MALE DOMINANCE, FEMALE MATE CHOICE, AND INTERSEXUAL CONFLICT IN THE ROSE BITTERLING (RHODEUS OCELLATUS)

Sex determination mechanisms and sex control approaches in aquaculture animals

Aquaculture is one of the most efficient modes of animal protein production and plays an important role in global food security. Aquaculture animals exhibit extraordinarily diverse sexual phenotypes and underlying mechanisms, providing an ideal system to perform sex determination research, one of the important areas in life science. Moreover, sex is also one of the most valuable traits because sexual dimorphism in growth, size, and other economic characteristics commonly exist in aquaculture animals. Here, we synthesize current knowledge of sex determination mechanisms, sex chromosome evolution, reproduction strategies, and sexual dimorphism, and also review several approaches for sex control in aquaculture animals, including artificial gynogenesis, application of sex-specific or sex chromosome-linked markers, artificial sex reversal, as well as gene editing. We anticipate that better understanding of sex determination mechanisms and innovation of sex control approaches will facilitate sustainable development of aquaculture.

Winner and loser effects influence subsequent mating interactions in crayfish

In species whose social structure includes dominance relationships, individuals are likely to engage in frequent agonistic interactions with conspecifics, and these interactions can have substantial effects on participants. For example, 'winner' and 'loser' effects, whereby winning or losing a contest increases the probability of winning or losing subsequent encounters, have been described in many species. However, a smaller body of research has shown that winning or losing a contest can lead to additional behavioral changes that affect other domains of an individual's social experiences. Here, we report on an experiment designed to evaluate the effects of prior contests on subsequent mating interactions in the crayfish (Faxonius virilis). We presented males with mating opportunities either immediately following or 7 days after a contest with a conspecific male. We predicted that winners would be more likely to mate than losers, because of either or both winner/loser effects and differences in male competitiveness. We found that, when presented with a mating opportunity immediately following a contest, winning males were more likely to mate than were losing males. We also found that these differences had eroded within 7 days, such that there was no significant difference in the proportions of winners and losers that mated after that period. We concluded that the changes in mating behavior that we observed immediately after a contest were likely due to relatively short-term winner and loser effects, rather than any differences in the males' competitiveness, which would presumably be of longer duration.

Female preference for dominant males in the Mexican mojarra cichlid fish, Cichlasoma istlanum

It is widely assumed that female preference and male competition operate simultaneously during sexual selection. Dominance is likely an honest indicator of male quality, and females can identify and choose the dominant male to reproduce with individuals with greater competitive abilities, thus improving the quality and competitiveness of their offspring. In this context, few studies have investigated female preference in relation to male fighting ability. The Mexican mojarra, Cichlasoma istlanum, is a cichlid species native to the Balsas River basin. It is territorial during reproduction and provides parental care. Males commonly engage in territorial defence, whereas females care directly for offspring. This study examined whether females prefer dominant males that exhibit more aggressive behaviour. The authors conducted experiments using groups of two males and one female to test competitive ability in males and female preference. They also quantified the time during which the female associated with the dominant male and the subordinate male after observing the outcome of a fight between the two males. They found that Mexican mojarra females preferred dominant males and that the time females spent associating with males was positively related with their aggressive behaviours during competition. These results indicate that dominant males were more attractive than subordinate males to female Mexican mojarra. The relationship between female preference and male dominance in the Mexican mojarra demonstrates the importance of male competitive ability for future parental care in reproduction.

Comparative transcriptome analysis on four types of gonadal tissues of blotched snakehead (Channa maculata)

Blotched snakehead (Channa maculata) is an economically important freshwater fish in China, of which males grow much faster than females. To illuminate the molecular mechanism of sex differentiation and gonad development, RNA-Sequencing was performed to identify sex-related genes and pathway in gonads of 6-month-old normal XX females (XX-F), normal XY males (XY-M), XY sex reversal females (XY-F) and YY super-males (YY-M). The analysis showed that many differentially expressed genes (DEGs) had similar expression patterns in XY-F and XX-F, which were different from XY-M and YY-M. qRT-PCR indicated that Amh, Dmrt1, and Sox9 had relatively high expression in testes of XY-M and YY-M. Taking Amh as an example, there was a relative fold change of 1.0 in XX-F, 2.1 fold change in XY-F, 36.1 fold change in XY-M, and 26.0 fold change in YY-M. Cyp19a1a, Figla, and Foxl2 were highly expressive in ovaries of XX-F and XY-F. Taking Figla as an example, there was a relative fold change of 557 in XX-F, 304.5 fold change in XY-F, 5.6 fold change in XY-M, and 4.4 fold change in YY-M. KEGG analysis revealed many DEGs distributed in pathways related to sex differentiation, steroid hormone synthesis and growth, etc. Significant variation and trends in relative expression levels tested by qRT-PCR were consistent with those recorded by RNA-Sequencing. This is the first time that transcriptome of snakehead has been investigated systematically and in an integrated way. Large quantities of candidate genes involved in sex differentiation, gonad development and growth dimorphism were identified. The study provides useful resources for understanding sex differentiation and growth dimorphism, potentially assisting mono-sex production of snakehead in aquaculture.

Reproductive dynamics shapes genomotype composition in an allopolyploid complex

Hybrid complexes are composed of organisms with multiple combinations of parental genomes (genomotypes) that interconnect through nets of crosses. Although several such complexes are well established without speciation or extinction, mechanisms shaping their dynamics remain poorly understood. In this study, we quantified the reproductive success of the allopolyploid Iberian fish Squalius alburnoides in experimental free-access and directional crosses involving the most common genomotypes. Specifically, we analysed the paternity of the offspring produced when females had free access to male genomotypes and quantified variations in egg allocation, fertilization rate, and offspring survival among crosses involving each male genomotype. The composition of the offspring produced from free-access crosses varied significantly from that expected from random mating, suggesting that offspring production and viability are not independent of parental male genomotype. Moreover, directional crosses producing the genomotype most commonly found in wild populations appeared to be the most successful, with females laying more eggs, and fertilization rate and offspring survival being the highest. These results suggest that reproductive dynamics plays a relevant role in structuring the genomotype composition of populations and opens a path to future research on the ecology and evolutionary biology of allopolyploids and their multiplicity of possible evolutionary pathways.

Sexual Selection on male cuticular hydrocarbons via male-male competition and female choice

Traditional views of sexual selection assumed that male–male competition and female mate choice work in harmony, selecting upon the same traits in the same direction. However, we now know that this is not always the case and that these two mechanisms often impose conflicting selection on male sexual traits. Cuticular hydrocarbons (CHCs) have been shown to be linked to both social dominance and male attractiveness in several insect species. However, although several studies have estimated the strength and form of sexual selection imposed on male CHCs by female mate choice, none have established whether these chemical traits are also subject to sexual selection via male–male competition. Using a multivariate selection analysis, we estimate and compare sexual selection exerted by male–male competition and female mate choice on male CHC composition in the broad‐horned flour beetle Gnatocerus cornutus. We show that male–male competition exerts strong linear selection on both overall CHC abundance and body size in males, while female mate choice exerts a mixture of linear and nonlinear selection, targeting not just the overall amount of CHCs expressed but the relative abundance of specific hydrocarbons as well. We discuss the potential implications of this antagonistic selection with regard to male reproductive success.

Cognitive ability is heritable and predicts the success of an alternative mating tactic

The ability to attract mates, acquire resources for reproduction, and successfully outcompete rivals for fertilizations may make demands on cognitive traits--the mechanisms by which an animal acquires, processes, stores and acts upon information from its environment. Consequently, cognitive traits potentially undergo sexual selection in some mating systems. We investigated the role of cognitive traits on the reproductive performance of male rose bitterling (Rhodeus ocellatus), a freshwater fish with a complex mating system and alternative mating tactics. We quantified the learning accuracy of males and females in a spatial learning task and scored them for learning accuracy. Males were subsequently allowed to play the roles of a guarder and a sneaker in competitive mating trials, with reproductive success measured using paternity analysis. We detected a significant interaction between male mating role and learning accuracy on reproductive success, with the best-performing males in maze trials showing greater reproductive success in a sneaker role than as a guarder. Using a cross-classified breeding design, learning accuracy was demonstrated to be heritable, with significant additive maternal and paternal effects. Our results imply that male cognitive traits may undergo intra-sexual selection.

The effect of size and sex ratio experiences on reproductive competition in Nicrophorus vespilloides burying beetles in the wild

Male parents face a choice: should they invest more in caring for offspring or in attempting to mate with other females? The most profitable course depends on the intensity of competition for mates, which is likely to vary with the population sex ratio. However, the balance of pay‐offs may vary among individual males depending on their competitive prowess or attractiveness. We tested the prediction that sex ratio and size of the resource holding male provide cues regarding the level of mating competition prior to breeding and therefore influence the duration of a male's biparental caring in association with a female. Male burying beetles, Nicrophorus vespilloides were reared, post‐eclosion, in groups that differed in sex ratio. Experimental males were subsequently translocated to the wild, provided with a breeding resource (carcass) and filmed. We found no evidence that sex ratio cues prior to breeding affected future parental care behaviour but males that experienced male‐biased sex ratios took longer to attract wild mating partners. Smaller males attracted a higher proportion of females than did larger males, securing significantly more monogamous breeding associations as a result. Smaller males thus avoided competitive male–male encounters more often than larger males. This has potential benefits for their female partners who avoid both intrasexual competition and direct costs of higher mating frequency associated with competing males.

Social pairing of Seychelles warblers under reduced constraints: MHC, neutral heterozygosity, and age

The prevalence and significance of precopulatory mate choice remains keenly debated. The major histocompatibility complex (MHC) plays a key role in vertebrate adaptive immunity, and variation at the MHC influences individual survival. Although MHC-dependent mate choice has been documented in certain species, many other studies find no such pattern. This may be, at least in part, because in natural systems constraints may reduce the choices available to individuals and prevent full expression of underlying preferences. We used translocations to previously unoccupied islands to experimentally reduce constraints on female social mate choice in the Seychelles warbler (Acrocephalus sechellensis), a species in which patterns of MHC-dependent extrapair paternity (EPP), but not social mate choice, have been observed. We find no evidence of MHC-dependent social mate choice in the new populations. Instead, we find that older males and males with more microsatellite heterozygosity are more likely to have successfully paired. Our data cannot resolve whether these patterns in pairing were due to male-male competition or female choice. However, our research does suggest that female Seychelles warblers do not choose social mates using MHC class I to increase fitness. It may also indicate that the MHC-dependent EPP observed in the source population is probably due to mechanisms other than female precopulatory mate choice based on MHC cues.

Complete mitochondrial genome of Rhodeus ocellatus (Cypriniformes: Cyprinidae)

The complete mitogenome sequence of Rhodeus ocellatus (Kner) was determined using the next-generation sequencing (NGS). The genome was 16 761 bp in length and contained 13 protein-coding genes, two ribosomal RNA genes, 22 transfer RNA genes and a control region. The overall nucleotide composition was 30.43% A, 27.50% T, 25.94% C, and 16.13% G, with an A + T bias of 57.93%. The gene composition and the arrangement of the R. ocellatus mitochondrial genome were similar to that of most other vertebrates. The complete mitochondrial genome sequence will help to study the evolutionary relationships and population genetics of Rhodeus fish.

Differentiation of visual spectra and nuptial colorations of two Paratanakia himantegus subspecies (Cyprinoidea: Acheilognathidae) in response to the distinct photic conditions of their habitats

BACKGROUND

Vision, an important sensory modality of many animals, exhibits plasticity in that it adapts to environmental conditions to maintain its sensory efficiency. Nuptial coloration is used to attract mates and hence should be tightly coupled to vision. In Taiwan, two closely related bitterlings (Paratanakia himantegus himantegus and Paratanakia himantegus chii) with different male nuptial colorations reside in different habitats. We compared the visual spectral sensitivities of these subspecies with the ambient light spectra of their habitats to determine whether their visual abilities correspond with photic parameters and correlate with nuptial colorations.

RESULTS

Theelectroretinogram (ERG) results revealed that the relative spectral sensitivity of P.h. himantegus was higher at 670 nm, but lower at 370 nm, than the sensitivity of P. h. chii. Both bitterlings could perceive and reflect UV light, but the UV reflection patterns differed between genders. Furthermore, the relative irradiance intensity of the light spectra in the habitat of P. h. himantegus was higher at long wavelengths (480-700 nm), but lower at short wavelengths (350-450 nm), than the light spectra in the habitats of P. h.chii.

CONCLUSIONS

Two phylogenetically closely related bitterlings, P. h. himantegus and P. h. chii, dwell in different waters and exhibit different nuptial colorations and spectral sensitivities, which may be the results of speciation by sensory drive. Sensory ability and signal diversity accommodating photic environment may promote diversity of bitterling fishes. UV light was demonstrated to be a possible component of bitterling visual communication. The UV cue may assist bitterlings in genderidentification.

Asymmetric dominance and asymmetric mate choice oppose premating isolation after allopatric divergence

Assortative mating promotes reproductive isolation and allows allopatric speciation processes to continue in secondary contact. As mating patterns are determined by mate preferences and intrasexual competition, we investigated male–male competition and behavioral isolation in simulated secondary contact among allopatric populations. Three allopatric color morphs of the cichlid fish Tropheus were tested against each other. Dyadic male–male contests revealed dominance of red males over bluish and yellow-blotch males. Reproductive isolation in the presence of male–male competition was assessed from genetic parentage in experimental ponds and was highly asymmetric among pairs of color morphs. Red females mated only with red males, whereas the other females performed variable degrees of heteromorphic mating. Discrepancies between mating patterns in ponds and female preferences in a competition-free, two-way choice paradigm suggested that the dominance of red males interfered with positive assortative mating of females of the subordinate morphs and provoked asymmetric hybridization. Between the nonred morphs, a significant excess of negative assortative mating by yellow-blotch females with bluish males did not coincide with asymmetric dominance among males. Hence, both negative assortative mating preferences and interference of male–male competition with positive assortative preferences forestall premating isolation, the latter especially in environments unsupportive of competition-driven spatial segregation.

Genetic basis and biotechnological manipulation of sexual dimorphism and sex determination in fish

Aquaculture has made an enormous contribution to the world food production, especially to the sustainable supply of animal proteins. The utility of diverse reproduction strategies in fish, such as the exploiting use of unisexual gynogenesis, has created a typical case of fish genetic breeding. A number of fish species show substantial sexual dimorphism that is closely linked to multiple economic traits including growth rate and body size, and the efficient development of sex-linked genetic markers and sex control biotechnologies has provided significant approaches to increase the production and value for commercial purposes. Along with the rapid development of genomics and molecular genetic techniques, the genetic basis of sexual dimorphism has been gradually deciphered, and great progress has been made in the mechanisms of fish sex determination and identification of sex-determining genes. This review summarizes the progress to provide some directive and objective thinking for further research in this field.

Phylogenetic relationships of Acheilognathidae (Cypriniformes: Cyprinoidea) as revealed from evidence of both nuclear and mitochondrial gene sequence variation: evidence for necessary taxonomic revision in the family and the identification of cryptic species

Bitterlings are relatively small cypriniform species and extremely interesting evolutionarily due to their unusual reproductive behaviors and their coevolutionary relationships with freshwater mussels. As a group, they have attracted a great deal of attention in biological studies. Understanding the origin and evolution of their mating system demands a well-corroborated hypothesis of their evolutionary relationships. In this study, we provide the most comprehensive phylogenetic reconstruction of species relationships of the group based on partitioned maximum likelihood and Bayesian methods using DNA sequence variation of nuclear and mitochondrial genes on 41 species, several subspecies and three undescribed species. Our findings support the monophyly of the Acheilognathidae. Two of the three currently recognized genera are not monophyletic and the family can be subdivided into six clades. These clades are further regarded as genera based on both their phylogenetic relationships and a reappraisal of morphological characters. We present a revised classification for the Acheilognathidae with five genera/lineages: Rhodeus, Acheilognathus (new constitution), Tanakia (new constitution), Paratanakia gen. nov., and Pseudorhodeus gen. nov. and an unnamed clade containing five species currently referred to as "Acheilognathus". Gene trees of several bitterling species indicate that the taxa are not monophyletic. This result highlights a potentially dramatic underestimation of species diversity in this family. Using our new phylogenetic framework, we discuss the evolution of the Acheilognathidae relative to classification, taxonomy and biogeography.

Reproductive behavior of the male rose bitterling Rhodeus ocellatus as influenced by the operational sex ratio

Background

Game theory predicts that as the operational sex ratio (OSR; the ratio of sexually active males to fertilizable females) increases, reproductive males may adopt alternative reproductive behaviors. Empirical studies indicated that alternative behaviors may differ for different species. In the present study, we investigated the influence of different male-biased OSRs on male reproductive behaviors shown by a freshwater fish, the rose bitterling Rhodeus ocellatus.

Results

Six levels of the OSR (1:1, 2:1, 3:1, 5:1, 7:1, and 9:1) were established, and male leading, aggression, and preoviposition ejaculation behaviors were recorded. Our experiments showed that at low OSRs, the leading behavior of male rose bitterling increased with an increasing OSR, reaching a maximum at an OSR of 3:1, and then decreased as the OSR further increased. In contrast, aggression and preoviposition ejaculation behavior continually increased with an increasing OSR. Notably, males possibly lost their territorial position to other males and became sneakers when facing more rivals.

Conclusions

These observations suggest that at lower OSRs, male rose bitterling mainly rely on leading behavior, while at higher OSRs, they increasingly adopt aggression and preoviposition ejaculation. Therefore, for the rose bitterling, (1) decreased leading behavior and (2) increased aggression and preoviposition ejaculation are two alternative responses in the face of an increasing OSR, which is a more straightforward strategy for facing an increased risk of sperm competition than that adopted by its congener, the European bitterling.

A sperm competition model for the European bitterling (Rhodeus amarus)

Sperm competition occurs when the spermatozoa of one male coincide with those of another to fertilise the same eggs. In some taxa males perform multiple ejaculations, which may function in sperm competition or in maintaining a baseline density of spermatozoa in the female reproductive tract to ensure fertilisation, a process that has been termed ‘topping up’. We investigated multiple ejaculations in the European bitterling (Rhodeus amarus), a freshwater fish that oviposits in freshwater mussels. We quantified spermatozoa in the mussel mantle cavity following ejaculation, and measured sperm motility parameters of males adopting different mating tactics. Following ejaculation spermatozoa density in the mussel increased linearly, peaked after 30 s, and then declined exponentially. Spermatozoa motility parameters did not differ between male mating tactics. We parameterised a model of sperm competition forR. amarus, which accurately predicted male fertilisation probability. We discuss these results in the context of multiple ejaculations and male mating tactics.

Exposure to an environmental estrogen breaks down sexual isolation between native and invasive species

Environmental change can increase the likelihood of interspecific hybridization by altering properties of mate recognition and discrimination between sympatric congeners. We examined how exposure to an environmentally widespread endocrine-disrupting chemical (EDC), bisphenol A (BPA), affected visual communication signals and behavioral isolation between an introduced freshwater fish and a native congener (genus: Cyprinella). Exposure to BPA induced changes in the expression of male secondary traits as well as male and female mate choice, leading to an overall reduction in prezygotic isolation between congeners. Changes in female mate discrimination were not tightly linked to changes in male phenotypic traits, suggesting that EDC exposure may alter female choice thresholds independently of the effects of exposure on males. These findings indicate that environmental exposure to EDCs can lead to population declines via the erosion of species boundaries and by promoting the establishment and spread of non-native species via hybridization.

Sexual conflict inGerris gillettei(Insecta: Hemiptera): influence of effective mating rate and morphology on reproductive success

In water striders, the interests of both sexes diverge over the decision to mate, leading to precopulatory sexual conflict. The influence of mating rate and key persistence and resistance traits on reproductive success has seldom been investigated in the context of multiple matings. We used amplified fragment length polymorphism (AFLP) based genetic parentage analyses to estimate mating and reproductive success in Gerris gillettei Lethierry and Severin, 1896, while allowing for free multiple matings. We tested the hypotheses that males should display stronger opportunity for sexual selection and steeper Bateman gradients. In each sex, persistence and resistance traits should also impact mating and reproductive success. Surprisingly, males and females had similarly high and variable effective mating rates (i.e., number of genetic partners), and both sexes produce more offspring when mating with more partners. As predicted, exaggerated persistence traits allowed males to mate with more partners and sire more offspring. However, we found no evidence for an impact of resistance traits for females. The mating environment may have favoured low resistance in females, but high promiscuity can be beneficial for females. This first description of the genetic mating system for a water strider species suggests that the determinants of fitness can be further deciphered using the sexual selection framework.

Female rose bitterling prefer MHC-dissimilar males: experimental evidence

The role of genetic benefits in female mate choice remains a controversial aspect of sexual selection theory. In contrast to "good allele" models of sexual selection, "compatible allele" models of mate choice predict that females prefer mates with alleles complementary to their own rather than conferring additive effects. While correlative results suggest complementary genetic effects to be plausible, direct experimental evidence is scarce. A previous study on the Chinese rose bitterling (Rhodeus ocellatus) demonstrated a positive correlation between female mate choice, offspring growth and survival, and the functional dissimilarity between the Major Histocompatibility Complex (MHC) alleles of males and females. Here we directly tested whether females used cues associated with MHC genes to select genetically compatible males in an experimental framework. By sequentially pairing females with MHC similar and dissimilar males, based on a priori known MHC profiles, we showed that females discriminated between similar and dissimilar males and deposited significantly more eggs with MHC dissimilar males. Notably, the degree of dissimilarity was an important factor for female decision to mate, possibly indicating a potential threshold value of dissimilarity for decision making, or of an indirect effect of the MHC.

Concordant female mate preferences in the cichlid fish Tropheus moorii

Discriminating female mate preferences enhance the variance in reproductive success among males of a population and create a potential for sexual selection, which can account for trait evolution and diversification. Fish color patterns are among the prime targets of mate choice-driven sexual selection. Populations of the cichlid Tropheus from Lake Tanganyika display remarkable geographic color pattern variation, but the role of female choice in their rapid and rich phenotypic diversification is unclear. Males and females establish a pair bond prior to spawning monogamously, but as brood care is strictly maternal, female investment in reproduction is high and the operational sex ratio is male-biased. Therefore, variance in male reproductive success can accrue if individual males succeed repeatedly in securing a mate. To test this prediction in the red colored Tropheus moorii "Chimba", four pairs of males were presented to a series of females and female mate preferences were inferred from pairwise interactions. There was a significant difference in mating success between the males of each pair (P < 0.001 over all trials), as-with one exception-females shared preferences for the same males. Male courtship activity was strongly correlated with female choice. Our experiment suggests that female choice contributes to the variance in male reproductive success in the tested population.

Effects of parental number and duration of the breeding period on the effective population size and genetic diversity of a captive population of the endangered Tokyo bitterling Tanakia tanago (Teleostei: Cyprinidae)

The maintenance of genetic diversity is one of the chief concerns in the captive breeding of endangered species. Using microsatellite and mtDNA markers, we examined the effects of two key variables (parental number and duration of breeding period) on effective population size (N(e) ) and genetic diversity of offspring in an experimental breeding program for the endangered Tokyo bitterling, Tanakia tanago. Average heterozygosity and number of alleles of offspring estimated from microsatellite data increased with parental number in a breeding aquarium, and exhibited higher values for a long breeding period treatment (9 weeks) compared with a short breeding period (3 weeks). Haplotype diversity in mtDNA of offspring decreased with the reduction in parental number, and this tendency was greater for the short breeding period treatment. Genetic estimates of N(e) obtained with two single-sample estimation methods were consistently higher for the long breeding period treatment with the same number of parental fish. Average N(e) /N ratios were ranged from 0.5 to 1.4, and were high especially in the long breeding period with small and medium parental number treatments. Our results suggest that the spawning intervals of females and alternative mating behaviors of males influence the effective size and genetic diversity of offspring in bitterling. To maintain the genetic diversity of captive T. tanago, we recommend that captive breeding programs should be conducted for a sufficiently long period with an optimal level of parental density, as well as using an adequate number of parents.

Intrasexual competition facilitates the evolution of alternative mating strategies in a colour polymorphic fish

Background

Intense competition for access to females can lead to males exploiting different components of sexual selection, and result in the evolution of alternative mating strategies (AMSs). Males of Poecilia parae, a colour polymorphic fish, exhibit five distinct phenotypes: drab-coloured (immaculata), striped (parae), structural-coloured (blue) and carotenoid-based red and yellow morphs. Previous work indicates that immaculata males employ a sneaker strategy, whereas the red and yellow morphs exploit female preferences for carotenoid-based colours. Mating strategies favouring the maintenance of the other morphs remain to be determined. Here, we report the role of agonistic male-male interactions in influencing female mating preferences and male mating success, and in facilitating the evolution of AMSs.

Results

Our study reveals variation in aggressiveness among P. parae morphs during indirect and direct interactions with sexually receptive females. Two morphs, parae and yellow, use aggression to enhance their mating success (i.e., number of copulations) by 1) directly monopolizing access to females, and 2) modifying female preferences after winning agonistic encounters. Conversely, we found that the success of the drab-coloured immaculata morph, which specializes in a sneak copulation strategy, relies in its ability to circumvent both male aggression and female choice when facing all but yellow males.

Conclusions

Strong directional selection is expected to deplete genetic variation, yet many species show striking genetically-based polymorphisms. Most studies evoke frequency dependent selection to explain the persistence of such variation. Consistent with a growing body of evidence, our findings suggest that a complex form of balancing selection may alternatively explain the evolution and maintenance of AMSs in a colour polymorphic fish. In particular, this study demonstrates that intrasexual competition results in phenotypically distinct males exhibiting clear differences in their levels of aggression to exclude potential sexual rivals. By being dominant, the more aggressive males are able to circumvent female mating preferences for attractive males, whereas another male type incorporates subordinate behaviours that allow them to circumvent male aggression and female mating preferences. Together, these and previous results indicate that exploiting different aspects of social interactions may allow males to evolve distinct mating strategies and thus the long term maintenance of polymorphisms within populations.

Effects of different mating scenarios on embryo viability in brown trout

Mating with attractive or dominant males is often predicted to offer indirect genetic benefits to females, but it is still largely unclear how important such non-random mating can be with regard to embryo viability. We sampled a natural population of adult migratory brown trout (Salmo trutta), bred them in vitro in a half-sib breeding design to separate genetic from maternal environmental effects, raised 2098 embryos singly until hatching, and exposed them experimentally to different levels of pathogen stress at a late embryonic stage. We found that the embryos' tolerance to the induced pathogen stress was linked to the major histocompatibility complex (MHC) of their parents, i.e. certain MHC genotypes appeared to provide better protection against infection than others. We also found significant additive genetic variance for stress tolerance. Melanin-based dark skin patterns revealed males with 'good genes', i.e. embryos fathered by dark coloured males had a high tolerance to infection. Mating with large and dominant males would, however, not improve embryo viability when compared to random mating. We used simulations to provide estimates of how mate choice based on MHC or melanin-based skin patterns would influence embryos' tolerance to the experimentally induced pathogen stress.

Sexual conflict inhibits female mate choice for major histocompatibility complex dissimilarity in Chinook salmon

In many species females prefer major histocompatibility complex (MHC) dissimilar mates, which may improve offspring resistance to pathogens. However, sexual conflict may interfere with female preference when males attempt to mate with all females, regardless of compatibility. Here we used semi-natural spawning channels to examine how mating behaviour and genetic similarity at the MHC class II peptide binding region affected parentage patterns in Chinook salmon (Oncorhynchus tshawytscha). We found that females directed aggression at more MHC-similar males than expected by chance, providing a possible mechanism of female MHC choice in salmon. Males also directed aggression towards MHC-similar females, which was consistent with males harassing unreceptive mates. Males' aggression was positively correlated with their reproductive success, and it appeared to overcome female aversion to mating with MHC-similar males, as females who were the target of high levels of male aggression had lower than expected MHC divergence in their offspring. Indeed, offspring MHC divergence was highest when the sex ratio was female-biased and male harassment was likely to be less intense. These data suggest that male harassment can reduce female effectiveness in selecting MHC-compatible mates, and sexual conflict can thus have an indirect cost to females.

Ovipositor ultrastructure of the striped bitterling Acheilognathus yamatsutae (Teleostei: Acheilognathinae) during spawning season

The ovipositor of striped bitterling Acheilognathus yamatsutae was subjected to ultrastructure and histochemical analysis during spawning season using light and electron microscopy. Although the ovipositor of A. yamatsutae is a long cylindrical tube with smooth external surface, it was possible to confirm the presence of well-developed fingerprint structure using scanning electron microscopy. Internal aspect analysis of ovipositor revealed formation of 5-8 longitudinal folds. Cross section analysis revealed that the ovipositor is composed of an outer epithelial layer, a mid connective tissue layer, and an inner epithelial layer. The outer epithelial layer contains 7-9 cell layers composed mainly of epithelial and mucous cells. Result of AB-PAS (pH 2.5) and AF-AB reaction showed that mucous cells contained mainly acidic carboxylated mucosubstances. The connective tissue layer was loose and made mainly of collagen fibers and some muscle fibers, along with blood vessels and a small number of chromatophores. The inner epithelial layer, which is a single layer, is composed of columnar epithelia. Observation under transmission electron microscope enabled distinction of the outer epithelial layer into superficial, intermediate and basal layers. Although the types of cells in the superficial tissue layer were diverse, they all shared the development of glycocalyx covered microridges. The majority of epithelial cells in the intermediate layer were cuboidal shaped, while those in the basal layer were columnar. Two types (A and B) of secretory cells were observed in the outer epithelial layer. The connective tissue layer had two types of chromatophores including xantophore and melanophore, in addition to a well-developed nerve fiber bundles. Columnar epithelial cells, mitochondria-rich cells and rodlet cells were observed in the inner epithelial layer. Microvilli were well developed on the free surface of columnar epithelial cells.