Control of social status by sex steroids: insights from teleost fishes

Many animals live in highly social environments, in which individuals must behave in a way that enables them to survive and live harmoniously among conspecifics. Dominance hierarchies are typical among social species and are essential for determining and preserving stability within social groups. Although there is considerable evidence that sex steroid hormones regulate behaviors associated with dominance, such as aggression and mating, fewer studies have examined the role of these hormones in controlling social status, especially in species that exhibit social hierarchies. Furthermore, despite this research, we know remarkably little about the precise neural and molecular mechanisms through which sex steroids modulate traits associated with social rank. Here, we review the neuroendocrine regulation of social status by sex steroids in teleost fishes, the largest and most diverse vertebrate group that shows extensive variation in reproductive systems and social structures between species. First, we describe the function of sex steroids and novel steroid-related genes that teleost fishes possess due to a lineage-specific whole-genome duplication event. Then, we discuss correlational, pharmacological, and molecular genetic studies on the control of social status by sex steroids in teleost fishes, including recent studies that have implemented gene editing technologies, such as CRISPR/Cas9. Finally, we argue that gene editing approaches in teleost studies, within both integrative and comparative frameworks, will be vital for elucidating the role of sex steroids in controlling social rank and characterizing their neural and molecular mechanisms of action. Collectively, ongoing and future research in these species will provide novel insight into the evolution of the regulation of social status by sex steroids and other neuroendocrine substrates across vertebrates.

Physiological and condition-related traits in the gynogenetic-sexual Carassius auratus complex: different investments promoting the coexistence of two reproductive forms?

BACKGROUND

Carassius auratus complex is an extraordinary species complex including the diploid and polyploid forms exhibiting asexual and sexual reproduction modes. The coexistence of both forms in the same habitats is currently reported. The stable coexistence of asexual and sexual forms assumes some disadvantages for asexuals that balance the costs of sex. In our study, we hypothesized and tested the differences in physiological (including heamatological and immunological), growth-related, condition-related, and fitness-related traits between gynogenetic females and sexuals.

RESULTS

Our results revealed similar growth performance in gynogenetic females and sexuals measured by body size and weight, or expressed by condition factor. The energy allocation in reproduction measured by the relative size of gonads revealed no difference between gynogenetic and sexual females; in addition, both females in spawning expressed the same estradiol levels in blood plasma. We found a gender specific trade-off between investment in reproduction and immunocompetence (measured by the spleen-somatic index). Higher aerobic performance expressed by the heart index and higher oxygen-carrying capacity were found in sexual males, with increasing values before and during spawning. Our study evidenced significantly lower aerobic performance but higher oxygen-carrying capacity per erythrocyte in gynogenetic females when compared to sexuals. IgM production differed between gynogens and sexuals of C. auratus complex.

CONCLUSIONS

Our study indicates that a similar amount of energy is invested by both gynogenetic and sexual females of C. auratus complex in reproductive behaviour. We suggest that lower aerobic performance in gynogens may represent their physiological disadvantage balancing the cost of sexual reproduction. A trade-off between the number of erythrocytes and the oxygen-carrying capacity per erythrocyte in sexual males and gynogenetic females may contribute to the coexistence of gynogenetic and sexual forms. In addition, the differences in specific immunity between gynogens and sexuals may also reduce the evolutionary disadvantage of sexual reproduction. In conclusion, we propose that several mechanisms contribute to the coexistence of the gynogenetic-sexual C. auratus complex.

Public information influences sperm transfer to females in sailfin molly males

In animals, including humans, the social environment can serve as a public information network in which individuals can gather public information about the quality of potential mates by observing conspecifics during sexual interactions. The observing individual itself is also a part of this information network. When recognized by the observed conspecifics as an audience, his/her presence could influence the sexual interaction between those individuals, because the observer might be considered as a potential mate or competitor. One of the most challenging questions in sexual selection to date is how the use of public information in the context of mate choice is linked to the fitness of individuals. Here, we could show that public information influences mate-choice behaviour in sailfin molly males, Poecilia latipinna, and influences the amount of sperm males transfer to a female partner. In the presence of an audience male, males spent less time with the previously preferred, larger of two females and significantly more time with the previously non-preferred, smaller female. When males could physically interact with a female and were faced with an audience male, three audience females or no audience, males transferred significantly more sperm to a female partner in the presence of an audience male than with female audience or no audience and spent less time courting his female partner. This is the first study showing that public information use turns into fitness investment, which is the crucial factor to understand the role of public information in the dynamic processes in sexual selection.

The role of androgens in species recognition and sperm production in Atlantic mollies (Poecilia mexicana)

Much is known about the role of hormones in the regulation of vertebrate mating behavior, including receptivity, and several components of mate choice. Hormones may modulate reproductive behavior in such a way to increase or decrease the individual's motivation, and therefore hormones may be important in mediating behavior associated with reproductive isolation. The mating complex of the all female gynogenetic Amazon mollies, Poecilia formosa, and their parental species (sailfin mollies, P. latipinna, and Atlantic mollies, P. mexicana) is a model system for studying ultimate mechanisms of species recognition. However, proximate mechanisms, such as variation in hormone levels, and the effect of hormones on sperm production have not been extensively examined. We predict that one or more of the sex steroid hormones in teleost fish (11-ketotestosterone (KT), testosterone (T), and estradiol (E)) will play a role in species recognition (during mate choice and/or sperm priming) for Atlantic mollies (the maternal parental species) that are sympatric with Amazon mollies. We sequentially paired male Atlantic mollies with female conspecifics and Amazon mollies and obtained water-borne hormone samples before and after mating for all fish. We measured circulating KT, T, and E from the water samples. Although we did not find an overall KT response to mating with conspecifics as has been found previously in sailfin mollies, male Atlantic mollies that mated more with conspecific females had lower postmating T levels. Additionally, males attempted to mate more with conspecific females that had lower postmating E levels, but attempted to mate more with Amazon mollies that had higher postmating KT levels. We also examined the effect of KT on sperm priming (a mechanism of premating mate choice), and found that KT levels of male Atlantic mollies prior to mating are correlated with the sperm priming response when males were paired with conspecific females, but this correlation was not found when males were paired with Amazon mollies. Our results indicate that male mating behavior is affecting or responding to both male and female hormones, but that the hormones alone are not playing a role in species recognition. Male Atlantic mollies may not discriminate against Amazon mollies as strongly as male sailfin mollies because Amazon mollies resemble their maternal parental species more than their paternal species.

Primary and secondary sexual characters in alternative reproductive tactics of Chinook salmon: Associations with androgens and the maturation-inducing steroid

The proximate mechanisms that underlie the evolution of within-sex variation in mating behavior, sexual characters and reproductive investment patterns are still poorly understood. Species exhibiting alternative reproductive tactics (ARTs) are ideal model systems to examine these mechanisms. Chinook salmon (Oncorhynchus tshawytscha) exhibits two distinct ARTs: hooknoses, which are large males that establish spawning dominance hierarchies via intense male-male competition and jacks, which are smaller precocious sneaking males that steal fertilizations via sperm competition. In this study, we examine plasma testosterone (T), 11-ketotestosterone (11-KT) and maturation-inducing steroid (MIS; 17α,20β-dihydroxy-4-pregnen-3-one) profiles of spawning hooknoses and jacks. Furthermore, we examine relationships between androgens and primary (gonad mass, gonadosomatic index and sperm traits) and secondary (total mass, body size, hump depth and kype length) sexual characters. Relationships between MIS and sperm traits are also examined. We found that hooknoses and jacks did not significantly differ in terms of plasma T, 11-KT or MIS concentrations. Moreover, we found significant positive relationships between levels of both androgens within each ART. There were no significant relationships between androgens, MIS and sperm traits. T and 11-KT concentrations co-varied positively with gonad investment and kype length in jacks. In hooknoses, 11-KT concentration was positively related to total mass, hump depth and condition factor. Overall, these findings suggest that there are differential androgen effects for each of the ARTs in Chinook salmon.

The role of the maturation-inducing steroid, 17,20beta-dihydroxypregn-4-en-3-one, in male fishes: a review

The major progestin in teleosts is not progesterone, as in tetrapods, but 17,20beta-dihydroxypregn-4-en-3-one (17,20beta-P) or, in certain species, 17,20beta,21-trihydroxy-pregn-4-en-3-one (17,20beta,21-P). Several functions for 17,20beta-P and 17,20beta,21-P have been proposed (and in some cases proved). These include induction of oocyte final maturation and spermiation (milt production), enhancement of sperm motility (by alteration of the pH and fluidity of the seminal fluid) and acting as a pheromone in male cyprinids. Another important function, initiation of meiosis (the first step in both spermatogenesis and oogenesis), has only very recently been proposed. This is a process that takes place at puberty in all fishes and once a year in repeat spawners. The present review critically examines the evidence to support the proposed functions of 17,20beta-P in males, including listing of the evidence for the presence of 17,20beta-P in the blood plasma of male fishes and discussion of why, in many species, it appears to be absent (or present at low and, in some cases, unvarying concentrations); consideration of the evidence, obtained mainly from in vitro studies, for this steroid being predominantly produced by the testis, for its production being under the control of luteinizing hormone (gonadotrophin II) and, at least in salmonids, for two cell types (Leydig cells and sperm cells) being involved in its synthesis; discussion of the factors involved in the regulation of the switch from androgen to 17,20beta-P production that seems to occur in many species just at the time of spermiation; discussion of the effects of in vivo injection and application of 17,20beta-P (and closely related compounds) in males; a listing of previously published evidence that supports the proposed new function of 17,20beta-P as an initiator of meiosis; finally, discussion of the evidence for environmental endocrine disruption by progestins in fishes.

Genetically Determined Variation in Stress Responsiveness in Rainbow Trout: Behavior and Neurobiology

It is becoming increasingly recognized that the diversity in stressors, their intensity, predictability and the context in which they are experienced, will result in behavioral and physiological responses just as diverse. In addition, stress responses are characterized by individual variations where the physiological and behavioral reactions are associated in such a manner that distinct stress coping styles encompassing suites of correlated traits can be identified. These are often referred to as proactive and reactive stress coping styles. Proactive coping is characterized by more aggression, higher general activity and higher sympathetic activation, whereas reactive coping is characterized by immobility, lack of initiative and a higher parasympathetic/hypothalamic activation. Stable coping styles appear to coexist within populations, and these strategies appear to be largely innate. Moreover, the physiological and behavioral traits of coping styles appear to be heritable. These stress coping styles have proven to play a major role in competitive ability and subsequent social position in different species of vertebrates. However, there are also studies showing that social position can affect parameters encompassing the stress coping style of individuals. In this regard it is important, but not always easy, to distinguish between causes and effects of behavioral and physiological responses to stressors. The question raised is to what extent and rigidness stress coping styles are guided by genetic factors.

Modulation of aggressive behaviour by fighting experience: mechanisms and contest outcomes

Experience in aggressive contests often affects behaviour during, and the outcome of, later contests. This review discusses evidence for, variations in, and consequences of such effects. Generally, prior winning experiences increase, and prior losing experiences decrease, the probability of winning in later contests, reflecting modifications of expected fighting ability. We examine differences in the methodologies used to study experience effects, and the relative importance and persistence of winning and losing experiences within and across taxa. We review the voluminous, but somewhat disconnected, literature on the neuroendocrine mechanisms that mediate experience effects. Most studies focus on only one of a number of possible mechanisms without providing a comprehensive view of how these mechanisms are integrated into overt behaviour. More carefully controlled work on the mechanisms underlying experience effects is needed before firm conclusions can be drawn. Behavioural changes during contests that relate to prior experience fall into two general categories. Losing experiences decrease willingness to engage in a contest while winning experiences increase willingness to escalate a contest. As expected from the sequential assessment model of contest behaviour, experiences become less important to outcomes of contests that escalate to physical fighting.A limited number of studies indicate that integration of multiple experiences can influence current contest behaviour. Details of multiple experience integration for any species are virtually unknown. We propose a simple additive model for this integration of multiple experiences into an individual's expected fighting ability. The model accounts for different magnitudes of experience effects and the possible decline in experience effects over time. Predicting contest outcomes based on prior experiences requires an algorithm that translates experience differences into contest outcomes. We propose two general types of model, one based solely on individual differences in integrated multiple experiences and the other based on the probability contests reach the escalated phase. The difference models include four algorithms reflecting possible decision rules that convert the perceived fighting abilities of two rivals into their probabilities of winning. The second type of algorithm focuses on how experience influences the probability that a subsequent contest will escalate and the fact that escalated contests may not be influenced by prior experience. Neither type of algorithm has been systematically investigated.Finally, we review models for the formation of dominance hierarchies that assume that prior experience influences contest outcome. Numerous models have reached varied conclusions depending on which factors examined in this review are included. We know relatively little about the importance of and variation in experience effects in nature and how they influence the dynamics of aggressive interactions in social groups and random assemblages of individuals. Researchers should be very active in this area in the next decade. The role of experience must be integrated with other influences on contest outcome, such as prior residency, to arrive at a more complete picture of variations in contest outcomes. We expect that this integrated view will be important in understanding other types of interactions between individuals, such as mating and predator-prey interactions, that also are affected significantly by prior experiences.

Discriminating males alter sperm production between species

Prezygotic reproductive isolation and its importance in speciation is traditionally approached from the viewpoint of those events that occur before mating. However, recent interest in sperm competition theory has shown that prezygotic isolation can be affected by mechanisms that occur after mating but before fertilization. One neglected aspect of these studies is how the cost of sperm production might play a role in species isolation. We examined differential sperm production in a species whose males are sexually parasitized by a unisexual gynogenetic species. Gynogens are clonal females that require sperm from males of closely related bisexual species to initiate embryogenesis. We tested for differential sperm production by male sailfin mollies (Poecilia latipinna) when they were in the presence of either a heterospecific, gynogenetic female (Poecilia formosa, Amazon molly) or a conspecific female. We found that previously demonstrated male mate choice for conspecific over heterospecific females also is revealed in sperm production. Males from both an allopatric and a sympatric population produce more sperm when in the presence of a conspecific female than when in the presence of a heterospecific female. We suggest that differential sperm production also could play a role in prezygotic reproductive isolation in bisexual species complexes that occur in sympatry.

Raffles, roles, and the outcome of sperm competition in sockeye salmon

In species with male alternative reproductive phenotypes, one phenotype is usually disadvantaged in mating competition. In salmonid fishes, large late-maturing males pair with nesting females and maintain close contact before and during spawning. Small early-maturing males have little contact with nesting females and, during spawning, begin to release sperm after the paired male. The effects of male phenotype and timing of ejaculation on success in sperm competition are not known. In this study, we determined paternity of offspring resulting from in vitro competitive fertilizations to examine these two aspects of sperm competition in sockeye salmon, Oncorhynchus nerka (Walbaum, 1792). When we fertilized eggs with mixtures of equal numbers of sperm from each of two male age classes, we found that success in sperm competition did not depend on male age. However, success in these competitive fertilizations did not conform to the fair raffle model of sperm competition, since paternity in most of the clutches was biased in favour of one male. When we added milt from two males sequentially to a batch of eggs, we found that sperm from the second male fertilized fewer eggs than sperm from the first male, but the difference was less than expected. In addition, a male's success when his milt was added first was not correlated with his success when his milt was added second.

Milt quality, parasites, and immune function in dominant and subordinate Arctic charr

Within a species, different males may display different mating strategies. For example, some males may be selected to invest in attractiveness and mate guarding, whereas others are selected for increased sperm production and sneaky breeding. In systems with a hierarchical structure, dominant males are expected to adopt mate-guarding behaviour and subordinate males sneaky-breeding behaviour. In this study, we kept wild-caught and sexually ripe male Arctic charr (Salvelinus alpinus) in size-matched pairs and determined social rank from the number of aggressive encounters. After 4 days, subordinate males showed symptoms of stress, with higher blood glucose and erythrocyte levels than dominant males. There were no differences between dominant and subordinate males in parasite intensities or immune activity, measured as levels of granulocytes and lymphocytes in blood. Although subordinate males had smaller testes than dominant males, they still had a higher density of sperm cells and higher sperm numbers relative to the size of their testes. These results can be explained as indicating adaptation of subordinate males for reproduction in an unfavourable role, always exposed to sperm competition and out of synchrony with females' egg release. Our results suggest that rapid changes in social rank may affect ejaculate production.

Isolation increases milt production in goldfish

Milt volume in goldfish is increased by female steroid and prostaglandin pheromones, by exposure to males with elevated gonadotropin levels, and by isolation from conspecifies. This study examined various aspects of the isolation effect on milt volume and serum gonadotropin II (GTH II). The latency of isolation-induced milt increase in this study (12-24 hr) was longer than the latencies to pheromone-induced milt increase in previous work (0.5-6.0 hr), was not affected by the time of day at which males were isolated, persisted for at least 72 hr in isolated males, and was terminated within 24 hr in males that were returned to groups. Isolated males maintained high milt production when separated from tank mates by a perforated barrier or when exposed to visual and odor cues from males in other tanks, suggesting that the unknown conspecific cues that maintain low milt production in groups operate at close range. Isolation appears to increase milt through a mechanism different from that mediating response to female pheromones because: (1) unlike female pheromones, which consistently increase serum GTH II, no isolation or regrouping treatment in this study affected male GTH II concentration; and (2) the effects of isolation and of exposure to the female pheromone 17 alpha, 20 beta-dihydroxy-4-pregnen-3-one (17,20 beta-P) are additive. Finally, males that were previously isolated or exposed to 17,20 beta-P increased milt (but not GTH II) in grouped males, suggesting that the effects of isolation and pheromone exposure can indirectly stimulate male conspecifics. Although the biological function of the isolation effect is not clear, we propose that it illustrates the effect of removal from inhibitory cues normally received from male conspecifies.

Social modulation of androgen levels in male teleost fish

Androgens are classically thought of as the sex steroids controlling male reproduction. However, in recent years evidence has accumulated showing that androgens can also be affected by the interactions between conspecifics, suggesting reciprocal interactions between androgens and behaviour. These results have been interpreted as an adaptation for individuals to adjust their agonistic motivation and to cope with changes in their social environment. Thus, male-male interactions would stimulate the production of androgens, and the levels of androgens would be a function of the stability of its social environment ['challenge hypothesis', Gen. Comp. Endocrinol. 56 (1984) 417]. Here the available data on social modulation of androgen levels in male teleosts are reviewed and some predictions of the challenge hypothesis are addressed using teleosts as a study model. We investigate the causal link between social status, territoriality and elevated androgen levels and the available evidence suggests that the social environment indeed modulates the endocrine axis of teleosts. The association between higher androgen levels and social rank emerges mainly in periods of social instability. As reported in the avian literature, in teleosts the trade-off between androgens and parental care is indicated by the fact that during the parental phase breeding males decreased their androgen levels. A comparison of androgen responsiveness between teleost species with different mating and parenting systems also reveals that parenting explains the variation observed in androgen responsiveness to a higher degree than the mating strategy. Finally, the adaptive value of social modulation of androgens and some of its evolutionary consequences are discussed.

Milt production in goldfish: regulation by multiple social stimuli

Previous studies in goldfish (Carassius auratus) demonstrate that milt (sperm and seminal fluid) volume is increased both by a preovulatory steroidal pheromone, 17 alpha,20 beta-dihydroxy-4-pregnen-3-one (17,20 beta-P), that acts by increasing serum concentrations of gonadotropin II (GTH-II), and by a post-ovulatory prostaglandin (PG) pheromone, that acts by stimulating sexual interactions. Here, we show that male goldfish also increase milt volume when isolated for 24 h, or when placed with another male injected with human chorionic gonadotropin (hCG) for 12 h. In contrast to the milt increase induced by pheromonal 17,20 beta-P, the milt increase following isolation or exposure to hCG-injected males was not associated with increased serum GTH-II. Serum growth hormone also was unaffected, although serum testosterone increased in isolated males. The absence of GTH-II increase following isolation or exposure to hCG-injected males and the long latency time for these two types of responses, suggests these effects are mediated by a novel mechanism as yet undescribed. The present findings suggest that sperm competition has selected for complex mechanisms regulating milt production and fertility in goldfish: males regulate milt production not only in response to stimulatory pheromonal cues from ovulatory females, but also in response to unknown stimulatory and inhibitory cues from male competitors.

Intermale competition in sexually mature arctic charr: effects on brain monoamines, endocrine stress responses, sex hormone levels, and behavior

Sexually mature Arctic charr (Salvelinus alpinus) males were allowed to interact in pairs for 4 days in the absence of females. Agonistic behavior was quantified, and at the end of the experiment, plasma levels of glucose, cortisol, testosterone (T), 11-ketotestosterone (11-KT), and 17alpha,20beta-dihydroxy-4-pregnen-3-one (17alpha, 20beta-P) were determined alongside brain concentrations of serotonin (5-HT), 5-hydroxyindoleacetic acid (5-HIAA, the major 5-HT metabolite), dopamine (DA), and 3,4-dihydroxyphenylacetic acid (DOPAC, a major DA metabolite). Plasma cortisol and glucose were significantly elevated in subordinate fish, and the number of aggressive acts received showed positive correlations with plasma levels of glucose on day 1, during the development of the dominance relationship, and of cortisol on day 4, when the dominance relationship was established. In contrast, plasma concentrations of T and 11-KT were significantly higher in dominant than in subordinate males, and there was a similar tendency in plasma concentrations of 17alpha,20beta-P. Further, plasma levels of these gonadal steroids were correlated with the number of aggressive acts performed on day 4, but not with the number of aggressive acts received. The plasma cortisol concentrations did not correlate with either 5-HIAA:5-HT or DOPAC:DA ratios in any of the brain parts analyzed. Plasma glucose levels showed positive correlations with brain 5-HIAA:5-HT ratios. Negative correlations were observed between 5-HIAA:5-HT ratios in the optic tectum and between plasma levels of T, 11-KT, and 17alpha,20beta-P. Telencephalic DOPAC/DA ratios displayed a negative correlation with plasma levels of T, 11-KT, and 17alpha,20beta-P, but only in dominant males.

Plasma gonadotropin II, sex steroids, and thyroid hormones in wild striped bass (Morone saxatilis) during spermiation and final oocyte maturation

The blood levels of gonadotropin II (GtH II), sex-steroid hormones, and thyroid hormones were determined in wild spermiating male striped bass (Morone saxatilis) in males and in females at various stages of final oocyte maturation (FOM), captured on their spawning grounds. The progression of spermiation was associated with increases in plasma GtH II and decreases in plasma testosterone (T), 11-ketotestosterone, and thyroxine (T4). Plasma triiodothyronine (T3) remained at high and relatively unchanged levels. Plasma levels of 17,20beta-dihydroxy-4-pregnen-3-one (17,20beta-P) and 17,20beta, 21-trihydroxy-4-pregnen-3-one (17,20beta,21-P), the proposed maturation-inducing steroids (MIS) in striped bass, were low and unchanged during the same period. It was concluded that low progestogen levels are adequate to induce spermiation in striped bass, and that higher levels may be associated with spawning behavior. In the females, based on the profiles of the studied hormones, FOM was separated into two phases. Early FOM, which included germinal vesicle (GV) migration and lipid-droplet coalescence, was associated with elevations in plasma GtH II, T, and estradiol 17beta. Late FOM, which included GV breakdown and yolk-globule coalescence, was associated with a further surge in plasma GtH II, increases in the levels of the two MIS, mainly 17, 20beta-P, and a drop in T4. Plasma T3 levels did not change during FOM. Examination of conjugated steroids demonstrated, in the males, a reduction in conjugated androgens at the peak of the spawning season and, in the females, a small increase in conjugated 17, 20beta-dihydroxylated and 5beta-reduced,3alpha-hydroxylated steroids after spawning. This is the most comprehensive report, to date, on the endocrine regulation of gonadal maturation in wild striped bass, demonstrating that a two-stage process of FOM is regulated by different endocrine signals, providing further evidence for the involvement of 17,20beta-P as a MIS in the females, and indicating that both males and females are in an euthyroid state during the spawning season.