Influence of the Male Ejaculate on Post-Mating Prezygotic Barriers in Field Crickets

Post-copulatory interactions between males and females involve highly coordinated, complex traits that are often rapidly evolving and divergent between species. Failure to produce and deposit eggs may be a common post-mating prezygotic barrier, yet little is known about what prevents the induction of egg-laying between species. The field crickets, Gryllus firmus and G. pennsylvanicus are isolated by a one-way reproductive incompatibility; G. pennsylvanicus males fail to fertilize G. firmus eggs or to induce normal egg-laying in G. firmus females. We use experimental crosses to elucidate the role of accessory gland-derived vs. testis-derived components of the G. firmus male ejaculate on egg-laying in conspecific and heterospecific crosses. Using surgical castrations to create ‘spermless’ males that transfer only seminal fluid proteins (SFPs) we test whether G. firmus male SFPs can induce egg-laying in conspecific crosses and rescue egg-laying in crosses between G. pennsylvanicus males and G. firmus females. We find G. firmus SFPs induce only a small short-term egg-laying response and that SFPs alone cannot explain the normal induction of egg-laying. Gryllus firmus SFPs also do not rescue the heterospecific cross. Testis-derived components, such as sperm or prostaglandins, most likely stimulate egg-laying or act as transporters for SFPs to targets in the female reproductive tract. These results highlight the utility of experimental approaches for investigating the phenotypes that act as barriers between species and suggest that future work on the molecular basis of the one-way incompatibility between G. firmus and G. pennsylvanicus should focus on divergent testis-derived compounds or proteins in addition to SFPs.     

BARRIERS TO GENE EXCHANGE BETWEEN CLOSELY RELATED CRICKET SPECIES. II. LIFE CYCLE VARIATION AND TEMPORAL ISOLATION

The closely related field crickets Gryllus firmus and Gryllus pennsylvanicus are known to hybridize in a zone of contact that extends over more than 800 km from the Blue Ridge in Virginia to southern Connecticut. Here I present evidence that the extent of temporal reproductive isolation of the two cricket species varies along the length of the zone. Adults of G. firmus and G. pennsylvanicus occur synchronously in Connecticut but G. firmus matures significantly later than G. pennsylvanicus along the Blue Ridge in Virginia. Variation in the extent of temporal isolation is a consequence of intrinsic differences in the egg-to-adult development time of G. firmus from the two localities. In laboratory rearing experiments, there is little variation in development time among crickets from G. pennsylvanicus populations ranging from Vermont to Virginia. However, G. firmus from Virginia takes significantly longer to mature than G. firmus from Connecticut. Comparisons between species from neighboring localities show that lowland Virginia G. firmus take much longer to develop and are larger as adults than montane Virginia G. pennsylvanicus. In Connecticut, crickets of the two species differ very little in development time and body size. Patterns of variation in development time and body size are compared with similar data for frogs along one of the same environmental gradients and for field crickets along similar gradients elsewhere. Although G. firmus and G. pennsylvanicus are temporally isolated in Virginia, adults of the two species do occur together. Examination of Esterase genotypes of field-inseminated females and their progeny from a mixed population on the Blue Ridge suggests that pre-mating barriers other than temporal isolation are not effective in preventing gene exchange.     

Female mate choice in relation to heterozygosity in Tribolium castaneum

Abstract Female mate choice, both before and after copulation, is pervasive among insect species. It is often hypothesized that females would preferentially mate with males that are genetically dissimilar to promote the genetic variability of the offspring. We used various strains of red flour beetle, Tribolium castaneum, and tested the effect of male and female genetic backgrounds on precopulatory and post‐copulatory female mate choice. Simultaneous mate choice experiments using previously well established pheromone assays did not detect female preference for males of different strains. Post‐copulatory female mate choice was examined through paternity analysis. Two parameters were used to measure post‐copulatory female mate choice, including male defence capacity (P₁, proportion of offspring sired by the first male when a female mated with two males consecutively) and offence capacity (P₂, proportion of offspring sired by the second of two males to mate with a female). When female and male beetle strains were same, defence capacity was significantly higher than when female and male strains were different. However, such a pattern was not observed for offence capacity. The results suggest that female precopulatory mate choice is not affected by genetic background, but the outcome of post‐copulatory processes depends on the genetic background of male and female beetles.     

Testing the viviparity-driven-conflict hypothesis: parent-offspring conflict and the evolution of reproductive isolation in a poeciliid fish

The evolution of viviparity increases the potential for genomic conflicts between mothers and offspring over the level of maternal investment. The viviparity-driven-conflict hypothesis predicts that such conflicts will drive the evolution of asymmetrical reproductive isolation between populations with divergent mating systems. We tested this hypothesis using crosses between populations of a poeciliid fish that differ in their level of polyandry. Our results support the prediction of an asymmetry in the rate of spontaneous abortion in reciprocal crosses, with the highest rate occurring in crosses between females from a relatively monandrous population and males from a relatively polyandrous population. The patterns of offspring size were not consistent with the pattern predicted by the viviparity-driven-conflict hypothesis: crosses between a monandrous female and a polyandrous male did not produce larger offspring than the reciprocal cross. This discrepancy was due to the presence of an effect of the maternal population on offspring size: polyandrous females produced larger offspring than monandrous females. In addition, offspring size was positively correlated with maternal size in crosses involving a polyandrous male. We discuss these results in light of models for intra- and intergenomic epistasis and the rapid origin of asymmetric reproductive isolation in viviparous taxa.     

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.     

Female freshwater crayfish adjust egg and clutch size in relation to multiple male traits

Females may invest more in reproduction if they acquire mates of high phenotypic quality, because offspring sired by preferred partners may be fitter than offspring sired by non-preferred ones. In this study, we tested the differential maternal allocation hypothesis in the freshwater crayfish, Austropotamobius italicus, by means of a pairing experiment aimed at evaluating the effects of specific male traits (body size, chelae size and chelae asymmetry) on female primary reproductive effort. Our results showed that females laid larger but fewer eggs for relatively small-sized, large-clawed males, and smaller but more numerous eggs for relatively large-sized, small-clawed males. Chelae asymmetry had no effects on female reproductive investment. While the ultimate consequences of this pattern of female allocation remain unclear, females were nevertheless able to adjust their primary reproductive effort in relation to mate characteristics in a species where inter-male competition and sexual coercion may mask or obscure their sexual preferences. In addition, our results suggest that female allocation may differentially affect male characters, thus promoting a trade-off between the expression of different male traits.     

Evidence of female cryptic choice in crayfish

To test whether male body size affects female reproductive investment in the polygamous crayfish Procambarus clarkii, we described mating behaviour of virgin females paired with either small or large males, and analysed the number, size and weight of both eggs and juveniles sired by either types of male. Along with confirming the overt selection by females of larger mates, we found that the size and weight of both the eggs and the juveniles were higher when sired by larger fathers. This suggests that P. clarkii females exert a form of cryptic choice for large males, seemingly adjusting the quantity of egg deutoplasm in function of the mate body size. The question of why females spend time and energy to brood low-fitness offspring is finally raised.     

The evolution of offspring size and number: a test of the Smith-Fretwell model in three species of crickets

Most models of parental investment in offspring assume a trade-off between propagule size and number, and an increasing concave down function relating offspring fitness to propagule size. In this study, we test these two fundamental assumptions, using three closely related species of crickets, Gryllus firmus, G. veletis, and G. pennsylvanicus. Egg weight, 35-day fecundity and 35-day egg biomass were estimated in a population of each species, and the relationships between these reproductive traits and date of egg laying and body size were estimated. The relationships between egg weight and offspring survival were also sought for eggs buried at different depths, soil moistures, and soil types (G. firmus and G. veletis), as well as in the field (G. pennsylvanicus). A trade-off between egg weight and 35-day fecundity was revealed in a multivariate analysis taking into account among-species variation in egg weight and body size. Independent of the environmental conditions affecting the eggs, a positive correlation existed between the number of larvae that emerged from the soil and propagule weight in each species. Therefore, these findings provide partial support for the assumptions considered in the models mentioned above. A single optimal egg size was favored in two out of the three sets of conditions in which the functions relating egg weight to larval survival could be derived. The conditions encountered by the eggs, however, influenced the average survival of the larvae, as well as the shape of the relationship between egg weight and offspring survival. This suggests that cricket eggs frequently face heterogeneous environments with respect to egg and hatchling survival; the implication of habitat heterogeneity on the evolution of an optimal egg size is considered. The relationships between the reproductive components and female age and size, as well as between egg size and variation in cricket life-history, are discussed in an ecological and evolutionary context.     

Paternal inheritance of growth in fish pursuing alternative reproductive tactics

In species with indeterminate growth, age‐related size variation of reproductive competitors within each sex is often high. This selects for divergence in reproductive tactics of same‐sex competitors, particularly in males. Where alternative tactics are fixed for life, the causality of tactic choice is often unclear. In the African cichlid Lamprologus callipterus, large nest males collect and present empty snail shells to females that use these shells for egg deposition and brood care. Small dwarf males attempt to fertilize eggs by entering shells in which females are spawning. The bourgeois nest males exceed parasitic dwarf males in size by nearly two orders of magnitude, which is likely to result from greatly diverging growth patterns. Here, we ask whether growth patterns are heritable in this species, or whether and to which extent they are determined by environmental factors. Standardized breeding experiments using unrelated offspring and maternal half‐sibs revealed highly divergent growth patterns of male young sired by nest or dwarf males, whereas the growth of female offspring of both male types did not differ. As expected, food had a significant modifying effect on growth, but neither the quantity of breeding substrate in the environment nor ambient temperature affected growth. None of the environmental factors tested influenced the choice of male life histories. We conclude that in L. callipterus growth rates of bourgeois and parasitic males are paternally inherited, and that male and female growth is phenotypically plastic to only a small degree.     

Body Weight and Rearing Conditions of Males, Female Choice and Paternities in a Small Mammal, Cavia aperea

Patterns of mate choice may be important determinants of a species' social organisation and mating system. At least two different aspects of female mate choice can be distinguished: choice of a social partner and choice of the genetic father of the offspring. Different characteristics of males can qualify them for these two roles. Although social and reproductive partners have been shown to differ in many species, social association times are often used in laboratory choice tests to infer reproductive preferences. The traits for which females may choose partners are diverse. Body size can correlate with the male's strength in defending resources or other abilities benefiting the female and her offspring. In species living in social groups, social skills learned from group members during infancy can be important for later reproductive success. In this laboratory study, we conducted choice tests with wild cavies, Cavia aperea , a harem-living species of South American rodents, to determine social preferences of females towards two simultaneously available males. For offspring sired during these tests, paternities were determined by microsatellite DNA profiling. Males used in the tests differed in body weight and in rearing conditions: Half of the males had been reared in the presence or absence of their father, respectively. Male rearing conditions had no effect on either female social preferences or paternities. Females significantly preferred heavier males as social partners. In five of six tests, the heavier male also sired the offspring. Sires were in most cases but not consistently socially preferred. Heavier males may be preferable as social partners because they are better able to provide females with resources or have more experience in paternal care or predator avoidance as weight correlates with age. When choosing reproductive partners, females may prefer other male traits and the distribution of paternities may also be influenced by sperm competition.     

Revisiting telegony: offspring inherit an acquired characteristic of their mother's previous mate

Newly discovered non‐genetic mechanisms break the link between genes and inheritance, thereby also raising the possibility that previous mating partners could influence traits in offspring sired by subsequent males that mate with the same female (‘telegony’). In the fly Telostylinus angusticollis, males transmit their environmentally acquired condition via paternal effects on offspring body size. We manipulated male condition, and mated females to two males in high or low condition in a fully crossed design. Although the second male sired a large majority of offspring, offspring body size was influenced by the condition of the first male. This effect was not observed when females were exposed to the first male without mating, implicating semen‐mediated effects rather than female differential allocation based on pre‐mating assessment of male quality. Our results reveal a novel type of transgenerational effect with potential implications for the evolution of reproductive strategies.     

Costs of weaponry: Unarmed males sire more offspring than armed males in a male‐dimorphic mite

Morphological structures used as weapons in male–male competition are not only costly to develop but are also probably costly to maintain during adulthood. Therefore, having weapons could reduce the energy available for other fitness‐enhancing actions, such as post‐copulatory investment. We tested the hypothesis that armed males make lower post‐copulatory investments than unarmed males, and that this difference will be most pronounced under food‐limited conditions. We performed two experiments using the male‐dimorphic bulb mite Rhizoglyphus robini, in which males are either armed “fighters” or unarmed “scramblers.” Firstly, we tested whether fighters and scramblers differed in their reproductive output after being starved or fed for 1 or 2 weeks. Secondly, we measured the reproductive output of scramblers and fighters (starved or fed) after one, two or three consecutive matings. Scramblers sired more offspring than fighters after 1 week, but scramblers and fighters only sired a few offspring after 2 weeks. Scramblers also sired more offspring than fighters at the first mating, and males rarely sired offspring after consecutive matings. Contrary to our hypothesis, the fecundity of starved and fed males did not differ. The higher reproductive output of scramblers suggests that, regardless of nutritional state, scramblers make larger post‐copulatory investments than fighters. Alternatively, (cryptic) female choice generally favours scramblers. Why the morphs differed in their reproductive output is unclear. Neither morph performed well relatively late in life or after multiple matings. It remains to be investigated to what extent the apparent scrambler advantage contributes to the maintenance and evolution of male morph expression.     

A TEST FOR ENVIRONMENTAL EFFECTS ON BEHAVIORAL ISOLATION IN TWO SPECIES OF KILLIFISH

Behavioral isolation is a common and potent mechanism of reproductive isolation. Determining the extent to which behavioral isolation varies with environmental conditions is critical to understanding speciation and the maintenance of species boundaries. Here, we tested the effect of salinity on behavioral isolation (female species recognition, male–male competition, and male species recognition) between two closely related killifish (Lucania goodei and L. parva) that differ in salinity tolerance. We performed no‐choice assays and behavioral trials where males could compete and court females in fresh water (0 ppt) and brackish water (15 ppt). We found high levels of behavioral isolation that did not vary as a function of salinity. In behavioral trials, male species recognition of females was strong and asymmetric between the two species. Lucania goodei males preferred conspecifics and rarely courted or mated with L. parva females. Lucania parva males preferred conspecifics but readily courted and mated with L. goodei females. This asymmetry matches previously documented asymmetries in hybrid offspring fitness. Crosses between L. parva males and L. goodei females produce fully viable/fertile hybrids, but crosses between L. goodei males and L. parva females produce males with reduced fertility. Hence, behavioral isolation may have evolved in part due to reinforcement.     

Melanin‐based coloration of sneaker male Atlantic salmon is linked to viability and emergence timing of their offspring

The ‘good genes’ hypothesis of sexual selection predicts that male ornaments are favoured by female mate choice because male ornament reveals genetic quality. In species with different male reproductive tactics, variation in genetic quality among ‘sneaking’ males has rarely been investigated, as usually ‘sneakers’ are thought not to be chosen by females. Here we focused on the alternative reproductive tactic in Atlantic salmon (Salmo salar Linnaeus, 1758) to test whether the skin colour of sneakers may reveal the performance traits of their offspring. A fully factorial breeding design was realized between 20 sneakers and two females using in vitro fertilization. We quantified the red and dark colorations of males and measured the survival of their progeny under semi‐natural conditions. In addition, the size of offspring and their emergence timing from the gravel nest were monitored in the laboratory. We found that darker males sired more viable offspring, whereas red coloration was negatively correlated with offspring survival. Nevertheless, darker and redder male pigmentations were linked to a delay in offspring emergence. These results demonstrate that colours can reveal individual genetic quality in an alternative male reproductive tactic, with male melanin‐based coloration being linked to both beneficial and detrimental effects for the offspring. Our results imply that sneaker ornaments may potentially play a role in both intra‐ and intersexual selection. © 2013 The Linnean Society of London, Biological Journal of the Linnean Society, 2014, 111 , 126–135.     

THE EFFECTS OF THE MATING SYSTEM ON PROGENY PERFORMANCE IN HYLA CRUCIFER (ANURA: HYLIDAE)

We performed a controlled mating experiment to determine whether genetic variation in larval traits in Hyla crucifer was predictable on the basis of mating status or body size of male parent. Larval growth rate was predictably related to body size of the sire. Males from the upper half of the body-size distribution sired offspring with 6% higher growth rates than those of offspring sired by males from the lower half of the body-size distribution. Offspring sired by males that obtained mates in nature had 3% higher growth rates than their half-siblings sired by males that did not mate in nature. Genetic variation for larval-period duration and size at metamorphosis was detected; however, neither mating status nor body size of sire could be used to predict values of these traits in the progeny. Although all three larval traits can affect fitness, there was no evidence that the offspring of some sires would always outperform the offspring of others in all three traits. The predictable association between adult male size and larval growth rate means that the H. crucifer mating system would have a directional effect on larval growth rate if male body size influences the outcome of male-male competition or female choice.     

No evidence for female kin association,indications for extragroup paternity,and sex‐biased dispersal patterns in wild western gorillas

Characterizing animal dispersal patterns and the rational behind individuals’ transfer choices is a long‐standing question of interest in evolutionary biology. In wild western gorillas (Gorilla gorilla), a one‐male polygynous species, previous genetic findings suggested that, when dispersing, females might favor groups with female kin to promote cooperation, resulting in higher‐than‐expected within‐group female relatedness. The extent of male dispersal remains unclear with studies showing conflicting results. To investigate male and female dispersal patterns and extragroup paternity, we analyzed long‐term field observations, including female spatial proximity data, together with genetic data (10 autosomal microsatellites) on individuals from a unique set of four habituated western gorilla groups, and four additional extragroup males (49 individuals in total). The majority of offspring (25 of 27) were sired by the group male. For two offspring, evidence for extragroup paternity was found. Contrarily to previous findings, adult females were not significantly more related within groups than across groups. Consistently, adult female relatedness within groups did not correlate with their spatial proximity inferred from behavioral data. Adult females were similarly related to adult males from their group than from other groups. Using R _ST statistics, we found significant genetic structure and a pattern of isolation by distance, indicating limited dispersal in this species. Comparing relatedness among females and among males revealed that males disperse farer than females, as expected in a polygamous species. Our study on habituated western gorillas shed light on the dispersal dynamics and reproductive behavior of this polygynous species and challenge some of the previous results based on unhabituated groups.     

Body Size and the Maintenance of Reproductive Isolation in Stickleback,Genus Gasterosteus

The blackspotted stickleback Gasterosteus wheatlandi and the widely studied threespine stickleback G. aculeatus are sympatric throughout the former’s range and share many aspects of life history and reproductive behaviour. These two species differ significantly in size, with G. wheatlandi of both sexes measured at approximately 60% of the standard length of their G. aculeatus counterparts. This study concentrated on G. wheatlandi courtship behaviour and investigated its role in the maintenance of reproductive isolation with G. aculeatus. Specifically, the roles that (1) female body size plays in influencing male courtship preferences and (2) male body size and behaviour play in female courtship preferences were investigated through dummy and live conspecific and heterospecific stimulus presentations. Male G. wheatlandi courtship preferences are consistent with previously described patterns for G. aculeatus. Males of both species preferentially approach and court the larger of two simultaneously presented live or dummy females. Thus, the smaller G. wheatlandi males are indiscriminate with respect to assortative mate choice; not only preferring to approach and court more fecund conspecific females but, more significantly, G. aculeatus‐sized females. In contrast, females of both species demonstrate strong assortative courtship preferences. When presented with pairs of flask‐enclosed males, females of both species preferentially orient and court the conspecific male over the heterospecific. Similarly, when presented with a conspecific male and a heterospecific male presented singly, females prefer to enter the nest of the conspecific. Systematic analysis of the interactions between these pairs of fish (one male, one female) demonstrates that the breakdown of courtship in heterospecific courtship occurs late in the courtship sequence when the widely differing forms of male leading behaviour results in drastically differing female responses. I suggest that, as previously described in G. aculeatus, the supernormality effect plays a significant role in mediating adaptive mate choice behaviour in G. wheatlandi. However, the added element of a larger sympatric species introduces a possible cost in time and energy devoted to courting heterospecific, and sympatric, females that the larger G. aculeatus do not likely incur. There is substantial evidence from many sympatric G. aculeatus species pairs that there is assortative mate choice based on size and/or courtship behaviour. Courtship trials suggest a more pervasive role for females in assortative mate choice. Whether it is male body size per se, or in combination with behaviour, morphology or other cues, is unresolved in the present study.     

Paternal body size affects reproductive success in laboratory-held zebrafish (<Emphasis Type="Italic">Danio rerio</Emphasis>)

Across many fish species, large females tend to exhibit higher individual reproductive success due to elevated fecundity and the provisioning of better conditioned eggs and offspring compared to small females. By contrast, effects of paternal body size on reproductive success are less well understood. We disentangled the maternal- and paternal-size dependent effects on reproductive output and early life history in zebrafish (Danio rerio). In the laboratory, females and males from four size categories (small, medium-sized, large and very large) were allowed to spawn freely in a full factorial design with 10 replicates per size combination. As expected, larger females produced more eggs and better conditioned offspring compared to smaller females. Male body size further contributed to zebrafish reproductive success: offspring sired by large males exhibited higher hatching probability and these offspring also hatched earlier and larger than offspring fertilized by small males. However, the largest males experienced lower mating success and received fewer eggs than males of the smaller size classes. While male body size substantially affected reproductive success in zebrafish, it remained unclear whether and to what degree direct paternal effects (e.g., related to sperm quality) or indirect paternal effects stemming from differential allocation patterns by females were the mechanism behind our findings. Answering this question constitutes an important future research topic.     

The unexpected but understandable dynamics of mating,paternity and paternal care in the ocellated wrasse

Although theory generally predicts that males should reduce paternal care in response to cues that predict increased sperm competition and decreased paternity, empirical patterns are equivocal. Some studies have found the predicted decrease in male care with increased sperm competition, while even more studies report no effect of paternity or sperm competition on male care. Here, we report the first example, to our knowledge, of paternal care increasing with the risk and intensity of sperm competition, in the ocellated wrasse (Symphodus ocellatus). Theory also predicts that if paternal care varies and is important to female fitness, female choice among males and male indicators traits of expected paternal care should evolve. Despite a non-random distribution of mating success among nests, we found no evidence for female choice among parental males. Finally, we document the highest published levels of extra-pair paternity for a species with exclusive and obligate male care: genetic paternity analyses revealed cuckoldry at 100 per cent of nests and 28 per cent of all offspring were not sired by the male caring for them. While not predicted by any existing theory, these unexpected reproductive patterns become understandable if we consider how male and female mating and parental care interact simultaneously in this and probably many other species.     

Small Subordinate Male Advantage in the Zebrafish

Dominance is an important determinant of reproductive success in many species, and size is usually an indicator of dominance status, with larger, dominant individuals physically and physiologically preventing smaller subordinates from mating. However, small size may be advantageous in some mating contexts because enhanced manoeuvrability enables males to get closer to females during mating. Here, we determined the paternity success and testes size of dominant and subordinate male zebrafish (Danio rerio), in pairs that controlled for social status. There was no statistical difference in both body size and testes size between dominant and subordinate males. Dominant males sired significantly more offspring than subordinates, but when subordinates were small, they had a greater share of the paternity than larger subordinates. Small male advantage may be one mechanism by which variation in body size is maintained in this species.