Egg hormones in a highly fecund vertebrate: do they influence offspring social structure in competitive conditions?

Who listens to mother? A whole-family perspective on the evolution of maternal hormone allocation

Maternal effects, or the influence of maternal environment and phenotype on offspring phenotype, may allow mothers to fine-tune their offspring's developmental trajectory and resulting phenotype sometimes long after the offspring has reached independence. However, maternal effects on offspring phenotype do not evolve in isolation, but rather within the context of a family unit, where the separate and often conflicting evolutionary interests of mothers, fathers and offspring are all at play. While intrafamilial conflicts are routinely invoked to explain other components of reproductive strategy, remarkably little is known about how intrafamilial conflicts influence maternal effects. We argue that much of the considerable variation in the relationship between maternally derived hormones, nutrients and other compounds and the resulting offspring phenotype might be explained by the presence of conflicting selection pressures on different family members. In this review, we examine the existing literature on maternal hormone allocation as a case study for maternal effects more broadly, and explore new hypotheses that arise when we consider current findings within a framework that explicitly incorporates the different evolutionary interests of the mother, her offspring and other family members. Specifically, we hypothesise that the relationship between maternal hormone allocation and offspring phenotype depends on a mother's ability to manipulate the signals she sends to offspring, the ability of family members to be plastic in their response to those signals and the capacity for the phenotypes and strategies of various family members to interact and influence one another on both behavioural and evolutionary timescales. We also provide suggestions for experimental, comparative and theoretical work that may be instrumental in testing these hypotheses. In particular, we highlight that manipulating the level of information available to different family members may reveal important insights into when and to what extent maternal hormones influence offspring development. We conclude that the evolution of maternal hormone allocation is likely to be shaped by the conflicting fitness optima of mothers, fathers and offspring, and that the outcome of this conflict depends on the relative balance of power between family members. Extending our hypotheses to incorporate interactions between family members, as well as more complex social groups and a wider range of taxa, may provide exciting new developments in the fields of endocrinology and maternal effects.

Early-life stress influences ion balance in developing zebrafish (Danio rerio)

As a key endocrine axis involved in responding to stress, the hypothalamic-pituitary-interrenal axis plays dual roles in mobilizing energy and maintaining ionic/osmotic balance in fishes. Although these roles have been examined independently in detail in adult fishes, less attention has been paid to the effects of an endogenous stress response during early life, particularly with respect to its potential effects on ionic/osmotic balance. The present study tested the hypothesis that exposure of zebrafish to stress during early development would alter ion balance later in life. Zebrafish at three developmental stages (4, 7, or 15 days post-fertilization, dpf) were subjected to an air-exposure stressor twice a day for 2 days, causing elevation of whole-body cortisol levels. Individuals stressed early in life exhibited decreased survival and growth, altered cortisol responses to a subsequent air-exposure stressor, and increased whole-body Na⁺ and Ca²⁺ concentrations. Changes in whole-body Ca²⁺ concentrations were accompanied by increased ionocyte abundance at 7 dpf and increased rates of Ca²⁺ uptake from the environment. Differences in whole-body ion concentrations at 15 and 35 dpf were not accompanied by altered ion uptake rates. Across all ages examined, air-exposure stress experienced at 7 dpf was particularly effective at eliciting phenotypic changes, suggesting a critical window at this age for a stress response to influence development. These findings demonstrate that early-life stress in zebrafish triggers developmental plasticity, with age-dependent effects on both the cortisol stress axis and ion balance.

Multiple Environmental Stressors Induce an Adaptive Maternal Effect

AbstractEvolution of adaptation requires predictability and recurrence of functional contexts. Yet organisms live in multifaceted environments that are dynamic and ever changing, making it difficult to understand how complex adaptations evolve. This problem is particularly apparent in the evolution of adaptive maternal effects, which are often assumed to require reliable and discrete cues that predict conditions in the offspring environment. One resolution to this problem is if adaptive maternal effects evolve through preexisting, generalized maternal pathways that respond to many cues and also influence offspring development. Here, we assess whether an adaptive maternal effect in western bluebirds is influenced by maternal stress pathways across multiple challenging environments. Combining 18 years of hormone sampling across diverse environmental contexts with an experimental manipulation of the competitive environment, we show that multiple environmental factors influenced maternal corticosterone levels, which, in turn, influenced a maternal effect on aggression of sons in adulthood. Together, these results support the idea that multiple stressors can induce a known maternal effect in this system. More generally, they suggest that activation of general pathways, such as the hypothalamic-pituitary-adrenal axis, may simplify and facilitate the evolution of adaptive maternal effects by integrating variable environmental conditions into preexisting maternal physiological systems.

Exposure to exogenous egg cortisol does not rescue juvenile Chinook salmon body size, condition, or survival from the effects of elevated water temperatures

Climate change is leading to altered temperature regimes which are impacting aquatic life, particularly for ectothermic fish. The impacts of environmental stress can be translated across generations through maternally derived glucocorticoids, leading to altered offspring phenotypes. Although these maternal stress effects are often considered negative, recent studies suggest this maternal stress signal may prepare offspring for a similarly stressful environment (environmental match). We applied the environmental match hypothesis to examine whether a prenatal stress signal can dampen the effects of elevated water temperatures on body size, condition, and survival during early development in Chinook salmon Oncorhynchus tshawytscha from Lake Ontario, Canada. We exposed fertilized eggs to prenatal exogenous egg cortisol (1,000 ng/ml cortisol or 0 ng/ml control) and then reared these dosed groups at temperatures indicative of current (+0°C) and future (+3°C) temperature conditions. Offspring reared in elevated temperatures were smaller and had a lower survival at the hatchling developmental stage. Overall, we found that our exogenous cortisol dose did not dampen effects of elevated rearing temperatures (environmental match) on body size or early survival. Instead, our eyed stage survival indicates that our prenatal cortisol dose may be detrimental, as cortisol-dosed offspring raised in elevated temperatures had lower survival than cortisol-dosed and control reared in current temperatures. Our results suggest that a maternal stress signal may not be able to ameliorate the effects of thermal stress during early development. However, we highlight the importance of interpreting the fitness impacts of maternal stress within an environmentally relevant context.

Social density, but not sex ratio, drives ecdysteroid hormone provisioning to eggs by female house crickets (Acheta domesticus)

Social environment profoundly influences the fitness of animals, affecting their probability of survival to adulthood, longevity, and reproductive output. The social conditions experienced by parents at the time of reproduction can predict the social environments that offspring will face. Despite clear challenges in predicting future environmental conditions, adaptive maternal effects provide a mechanism of passing environmental information from parent to offspring and are now considered pervasive in natural systems. Maternal effects have been widely studied in vertebrates, especially in the context of social environment, and are often mediated by steroid hormone (SH) deposition to eggs. In insects, although many species dramatically alter phenotype and life-history traits in response to social density, the mechanisms of these alterations, and the role of hormone deposition by insect mothers into their eggs, remains unknown. In the experiments described here, we assess the effects of social environment on maternal hormone deposition to eggs in house crickets (Acheta domesticus). Specifically, we tested the hypotheses that variable deposition of ecdysteroid hormones (ESH) to eggs is affected by both maternal (a) social density and (b) social composition. We found that while maternal hormone deposition to eggs does not respond to social composition (sex ratio), it does reflect social density; females provision their eggs with higher ESH doses under low-density conditions. This finding is consistent with the interpretation that variable ESH provisioning is an adaptive maternal response to social environment and congruent with similar patterns of variable maternal provisioning across the tree of life. Moreover, our results confirm that maternal hormone provisioning may mediate delayed density dependence by introducing a time lag in the response of offspring phenotype to population size.

Effects of maternal cortisol treatment on offspring size, responses to stress, and anxiety-related behavior in wild largemouth bass (Micropterus salmoides)

Cortisol, the main glucocorticoid stress hormone in teleost fish, is of interest as a mediator of maternal stress on offspring characteristics because it plays an organizational role during early development. The present study tested the hypothesis that maternal exposure to cortisol treatment prior to spawn affects offspring phenotype using wild largemouth bass (Micropterus salmoides). Baseline and stress-induced cortisol concentrations, body size (i.e. length and mass), and behavior (i.e. anxiety, exploration, boldness, and aggression) were assessed at different offspring life-stages and compared between offspring of control and cortisol-treated females. Cortisol administration did not affect spawning success or timing, nor were whole-body cortisol concentrations different between embryos from cortisol-treated and control females. However, maternal cortisol treatment had significant effects on offspring stress responsiveness, mass, and behavior. Compared to offspring of control females, offspring of cortisol-treated females exhibited larger mass right after hatch, and young-of-the-year mounted an attenuated cortisol response to an acute stressor, and exhibited less thigmotaxic anxiety, exploratory behavior, boldness and aggression. Thus, offspring phenotype was affected by elevated maternal cortisol levels despite the absence of a significant increase in embryo cortisol concentrations, suggesting that a mechanism other than the direct deposition of cortisol into eggs mediates effects on offspring. The results of the present raise questions about the mechanisms through which maternal stress influences offspring behavior and physiology, as well as the impacts of such phenotypic changes on offspring fitness.

Prenatal Stress Exposure Generates Higher Early Survival and Smaller Size without Impacting Developmental Rate in a Pacific Salmon

Prenatal exposure to elevated glucocorticoids can act as a signal of environmental stress, resulting in modifications to offspring phenotype. While "negative" phenotypic effects (i.e., smaller size, slower growth) are often reported, recent research coupling phenotype with other fitness-related traits has suggested positive impacts of prenatal stress. Using captive Chinook salmon (Oncorhynchus tshawytscha), we treated eggs with biologically relevant cortisol levels-low (300 ng mL^-1 ), high (1,000 ng mL^-1 ), or control (0 ng mL^-1 )-to examine the early-life impacts of maternally transferred stress hormones on offspring. Specifically, we measured early survival, rate of development, and multiple measures of morphology. Low and high cortisol dosing of eggs resulted in significantly higher survival compared to controls (37% and 24% higher, respectively). Fish reared from high dose eggs were structurally smaller compared to control fish, but despite this variation in structural size, exposure to elevated cortisol did not impact developmental rate. These results demonstrate that elevations in egg cortisol can positively influence offspring fitness through an increase in early survival while also altering phenotype at a critical life-history stage. Overall, these results suggest that exposure to prenatal stress may not always produce apparently negative impacts on offspring fitness and further proposes that complex phenotypic responses should be examined in relevant environmental conditions.

Glucocorticoids in Fish Eggs: Variation, Interactions with the Environment, and the Potential to Shape Offspring Fitness

Wild and captive vertebrates face multiple stressors that all have the potential to induce chronic maternal stress (i.e., sustained, elevated plasma glucocorticoids), resulting in embryo exposure to elevated maternally derived glucocorticoids. In oviparous taxa such as fish, maternally derived glucocorticoids in eggs are known for their capacity to shape offspring phenotype. Using a variety of methodologies, scientists have quantified maternally derived levels of egg cortisol, the primary glucocorticoid in fishes, and examined the cascading effects of egg cortisol on progeny phenotype. Here we summarize and interpret the current state of knowledge on egg cortisol in fishes and the relationships linking maternal stress/state to egg cortisol and offspring phenotype/fitness. Considerable variation in levels of egg cortisol exists across species and among females within a species; this variation is hypothesized to be due to interspecific differences in reproductive life history and intraspecific differences in female condition. Outcomes of experimental studies manipulating egg cortisol vary both inter- and intraspecifically. Moreover, while exogenous elevation of egg cortisol (as a proxy for maternal stress) induces phenotypic changes commonly considered to be maladaptive (e.g., smaller offspring size), emerging work in other taxa suggests that there can be positive effects on fitness when the offspring's environment is taken into account. Investigations into (i) mechanisms by which egg cortisol elicits phenotypic change in offspring (e.g., epigenetics), (ii) maternal and offspring buffering capacity of cortisol, and (iii) factors driving natural variation in egg cortisol and how this variation affects offspring phenotype and fitness are all germane to discussions on egg glucocorticoids as signals of maternal stress.

Evolutionary implications of interspecific variation in a maternal effect: a meta-analysis of yolk testosterone response to competition

Competition between conspecifics during the breeding season can result in behavioural and physiological programming of offspring via maternal effects. For birds, in which maternal effects are best studied, it has been claimed that exposure to increased competition causes greater deposition of testosterone into egg yolks, which creates faster growing, more aggressive offspring; such traits are thought to be beneficial for high-competition environments. Nevertheless, not all species show a positive relationship between competitive interactions and yolk testosterone, and an explanation for this interspecific variation is lacking. We here test if the magnitude and direction of maternal testosterone allocated to eggs in response to competition can be explained by life-history traits while accounting for phylogenetic relationships. We performed a meta-analysis relating effect size of yolk testosterone response to competition with species coloniality, nest type, parental effort and mating type. We found that effect size was moderated by coloniality and nest type; colonial species and those with open nests allocate less testosterone to eggs when in more competitive environments. Applying a life-history perspective helps contextualize studies showing little or negative responses of yolk testosterone to competition and improves our understanding of how variation in this maternal effect may have evolved.

State-dependent behavior and alternative behavioral strategies in brown trout (Salmo trutta L.) fry

ABSTRACT

Animals generally adjust their behavior in response to bodily state (e.g., size and energy reserves) to optimize energy intake in relation to mortality risk, weighing predation probability against the risk of starvation. Here, we investigated whether brown trout Salmo trutta adjust their behavior in relation to energetic status and body size during a major early-life selection bottleneck, when fast growth is important. Over two consecutive time periods (P1 and P2; 12 and 23 days, respectively), food availability was manipulated, using four different combinations of high (H) and low (L) rations (i.e., HH, HL, LH, and LL; first and second letter denoting ration during P1 and P2, respectively). Social effects were excluded through individual isolation. Following the treatment periods, fish in the HL treatment were on average 15-21 % more active than the other groups in a forced open-field test, but large within-treatment variation provided only weak statistical support for this effect. Furthermore, fish on L-ration during P2 tended to be more actively aggressive towards their mirror image than fish on H-ration. Body size was related to behavioral expression, with larger fish being more active and aggressive. Swimming activity and active aggression were positively correlated, forming a behavioral syndrome in the studied population. Based on these behavioral traits, we could also distinguish two behavioral clusters: one consisting of more active and aggressive individuals and the other consisting of less active and aggressive individuals. This indicates that brown trout fry adopt distinct behavioral strategies early in life.

SIGNIFICANCE STATEMENT

This paper provides information on the state-dependence of behavior in animals, in particular young brown trout. On the one hand, our data suggest a weak energetic state feedback where activity and aggression is increased as a response to short term food restriction. This suggests a limited scope for behavioral alterations in the face of starvation. On the other hand, body size is linked to higher activity and aggression, likely as a positive feedback between size and dominance. The experiment was carried out during the main population survival bottleneck, and the results indicate that growth is important during this stage, as 1) behavioral compensation to increase growth is limited, and 2) growth likely increases the competitive ability. However, our data also suggests that the population separates into two clusters, based on combined scores of activity and aggression (which are positively linked within individuals). Thus, apart from an active and aggressive strategy, there seems to be another more passive behavioral strategy.

Programming of the hypothalamic-pituitary-interrenal axis by maternal social status in zebrafish (Danio rerio)

The present study examined the effects of maternal social status, with subordinate status being a chronic stressor, on development and activity of the stress axis in zebrafish embryos and larvae. Female zebrafish were confined in pairs for 48 h to establish dominant/subordinate hierarchies; their offspring were reared to 144 h post-fertilization (hpf) and sampled at five time points over development. No differences were detected in maternal cortisol contribution, which is thought to be an important programmer of offspring phenotype. However, once zebrafish offspring began to synthesize cortisol de novo (48 hpf), larvae of dominant females exhibited significantly lower baseline cortisol levels than offspring of subordinate females. These lower cortisol levels may reflect reduced hypothalamic-pituitary-interrenal (HPI) axis activity, because corticotropin-releasing factor (crf) and cytochrome p450 side chain cleavage enzyme (p450scc) mRNA levels also were lower in larvae from dominant females. Moreover, baseline mRNA levels of HPI axis genes continued to be affected by maternal social status beyond 48 hpf. At 144 hpf, stress-induced cortisol levels were significantly lower in offspring of subordinate females. These results suggest programming of stress axis function in zebrafish offspring by maternal social status, emphasizing the importance of maternal environment and experience on offspring stress axis activity.

Does individual variation in metabolic phenotype predict fish behaviour and performance?

There is increasing interest in documenting and explaining the existence of marked intraspecific variation in metabolic rate in animals, with fishes providing some of the best-studied examples. After accounting for variation due to other factors, there can typically be a two to three-fold variation among individual fishes for both standard and maximum metabolic rate (SMR and MMR). This variation is reasonably consistent over time (provided that conditions remain stable), and its underlying causes may be influenced by both genes and developmental conditions. In this paper, current knowledge of the extent and causes of individual variation in SMR, MMR and aerobic scope (AS), collectively its metabolic phenotype, is reviewed and potential links among metabolism, behaviour and performance are described. Intraspecific variation in metabolism has been found to be related to other traits: fishes with a relatively high SMR tend to be more dominant and grow faster in high food environments, but may lose their advantage and are more prone to risk-taking when conditions deteriorate. In contrast to the wide body of research examining links between SMR and behavioural traits, very little work has been directed towards understanding the ecological consequences of individual variation in MMR and AS. Although AS can differ among populations of the same species in response to performance demands, virtually nothing is known about the effects of AS on individual behaviours such as those associated with foraging or predator avoidance. Further, while factors such as food availability, temperature, hypoxia and the fish's social environment are known to alter resting and MMRs in fishes, there is a paucity of studies examining how these effects vary among individuals, and how this variation relates to behaviour. Given the observed links between metabolism and measures of performance, understanding the metabolic responses of individuals to changing environments will be a key area for future research because the environment will have a strong influence on which animals survive predation, become dominant and ultimately have the highest reproductive success. Although current evidence suggests that variation in SMR may be maintained within populations via context-dependent fitness benefits, it is suggested that a more integrative approach is now required to fully understand how the environment can modulate individual performance via effects on metabolic phenotypes encompassing SMR, MMR and AS.

Effects of maternal stress and cortisol exposure at the egg stage on learning, boldness and neophobia in brook trout

The environment experienced by females can have long-lasting effects on offspring phenotype. The objective of this study was to determine if maternal stress-induced behaviour reprogramming in offspring is found in brook char and to test whether cortisol is the main mediator, by separating the potential effects of cortisol from that of other potential maternal factors. We exposed female brook trout (Salvelinus fontinalis) to different parallel treatments during the oogenesis period: undisturbed as controls (1) fed cortisol through food (2) or physically stressed by handling once a week (3). Additionally, we exposed half of the control eggs to a cortisol suspension before fertilisation (4). Cortisol consumption and handling did not elevate either maternal plasma or egg cortisol, although egg cortisol level was significantly increased when eggs were bathed in the suspension. We measured spatial learning and memory, boldness and neophobia in 6 month-old offspring and found no effects of treatments on learning, memory or behaviour. Our results suggest that the relationship between maternal stress, circulating and egg cortisol levels, other maternal factors, and behavioural reprogramming is context and species-specific.

Changes in the concentrations of four maternal steroids during embryonic development in the threespined stickleback (Gasterosteus aculeatus)

Embryonic exposure to steroids often leads to long-term phenotypic effects. It has been hypothesized that mothers may be able to create a steroid environment that adjusts the phenotypes of offspring to current environmental conditions. Complicating this hypothesis is the potential for developing embryos to modulate their early endocrine environment. This study utilized the threespined stickleback (Gasterosteus aculeatus) to characterize the early endocrine environment within eggs by measuring four steroids (progesterone, testosterone, estradiol, and cortisol) of maternal origin. We then examined how the concentrations of these four steroids changed over the first 12 days post fertilization (dpf). Progesterone, testosterone, estradiol, and cortisol of maternal origin could be detected within unfertilized eggs and levels of all four steroids declined in the first 3 days following fertilization. While levels of progesterone, testosterone, and estradiol remained low after the initial decline, levels of cortisol rose again by 8 dpf. These results demonstrate that G. aculeatus embryos begin development in the presence of a number of maternal steroids but levels begin to change quickly following fertilization. This suggests that embryonic processes change the early endocrine environment and hence influence the ability of maternal steroids to affect development. With these findings, G. aculeatus becomes an intriguing system in which to study how selection may act on both maternal and embryonic processes to shape the evolutionary consequence of steroid-mediated maternal effects.

Mild maternal stress disrupts associative learning and increases aggression in offspring

Maternal stress has been shown to affect behaviour of offspring in a wide range of animals, but this evidence has come from studies that exposed gestating mothers to acute or severe stressors, such as restraint or exposure to synthetic stress hormones. Here we show that exposure of mothers to even a mild stressor reduces associative learning and increases aggression in offspring. Female guppies were exposed to routine husbandry procedures that produced only a minimal, non-significant, elevation of the stress hormone cortisol. In contrast to controls, offspring from mothers that experienced this mild stress failed to learn to associate a colour cue and food reward, and showed a greater amount of inter-individual variation in behaviour compared with control offspring. This mild stress also resulted in offspring that were more aggressive towards their own mirror image than controls. While it is possible that these results could represent the transmission of beneficial maternal characteristics to offspring born into unpredictable environments, the potential for mild maternal stress to affect offspring performance also has important implications for research into the trans-generational effects of stress.

The association between parental life history and offspring phenotype

In many taxa there is considerable intraspecific variation in life-history strategies from within a single population, reflecting alternative routes through which organisms can achieve successful reproduction. Atlantic salmon Salmo salar (Linnaeus) show some of the greatest within-population variability in life history strategies amongst vertebrates, with multiple discrete male and female life histories co-existing and interbreeding on many spawning grounds, although the effect of the various combinations of life histories on offspring traits remains unknown. Using crosses of wild fish we show here that the life history strategy of both parents was significantly associated with a range of offspring traits. Mothers that had spent longer at sea (two versus one year) produced offspring which were heavier, longer and in better condition at the time of first feeding. However, these relationships disappeared shortly after fry had begun feeding exogenously. At this stage, the juvenile rearing environment (i.e. time spent in fresh water as juveniles) of the mother was a better predictor of offspring traits, with mothers that were faster to develop in fresh water (migrating to sea after two rather than three years of age) producing offspring that had higher maximal metabolic rates, aerobic scopes, and that grew faster. Faster developing fathers (1 year old sneaker males) tended to produce offspring that had higher maximal metabolic rates, were in better body condition and grew faster. The results suggest that both genetic effects and those related to parental early and late life history contribute to offspring traits.

Mother knows best, even when stressed? Effects of maternal exposure to a stressor on offspring performance at different life stages in a wild semelparous fish

The environment mothers are exposed to has resonating effects on offspring performance. In iteroparous species, maternal exposure to stressors generally results in offspring ill-equipped for survival. Still, opportunities for future fecundity can offset low quality offspring. Little is known, however, as to how intergenerational effects of stress manifest in semelparous species with only a single breeding episode. Such mothers would suffer a total loss of fitness if offspring cannot survive past multiple life stages. We evaluated whether chronic exposure of female sockeye salmon (Oncorhynchus nerka) to a chase stressor impaired offspring performance traits. Egg size and early offspring survival were not influenced by maternal exposure to the repeated acute stressor. Later in development, fry reared from stressed mothers swam for shorter periods of time but possessed a superior capacity to re-initiate bouts of burst swimming. In contrast to iteroparous species, the mechanisms driving the observed effects do not appear to be related to cortisol, as egg hormone concentrations did not vary between stressed and undisturbed mothers. Sockeye salmon appear to possess buffering strategies that protect offspring from deleterious effects of maternal stress that would otherwise compromise progeny during highly vulnerable stages of development. Whether stressed sockeye salmon mothers endow offspring with traits that are matched or mismatched for survival in the unpredictable environment they encountered is discussed. This study highlights the importance of examining intergenerational effects among species-specific reproductive strategies, and across offspring life history to fully determine the scope of impact of maternal stress.

Among-sibling differences in the phenotypes of juvenile fish depend on their location within the egg mass and maternal dominance rank

We investigated whether among-sibling differences in the phenotypes of juvenile fish were systematically related to the position in the egg mass where each individual developed during oogenesis. We sampled eggs from the front, middle and rear thirds of the egg mass in female brown trout of known dominance rank. In the resulting juveniles, we then measured traits that are related to individual fitness: body size, social status and standard metabolic rate (SMR). When controlling for differences among females in mean egg size, siblings from dominant mothers were initially larger (and had a lower mass-corrected SMR) if they developed from eggs at the rear of the egg mass. However, heterogeneity in the size of siblings from different positions in the egg mass diminished in lower-ranking females. Location of the egg within the egg mass also affected the social dominance of the resulting juvenile fish, although the direction of this effect varied with developmental age. This study provides the first evidence of a systematic basis for among-sibling differences in the phenotypes of offspring in a highly fecund organism.