generating sperm competition. Males produce two sperm morphs, fertile eupyrene sperm and non-fertilizing oligopyrene sperm. Non-fertilizing sperm may play a role in sperm competition and therefore, like fertilizing sperm, the number produced could vary relative to sperm competition risk. In addition, trade-offs between sperm number and sperm size may be expected. We manipulated the sex ratio of sexually mature snails and found the presence of rivals affected the ratio of oligopyrene/eupyrene sperm males produced. In experimental and natural populations, the number of oligopyrene sperm produced, but not the number of eupyrene sperm, was significantly higher when the sex ratio was male biased. Testis mass did not vary between experimental treatments. We also found a negative relationship between the number and size of oligopyrene sperm produced, which is consistent with evolutionary models of sperm competition, and is, to our knowledge, the first intraspecific demonstration of a trade-off between these traits.

Conspecific sperm precedence in Drosophila.

Traits that influence the interactions between males and females can evolve very rapidly through sexual selection or sexually antagonistic coevolution. Rapid change in the fertilization systems of independent populations can give rise to reproductive incompatibilities between populations, and may contribute to speciation. Here I provide evidence for cryptic reproductive divergence among three sibling species of Drosophila that leads to a form of postmating isolation. When a female mates with both a conspecific and a heterospecific male, the conspecific sperm fertilize the vast majority of the eggs, regardless of the order of the matings. Heterospecific sperm fertilize fewer eggs after these double matings than after single matings. Experiments using spermless males show that the seminal fluid of the conspecific male is largely responsible for this conspecific sperm precedence. Moreover, when two males of the same species mate sequentially with a female from a different species, a highly variable pattern of sperm precedence replaces the second-male sperm precedence that is consistently found within species. These results indicate that females mediate sperm competition, and that second-male sperm precedence is not an automatic consequence of the mechanics of sperm storage.

Male mating behaviour and patterns of sperm precedence in Arctic ground squirrels

Ground squirrels (genusSpermophilus) display considerable interspecific variation in male reproductive behaviour. Although male Arctic ground squirrels,S. parryii plesius, are territorial during the mating period, males in several other species ofSpermophilusare not. To begin exploring the reasons for this behavioural difference, patterns of mating behaviour and sperm precedence were characterized for a free-living population ofS. parryii plesius. Twenty females that were monitored continuously throughout behavioural oestrus consorted and presumably copulated with a mean of 1.9±0.8 males during a single period of sexual receptivity. A combination of electrophoretic and DNA fingerprinting analyses revealed that more than 90% of pups in litters (N=11) reared by females that consorted with two or more males were sired by a female's first mate. The percentage of young sired by each of a female's consorts did not vary with either consortship duration or the interval between consortships by different males. Instead, the only consistent correlate of paternity was consortship order. Both pre-consortship calling and post-consortship defence of females were significantly associated with first consortships, suggesting that males were able to distinguish unmated females from females that had already copulated. Because the probability of siring young should influence which females are preferred as mates and how males compete for access to those females, comparative studies of sperm precedence patterns may yield insights into the evolution of interspecific variation in male mating behaviour.

Molecular genetic analysis of sperm competition in the damselfly Ischnura elegans (Vander Linden).

Sperm competition can be a powerful selective force in the evolution of mating systems. Several odonate species have attracted study to assess the extent and mechanism of last-male sperm precedence. Members of the genus Ischnura (Zygoptera) display a particularly interesting range of mating systems, and Ischnura elegans was selected for study. Polymorphic microsatellites were cloned, sequenced and used to determine paternity of I. elegans larvae, to reveal patterns of sperm precedence. More than 3000 larvae, collected from both wild and captively bred I. elegans females, were typed for one or two microsatellite loci and paternity was determined by comparison with parental genotypes. Microsatellite typing showed that most wild-caught females had mated with several males. Analysis of offspring from females which mated in captivity showed that multiple-matings result in a large proportion of last-male sperm precedence (mean value for immediate last male precedence is 0.79 +/- 0.2 (+/- s.d.; n = 11, range = 0.44-1)). There is appreciable variation in the extent and patterns of immediate and longer-term precedence, which could reflect differences in male sperm removal ability or selective use of sperm by females.

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.

Extreme variability in sperm precedence in the fungus beetle, Bolitotherus cornutus (Coleoptera Tenebrionidae)

Most previous studies of sperm precedence have reported only average values for species. Intraspecific variation in sperm precedence, however, is necessary for the evolution of adaptations that alter sperm precedence patterns. To examine intraspecific variation in sperm precedence, virgin female fungus beetles were mated to two different males in succession, and all females, males, and offspring were genotyped at eight allozyme loci. The proportion of offspring fathered by the second male varied from zero to one. Future studies should not only measure intraspecific variability in sperm precedence, but should attempt to determine causes of this variability.

Female mediation of sperm competition in the millipede Alloporus uncinatus (Diplopoda: Spirostreptidae)

The tropical spirostreptid millipede Alloporus uncinatus has a polygynandrous mating pattern that is apparently shaped by sperm competition (Telford and Dangerfield 1990, 1993a). In the present study radioisotopic labelling of ejaculates was used to quantify the temporal effects of double mating sequences on sperm precedence patterns. Ejaculates of successive males mix completely within the sperm storage organs and are concentrated close to the site of fertilisation, the spermathecal-oviduct junction. When matings follow each other immediately (experiment 1), equal volumes of the ejaculates of each male are stored within the spermathecae, and both males have equal probability of paternity. Where matings are separated by a 24-h delay (experiment 2), the volumetric contribution of the first male is reduced by 54.8 % and last male precedence operates. Although the distal ends of the gonopods play a primary role in the redistribution of ejaculates (Barnett and Telford 1994), by using a single mating sequence we show that they are not responsible for the reduction in ejaculate volume. The most likely explanation is absorption and/or ejection of the first male's ejaculate by the female. Genital functional morphology is used to reconstruct the mechanism of ejaculate distribution.

The disposition and genetic diversity of stored sperm in females of the damselfly Calopteryx splendens xanthostoma (Charpentier)

Female Calopteryx splendens xanthostoma have discrete sperm storage organs and secure oviposition in one of two ways: a female either mates with a male who subsequently guards her whilst she oviposits (MAG), or females can secure access to an oviposition site without copulating with the resident male (SAG). Random amplified polymorphic DNA (RAPD ) analysis of the genetic diversity of sperm from individual sperm storage organs revealed that the spermathecae contained sperm of a greater genetic diversity than sperm in the bursa copulatrix. Males cannot remove sperm from the spermathecae during copulation, and we propose that the spermathecae might function as a sperm cache for the sperm of previous mates. Females that conduct SAG ovipositions have a higher genetic diversity of stored sperm than MAG females. We suggest that females which gain access to oviposition sites without remating have the potential to exercise post-copulatory choice during oviposition, on the basis of: (i) the observed patterns of sperm precedence in other calopterygids; (ii) the variation in genetic diversity of stored sperm; and (iii) the avoidance of recopulation in SAG females.

Sperm competition: evolutionary causes and consequences

The interaction between functional and mechanistic approaches to sperm competition and between male and female perspectives are described and illustrated by a study of the zebra finch, Taeniopygia guttata. Sperm competition experiments in the laboratory show that last male sperm precedence occurs (as it does in many other taxa) although the mechanism is unknown (as in most other taxa). Empirically-derived values were used to construct a mathematical model of sperm competition in the zebra finch. The model indicates that precedence occurs as a consequence of: (i) the temporal pattern of pair copulations; (ii) the rate at which sperm are lost from the female tract; and (iii) more sperm being transferred during extra-pair copulations than during pair copulations. The latter effect is a consequence of males seeking extra-pair copulations after their own pair copulation period has ended. The effect of sperm numbers on the pattern of sperm precedence may be further increased by: (i) extra-pair males increasing ejaculate size (sperm numbers) (for which there is no evidence); (ii) extra-pair males being of a better quality and transferring more sperm or better quality sperm (for which there is some evidence); and (iii) cryptic female choice. Females eject over 99% of sperm following insemination; if they eject fewer sperm from males chosen as extra-pair copulation partners, the potential for cryptic female choice is considerable. However, this is still being investigated. The model also predicts the optimal time for an extra-pair copulation to occur (from either a male or female perspective). A comparison between the predicted and observed pattern suggests that the optimal timing of extra-pair copulations is constrained in both sexes.

Fitness consequences of differences in male mating behaviour in relation to female reproductive status in flour beetles

In those species where females mate multiply, males that mate with non-virgin will generally share fertilizations with previous mates. As a result, males are expected to have evolved mechanisms to assess female reproductive status to maximize their fertilization success. This study examine dwhether male flour beetles, Tribolium castaneum, can distinguish between virgin and previously mated females by comparing mate behaviour of males given equal access to females differing in reproductive status. Males preferentially approached and contacted virgin females, and copulated significantly more often with virgin females compared to previously mated females. Female reproductive status did not appear to affect the probability of a copulation attempt once contact had been made with the female, or the probability that a copulation attempt woulf be successful. Males mating with virgin females gained a significant reproductive advantage compared to matings with previously mated females, siring more progeny with each mating. Possible factors that may influence selection for male ability to distinguish and preferentially mate with virgin females are discussed, including patterns of sperm precedence and female re-mating intervals.

Why do male Callosobruchus maculatus beetles inseminate so many sperm?

Male Callosobruchus maculatus F. (Coleoptera: Bruchidae) inseminate more sperm than females can effectively store in their spermathecae. This study examines the adaptive significance of “excess” sperm transfer by measuring components of male and female reproductive success in response to manipulating the number of sperm inseminated. The number of sperm transferred during copulation was reduced from 56,000 ±4,462 to 8,700±1,194 by sequentially mating males to virgin females. Reducing the number of sperm inseminated by the first male to mate had no effect on the extent of sperm precedence, but reducing the number of sperm inseminated by the second male resulted in a significant reduction in the extent of sperm precedence. When large numbers of sperm are inseminated the remating refractory period of females is increased. These results indicate that males transferring large numbers of sperm during copulation have a two-fold advantage at fertilization; they are more effective at preempting previously stored sperm and they are likely to father more offspring by delaying the time of female remating. The transfer of “excess” sperm does not appear to serve as nonpromiscuous male mating effort; the number of eggs laid, their fertility and the subsequent survival of zygotes were unaffected by manipulating the number of sperm inseminated. The underlying mechanisms of sperm precedence were also examined. Simple models of sperm displacement failed to accurately predict the patterns of sperm precedence observed in this species. However, the results do not provide conclusive evidence against the models but rather serve to highlight our limited understanding of the movement of sperm within the female's reproductive tract.

Last-male sperm precedence breaks down when females mate with three males

Females of many species commonly mate with several males, yet our knowledge of sperm precedence patterns is based almost exclusively on laboratory experiments in which females were mated to only two males. In both birds and insects, these investigations have generally shown strong mating order effects, usually with the second male to mate siring most of the offspring. In the harlequin beetle-riding pseudoscorpion, single-locus minisatellite DNA profiling has recently revealed extensive multiple paternity within the broods of field-inseminated females. Here, we report the findings of a sperm precedence experiment in which we investigated this unusual absence of mating order effects. By allowing females to mate with only two males, we were able experimentally to induce the pattern of strong last-male sperm precedence typical of other two-male mating studies. By contrast, females mated to three males produced broods exhibiting the same highly mixed paternity detected in this species in nature. The elimination of mating order constraints on sperm utilization when females mate with several males suggests that the opportunity for post-copulatory sexual selection may be much greater in nature than is evident from two-male mating experiments.

Postcopulatory, prezygotic isolation: intraspecific and interspecific sperm precedence in Tribolium spp., flour beetles.

We investigated intraspecific and interspecific patterns of paternity in Tribolium castaneum, the flour beetle, by mating females either to pairs of conspecific males or to one conspecific and one heterospecific male of the closely related species, T. freemani. Females of both species store sperm in the spermatheca after copulation and postcopulatory, prezygotic reproductive isolation has been reported between this pair of species. When conspecific males of contrasting genotype are mated simultaneously to T. castaneum females, we observed extremely high levels of variation among females in the pattern of sperm precedence as shown by the offspring genotypes. In contrast, T. castaneum females mated simultaneously to a conspecific and a heterospecific male produced over 99 per cent conspecific progeny. When conspecific males were mated sequentially to T. castaneum females, within 3-7 days, all offspring were sired by the second male. In contrast, when a conspecific male was replaced with a T. freemani male, most females continued to produce only first-male, conspecific offspring for the next 10 days. When a heterospecific male was replaced with a conspecific male, T. castaneum females changed from producing hybrid to conspecific offspring within 3 days. We discuss the evolutionary implications of these findings.

Correlation between spermathecal morphology and mating systems in spiders

This study tested predictions regarding male mating preferences which were based on some aspects of female reproductive morphology which may influence sperm precedence patterns in six species of spiders. Males of two species, whose 'conduit' spermathecal design has been associated in previous studies with first male sperm precedence, showed the predicted preference for associating with immature females about to moult to maturity rather than mature females. Those of a third species, however, associated indiscriminately with mature and penultimate instar females. As predicted, males of three other species with 'cul-de-sac' spermathecal morphology did not associate preferentially with immature females. Immature females were avoided in two of the species, but not in the third. One of the species with cul-de-sac spermathecae showed, as predicted, lack of a strong first male advantage in sperm precedence. These data give only limited confirmation of the predictions.

Sperm displacement in a millipede? An investigation into the genital morphology of the southern African spirostreptid millipede Orthoporus pyrhocephalus

Male and female genital morphology of the spirostreptid millipede Orthoporus pyrhocephalus was examined by conventional and scanning electron microscopy techniques. The relative syncopulatory positions of the genitalia were established through dissections of pairs in copula. Male gonopods bear scoops and regions of pitted spines. both of which are similar in form to morphological structures implicated in insect sperm displacement. Consideration of the position of the oviduct‐spermathecal junction in females predicts that displacement is restricted to a sperm removal mechanism. The importance of spermathecal shape in determining sperm precedence patterns is discussed. The orientation of the genitalia during copulation indicates that sperm displacement is plausible and a mechanism of gonopod action is suggested.

Correlation between spermathecal morphology and mating systems in spiders

This study tested predictions regarding male mating preferences which were based on some aspects of female reproductive morphology which may influence sperm precedence patterns in six species of spiders. Males of two species, whose 'conduit' spermathecal design has been associated in previous studies with first male sperm precedence, showed the predicted preference for associating with immature females about to moult to maturity rather than mature females. Those of a third species, however, associated indiscriminately with mature and penultimate instar females. As predicted, males of three other species with 'cul-de-sac' spermathecal morphology did not associate preferentially with immature females. Immature females were avoided in two of the species, but not in the third. One of the species with cul-de-sac spermathecae showed, as predicted, lack of a strong first male advantage in sperm precedence. These data give only limited confirmation of the predictions.

Patterns of Sperm Storage in Relation to Sperm Competition in Passerine Birds

Using a technique for observing sperm storage tubules (SSTs) in whole-mount preparations of oviduct tissue, we estimated the number and length of SSTs across 20 species of passerine birds in seven subfamilies. The average number of SSTs per female varied ninefold across the species examined and was correlated positively with body mass and relative egg size, suggesting that much of the interspecific variation in the number of SSTs may be a consequence of allometry. However, after controlling for body mass, the number of SSTs per female was also negatively correlated with the average length of SSTs and we suggest this pattern could result from selection for greater sperm length when access to sperm storage sites is limited by females. SST length varied eight-fold across the species examined but unlike the number of SSTs, SST length was not correlated with body mass. Instead, SST length was strongly and positively correlated with sperm length, suggesting a history of coevolution between male gametes and sperm storage sites in females that is independent of body size. Neither mating system nor any of four other morphological variables (testis length, oviduct length, clutch size, sperm storage capacity) was consistently correlated with either the number or size of SSTs. Variation between species in the potential for sperm layering within an SST, however, indicates that the pattern of sperm precedence (i.e., the pattern of paternity relative to mating order) may vary from species to species. We propose that a conflict of interest between the sexes with respect to sperm storage could lead to an arms race between sperm length and SST length and may help explain both the diversity in sperm storage patterns among species and the influence of sperm storage on the evolution of mating behavior in birds.

Integration of Immediate and Long Term Sperm Precedence Patterns and Mating Costs in an Optimization Model of Insect Copulation Duration

The optimal copulation duration ( t) in insects is strongly influenced by the existence and importance of sperm precedence mechanisms. Using an optimization model, we predict the effects of sperm precedence patterns on the copulation duration for which "number of fertilized eggs"/"costs" is maximized. Effects of the costs of territoriality and clutch size were also analysed. Initially, we assumed that males can only respond to the population average of any particular parameter, and, hence, that each male has only one t opt (only inter-individual differences). We extended these conclusions to situations in which the male can adjust his behaviour to exact values of particular parameters, resulting in a variable t opt for each male (intra-individual differences). For inter-individual comparisons, the model predicts the following: (i) The existence of (long-term) sperm mixing within the female genital tract will increase t opt; (ii) In addition to this effect, sperm mixing combined with either a higher probability of a take-over between or during the first oviposition bouts, or a prolonged expected time between copulation and oviposition, will result in an increase of t opt . However, in the absence of long-term sperm mixing neither of these behavioural characteristics will influence t opt ; (iii) t opt also increases with the mate encounter time for the male. It is not influenced by the expected clutch size of the female; (iv) Both energetic and territorial costs will reduce t opt when high. For cases in which intra-individual adjustment of t opt is possible, we made the following predictions: (i) The presence of a higher than average probability of a take-over of the female before or during the following oviposition bouts, will reduce t opt; (ii) A larger than average clutch size will increase t opt.

Sources of Intraspecific Variation in Sperm Precedence in Red Flour Beetles

Sperm precedence, or differential fertilization success among mating males, is likely to be important in mediating male reproductive success in many species. Numerous studies have provided measures of sperm precedence for diverse taxonomic groups, but previous discussions have focused on species-specific mean P2 values. In this study, which systematically examined sources of variation in sperm precedence in Tribolium castaneum, consistent differences among male pairs represent a significant percentage (17.8%) of the total variation in sperm precedence. These results in T. castaneum and a reexamination of published studies reveal a high degree of intraspecific variation in sperm precedence, suggesting that mean values are insufficient to adequately characterize sperm-precedence patterns. Such individual variation in sperm precedence may profoundly influence differential reproductive success among males.

Extra-pair paternity and intraspecific brood parasitism in wild zebra finches Taeniopygia guttata, revealed by DNA fingerprinting

The frequency of extra-pair parentage in a wild population of zebra finches Taeniopygia guttata was examined by DNA fingerprinting. A total of 25 families, comprising 16 pairs of parents and 92 offspring (in broods of 1 to 6) were examined. Ten cases of extra-pair parentage, presumed to constitute intraspecific brood parasitism, were detected (10.9% of offspring or 36% of broods), including one possible instance of ‘quasi-parasitism’ (parasitism by a female fertilized by the male nest owner). The average number of parasitic eggs per clutch detected by fingerprinting was 1.10±0.32 SD, very similar to the one egg difference in average clutch size between parasitised (6.0±0.82) and unparasitised nests (5.0±0.95). Two cases of extra-pair paternity (EPP) were detected among 82 offspring whose maternity was confirmed: 2.4% of offspring, or 8% of broods. In both cases EPP accounted for only a single offspring within a brood. Behavioural observations show that EPP occurs through extra-pair copulation rather than rapid mate switching. The results are discussed in the light of what is known about the fertile period and sperm precedence patterns in this species.