Yolk steroids in great tit Parus major eggs: variation and covariation between hormones and with environmental and parental factors

Maternal deposition of hormones and contaminants shape the gonadal transcriptome in American alligators

Environmental conditions influence the maternal deposition of hormones into eggs, which is hypothesized to adaptively modify developmental outcomes in offspring. However, most ecosystems harbour environmental contaminants capable of disrupting endocrine signaling, and maternal exposure to these compounds has the potential to further alter offspring traits. Studies rarely examine maternally derived hormones and contaminants along with offspring phenotypes, and we know little about their interrelationships and potential interactions. Here, we measure yolk concentrations of 24 endocrine-disrupting compounds (EDCs) and 28 steroid hormones along with gonadal transcriptomes from two populations of the American alligator (Alligator mississippiensis) that differ in reproductive development and exposure to EDCs. Using a network-based approach, we identify gene expression modules associated with hormones and contaminants independently, in combination, or by potential indirect influences of EDCs on maternal hormone deposition. We find that yolk concentrations of both 17β-oestradiol and etiocholanolone differ across populations and explain substantial variation in gene expression. We further provide evidence for the indirect effect of the pesticide, methoxychlor, on gonadal gene expression through its relationship with 17β-oestradiol. Our results reveal novel pathways by which maternal exposure to environmental contaminants interacts with hormone provisioning to affect offspring sexual development.

Female red-winged blackbirds (Agelaius phoeniceus) do not alter nest site selection, maternal programming, or hormone-mediated maternal effects in response to perceived nest predation or brood parasitism risk

Predation or brood parasitism risks can change the behaviors and reproductive decisions in many parental animals. For oviparous species, mothers can mitigate their reproductive success in at least three ways: (1) by avoiding nest sites with high predation or parasitism risks, (2) through hormonal maternal effects that developmentally prime offspring for survival in risky environments, or (3) by investing less in reproduction when predation or parasitism risks are high. Here, we tested if perceived predation and parasitism risks can induce any of these behavioral or physiological responses by exposing female red-winged blackbirds (Agelaius phoeniceus) to playbacks of two major nest threats, a predator (Cooper's hawk, Accipiter cooperii) and an obligate brood parasite (brown-headed cowbird; Molothrus ater), as well as two controls (harmless Eastern meadowlark, Sturnella magna; and silence). We found that female blackbirds did not avoid nesting at sites treated with predator or brood parasite playbacks, nor were females more likely to abandon nesting attempts at these sites. Egg size and yolk hormone profiles, which are common proxies for maternal investment in oviparous species, were statistically similar across treatment sites. Instead, we found intraclutch variation in yolk steroid hormone profiles: concentrations of three progestogens (pregnanedione, 17α-hydroxypregnenolone, and deoxycorticosterone) and two androgens (testosterone and androstenedione) were higher in third-laid than first-laid eggs. Our study largely confirms previous findings of consistent intraclutch yolk hormone variation in this species, in birds in general, and in other oviparous lineages, but uniquely reports on several yolk steroid hormones largely overlooked in the literature on hormone-mediated maternal effects.

Does contaminant exposure disrupt maternal hormones deposition? A study on per- and polyfluoroalkyl substances in an Arctic seabird

Maternal effects are thought to be essential tools for females to modulate offspring development. The selective deposition of avian maternal hormones could therefore allow females to strategically adjust the phenotype of their offspring to the environmental situation encountered. However, at the time of egg formation, several contaminants are also transferred to the egg, including per- and polyfluoroalkyl substances (PFAS) which are ubiquitous organic contaminants with endocrine disrupting properties. It is, however, unknown if they can disrupt maternal hormone deposition. In this study we explored relationships between female PFAS burden and maternal deposition in the eggs of steroids (dihydrotestosterone, androstenedione and testosterone), glucocorticoids (corticosterone) and thyroid hormones (triiodothyronine and thyroxine) in a population of the Arctic-breeding black-legged kittiwake (Rissa tridactyla). Egg yolk hormone levels were unrelated to female hormone plasma levels. Second-laid eggs had significantly lower concentrations of androstenedione than first-laid eggs. Triiodothyronine yolk levels were decreasing with increasing egg mass but increasing with increasing females' body condition. Testosterone was the only transferred yolk hormone correlated to maternal PFAS burden: specifically, we found a positive correlation between testosterone in yolks and circulating maternal perfluorononanoic acid (PFNA), perfluorodecanoic acid (PFDcA) and perfluoroundecanoic acid (PFUnA) in first-laid eggs. This correlative study provides a first insight into the potential of some long-chain perfluoroalkyl acids to disrupt maternal hormones deposition in eggs and raises the question about the consequences of increased testosterone deposition on the developing embryo.

Yolk corticosterone and progesterone levels in Greater Rhea (Rhea americana) eggs vary in a changing social environment

Maternal hormones in avian egg yolks may signal and prepare offspring for the prevailing conditions. However, this adjustment requires some degree of flexibility in regulating yolk hormone deposition. The Greater Rhea (Rhea americana) has a particular mating system that combines mixed polygyny and polyandry, communal nesting, and exclusive paternal care of chicks. In this species, we previously found that yolk hormone deposition varies among eggs of different captive populations and could influence chicks' physiology and behavior. However, it is still unknown whether females can modify yolk hormone deposition in a changing social environment. Using a captive population of Greater Rheas, in this study, we quantified yolk hormone levels before and after a reduction in the number of females present in the population. We found that females deposited on average higher yolk corticosterone and lower yolk progesterone after the change in their social environment. Since corticosterone deposited into the yolk comes exclusively from the female's plasma, our results suggest that females had, on average, higher plasma corticosterone levels. The change in the number of females may increase the events of male-male competitions, courtships, and matings, leading to an increase of corticosterone in the females' plasma and then into their eggs. Since we previously found that higher yolk corticosterone and lower yolk progesterone were associated with the production of chicks that have an attenuated stress response, the present study results suggest that yolk hormone deposition is mediated by flexible mechanisms that could adjust development to the prevailing conditions.

Effect of prenatal glucocorticoids and thyroid hormones on developmental plasticity of mitochondrial aerobic metabolism, growth and survival: an experimental test in wild great tits

Developmental plasticity is partly mediated by transgenerational effects, including those mediated by the maternal endocrine system. Glucocorticoid and thyroid hormones may play central roles in developmental programming through their action on metabolism and growth. However, the mechanisms by which they affect growth and development remain understudied. One hypothesis is that maternal hormones directly affect the production and availability of energy-carrying molecules (e.g. ATP) by their action on mitochondrial function. To test this hypothesis, we experimentally increased glucocorticoid and thyroid hormones in wild great tit eggs (Parus major) to investigate their impact on offspring mitochondrial aerobic metabolism (measured in blood cells), and subsequent growth and survival. We show that prenatal glucocorticoid supplementation affected offspring cellular aerobic metabolism by decreasing mitochondrial density, maximal mitochondrial respiration and oxidative phosphorylation, while increasing the proportion of the maximum capacity being used under endogenous conditions. Prenatal glucocorticoid supplementation only had mild effects on offspring body mass, size and condition during the rearing period, but led to a sex-specific (females only) decrease in body mass a few months after fledging. Contrary to our expectations, thyroid hormone supplementation did not affect offspring growth or mitochondrial metabolism. Recapture probability as juveniles or adults was not significantly affected by prenatal hormonal treatment. Our results demonstrate that prenatal glucocorticoids can affect post-natal mitochondrial density and aerobic metabolism. The weak effects on growth and apparent survival suggest that nestlings were mostly able to compensate for the transient decrease in mitochondrial aerobic metabolism induced by prenatal glucocorticoids.

Differential effects of steroid hormones on levels of broad-sense heritability in a wild bird: possible mechanism of environment × genetic variance interaction?

Genetic variation is one of the key concepts in evolutionary biology and an important prerequisite of evolutionary change. However, we know very little about processes that modulate its levels in wild populations. In particular, we still are to understand why genetic variances often depend on environmental conditions. One of possible environment-sensitive modulators of observed levels of genetic variance are maternal effects. In this study we attempt to experimentally test the hypothesis that maternally transmitted agents (e.g. hormones) may influence the expression of genetic variance in quantitative traits in the offspring. We manipulated the levels of steroid hormones (testosterone and corticosterone) in eggs laid by blue tits in a wild population. Our experimental setup allowed for full crossing of genetic and rearing effects with the experimental manipulation. We observed that birds treated with corticosterone exhibited a significant decrease in broad-sense genetic variance of tarsus length, and an increase in this component in body mass on the 2nd day post-hatching. Our study indicates, that maternally transmitted substances such as hormones may have measurable impact on the levels of genetic variance and hence, on the evolutionary potential of quantitative traits.

Experimental manipulation of maternal corticosterone: Hormone transfer to the yolk in the zebra finch Taeniopygia guttata

Maternally-derived hormones affect offspring physiological and behavioural phenotype, plausibly as an adaptive response to maternal environmental conditions. Corticosterone (CORT), the principal avian glucocorticoid produced in response to stress, is recognised as a potential mediator of such maternal reproductive effects. Maternally-derived yolk CORT is implicated in mediating offspring growth and hatchling begging behaviour. However, determining the potential for maternal effects in opportunistic breeders subject to variable environments relies on understanding whether natural variation in maternal circulating hormones may directly impact the embryo during development. Therefore, we tested whether elevated maternal CORT concentrations increase yolk CORT concentrations in zebra finch (Taeniopygia guttata) eggs. We remotely dosed breeding females with biologically-relevant doses of CORT, or the oil vehicle, 0-3 h prior to the predicted time of ovulation, and allowed pairs to produce two clutches, one under each treatment, in a crosswise, balanced design. CORT dosing elevated maternal plasma CORT and increased mean yolk CORT by a factor of 1.75 compared to the egg yolks of control mothers. Importantly, CORT concentrations did not differ between inner and outer layers of yolk. We found no egg lay order effect and maternal CORT dosing did not influence reproductive outputs (clutch initiation date, clutch size or egg mass). Our results confirm the direct impact of biologically-relevant increases in maternal CORT on yolk CORT, providing evidence that maternal CORT concentrations during yolk deposition to the follicle alters embryonic exogenous CORT exposure. Further research is required to determine the impact of maternal CORT on embryonic developmental programming.

Natural variation in yolk fatty acids, but not androgens, predicts offspring fitness in a wild bird

BACKGROUND

In egg-laying animals, mothers can influence the developmental environment and thus the phenotype of their offspring by secreting various substances into the egg yolk. In birds, recent studies have demonstrated that different yolk substances can interactively affect offspring phenotype, but the implications of such effects for offspring fitness and phenotype in natural populations have remained unclear. We measured natural variation in the content of 31 yolk components known to shape offspring phenotypes including steroid hormones, antioxidants and fatty acids in eggs of free-living great tits (Parus major) during two breeding seasons. We tested for relationships between yolk component groupings and offspring fitness and phenotypes.

RESULTS

Variation in hatchling and fledgling numbers was primarily explained by yolk fatty acids (including saturated, mono- and polyunsaturated fatty acids) - but not by androgen hormones and carotenoids, components previously considered to be major determinants of offspring phenotype. Fatty acids were also better predictors of variation in nestling oxidative status and size than androgens and carotenoids.

CONCLUSIONS

Our results suggest that fatty acids are important yolk substances that contribute to shaping offspring fitness and phenotype in free-living populations. Since polyunsaturated fatty acids cannot be produced de novo by the mother, but have to be obtained from the diet, these findings highlight potential mechanisms (e.g., weather, habitat quality, foraging ability) through which environmental variation may shape maternal effects and consequences for offspring. Our study represents an important first step towards unraveling interactive effects of multiple yolk substances on offspring fitness and phenotypes in free-living populations. It provides the basis for future experiments that will establish the pathways by which yolk components, singly and/or interactively, mediate maternal effects in natural populations.

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.

Inter-Individual Variation in Anti-Parasitic Egg Rejection Behavior: A Test of the Maternal Investment Hypothesis

Hosts of avian brood parasites may reduce or forego the costs of caring for foreign young by rejecting parasitic eggs from the nest. Yet, many host species accept parasitic eggs and, even among rejecter species, some individuals go on to incubate and hatch them. The factors explaining the variation in egg rejection between species have received much theoretical and empirical attention, but the causes of intraspecific variation in different individuals’ propensity for accepting parasitic eggs are less well understood. Here we tested the maternal investment hypothesis, which predicts that hosts with costlier clutches will be more likely to reject parasitic eggs from their nest. We studied variation in the egg rejection responses of American robins (Turdus migratorius), a robust egg-rejecter host of the brood parasitic brown-headed cowbird (Molothrus ater), to 3D-printed cowbird-sized eggs which were painted dark blue, a color known to induce variable and repeatable egg rejection responses in individual robins. Costlier clutch investment was estimated by earlier laying date, larger clutch size, heavier unincubated yolk mass, and variable yolk steroid hormone concentrations. There was no statistical support for most of our predictions. However, we detected more concentrated and greater overall amount of deoxycorticosterone deposited in egg yolks of rejecters relative to acceptors, although this accounted for no more than 14% of variance in the data. Future work should test experimentally the potential physiological linkage between maternal egg yolk steroid investment and egg rejection propensity in this and other host species of avian brood parasites.

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.

Effects of predation risk on egg steroid profiles across multiple populations of threespine stickleback

Predation often has consistent effects on prey behavior and morphology, but whether the physiological mechanisms underlying these effects show similarly consistent patterns across different populations remains an open question. In vertebrates, predation risk activates the hypothalamic-pituitary-adrenal (HPA) axis, and there is growing evidence that activation of the maternal HPA axis can have intergenerational consequences via, for example, maternally-derived steroids in eggs. Here, we investigated how predation risk affects a suite of maternally-derived steroids in threespine stickleback eggs across nine Alaskan lakes that vary in whether predatory trout are absent, native, or have been stocked within the last 25 years. Using liquid chromatography coupled with mass spectroscopy (LC-MS/MS), we detected 20 steroids within unfertilized eggs. Factor analysis suggests that steroids covary within and across steroid classes (i.e. glucocorticoids, progestogens, sex steroids), emphasizing the modularity and interconnectedness of the endocrine response. Surprisingly, egg steroid profiles were not significantly associated with predator regime, although they were more variable when predators were absent compared to when predators were present, with either native or stocked trout. Despite being the most abundant steroid, cortisol was not consistently associated with predation regime. Thus, while predators can affect steroids in adults, including mothers, the link between maternal stress and embryonic development is more complex than a simple one-to-one relationship between the population-level predation risk experienced by mothers and the steroids mothers transfer to their eggs.

Experimental increase in temperature affects eggshell thickness, and not egg mass, eggshell spottiness or egg composition in the great tit (Parus major)

Phenotypic effects of global warming have been documented in many different taxa. However, the importance of transgenerational phenotypic plasticity in these adaptations are seldom studied. In birds, temperature could affect egg characteristics. Higher temperatures during egg-laying may reduce maintenance costs for females and allow a higher investment in reproduction. Yet, females may also use temperatures as a cue for the risk of mismatch latter in the season. Thus, higher temperatures may be correlated to an acceleration of embryonic development (e.g. via hormonal manipulation). We performed an experiment in which night-time temperature was increased in the nestbox by approximately 1 °C throughout the entire laying period in great tits (Parus major). We collected one pre-treatment egg (beginning of the laying sequence) and one post-treatment egg (end of the laying sequence). Egg content (yolk androgens and lysozymes in the albumen), eggshell coloration, eggshell mass, egg mass, and shape were not affected by the treatment. However, last-laid eggs in clutches from control nestboxes had a thicker eggshell than last-laid eggs from heated nestboxes, suggesting a putative slight decrease of maternal investment with the experimental increase of temperature. We also observed effects of the laying sequence on egg characteristics. Eggs that were laid late in the laying sequence were heavier, larger, had larger spots and higher yolk androgens than eggs laid earlier. Lysozyme concentration decrease with the laying sequence in late clutches only. Thus, effects of temperature may also change with the laying sequence and it would be interesting in the future to tests the effects on first-laid eggs.

Quantification of 27 yolk steroid hormones in seven shrubland bird species: interspecific patterns of hormone deposition and links to life history, development, and predation risk

Steroid hormones play critical organizational and activational roles during vertebrate development, impacting everything from sexual differentiation to metabolic activity. For oviparous species such as birds, these hormones are transferred from female to egg during follicle maturation, and differences in relative and absolute concentrations of the steroid hormones may reflect differences in life history, developmental, and ecological conditions. Prior work on yolk steroid hormones has focused on a handful of candidate hormones (e.g., testosterone, androstenedione, and corticosterone), but we used high-performance liquid chromatography with tandem mass spectroscopy (LC–MS–MS) to quantify 27 yolk steroids from the eggs of seven shrubland bird species (American Robin, Turdus migratorius Linnaeus, 1766; Brown-headed Cowbird, Molothrus ater (Boddaert, 1783); Brown Thrasher, Toxostoma rufum (Linnaeus, 1758); Eastern Towhee, Pipilo erythrophthalmus (Linnaeus, 1758); Field Sparrow, Spizella pusilla (A. Wilson, 1810); Gray Catbird, Dumetella carolinensis (Linnaeus, 1766); Northern Cardinal, Cardinalis cardinalis (Linnaeus, 1758)). In addition to comparing steroid profiles across species, we conducted exploratory analyses to determine how the hormones clustered using a principal component (PC) approach and if PCs were correlated with aspects of egg resources (relative egg size, proportion yolk), life-history traits (embryonic and nestling development speed), and nest-predation risk (daily survival rate (DSR)). We documented substantial interspecific variation in both absolute and proportional endocrine profiles. PCAs indicated that glucocorticoids generally clustered together (PC1), but other classes of steroids did not. PC2 and PC3 strongly covaried with egg resources, DSR, and development speed, suggesting that they reflect adaptive patterns of maternal hormone deposition.

The mean and variance of climate change in the oceans: hidden evolutionary potential under stochastic environmental variability in marine sticklebacks

Increasing climate variability may pose an even greater risk to species than climate warming because temperature fluctuations can amplify adverse impacts of directional warming on fitness-related traits. Here, the influence of directional warming and increasing climate variability on marine stickleback fish (Gasterosteus aculeatus) offspring size variation was investigated by simulating changes to the mean and variance of ocean temperatures predicted under climate change. Reproductive traits of mothers and offspring size reaction norms across four climate scenarios were examined to assess the roles of standing genetic variation, transgenerational and within-generation plasticity in adaptive potential. Mothers acclimated to directional warming produced smaller eggs than mothers in constant, ambient temperatures, whereas mothers in a predictably variable environment (weekly change between temperatures) produced a range of egg sizes, possibly reflecting a diversified bet hedging strategy. Offspring size post-hatch was mostly influenced by genotype by environment interactions and not transgenerational effects. Offspring size reaction norms also differed depending on the type of environmental predictability (predictably variable vs. stochastic), with offspring reaching the largest sizes in the stochastic environment. Release of cryptic genetic variation for offspring size in the stochastic environment suggests hidden evolutionary potential in this wild population to respond to changes in environmental predictability.

Temperature-induced variation in yolk androgen and thyroid hormone levels in avian eggs

Global warming has substantially changed the environment, but the mechanisms to cope with these changes in animals, including the role of maternal effects, are poorly understood. Maternal effects via hormones deposited in eggs, have important environment-dependent effects on offspring development and fitness: thus females are expected to adjust these hormones to the environment, such as the ambient temperature. Longer-term temperature variation could function as a cue, predicting chick rearing conditions to which yolk hormone levels are adjusted, while short-term temperature variation during egg formation may causally affect hormone transfer to eggs. We studied the effects of ambient temperature on yolk androgens (testosterone and androstenedione) and thyroid hormones (thyroxine and triiodothyronine) in great tits (Parus major) using data from unmanipulated clutches from a wild population and from aviary birds (ad libitum food) exposed to different experimental temperature treatments during five years. Both in the wild and in captivity, longer-term pre-laying ambient temperature was not associated with clutch mean yolk hormone levels, while the way androstenedione and thyroxine levels varied across the laying sequence did associate with pre-laying temperature in the wild. Yolk testosterone levels were positively correlated with short-term temperature (during yolk formation) changes within clutches in both wild and captivity. We also report, for the first time in a wild bird, that yolk thyroxine levels correlated with a key environmental factor: thyroxine levels were negatively correlated with ambient temperature during egg formation. Thus, yolk hormone levels, especially testosterone, seem to be causally affected by ambient temperature. These short-term effects might reflect physiological changes in females with changes in ambient temperature. The adaptive value of the variation with ambient temperatures pre-laying or during egg formation should be studied with hormone manipulations in different thermal environments.