Adaptive sex differences in growth of pre-ovulation oocytes in a passerine bird

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.

Does paternal immunocompetence affect offspring vulnerability to maternal androgens? A study in domestic chickens

Exposure of yolk androgens can positively stimulate chick growth and competitive ability, but may negatively affect immunity. It has been hypothesized that only chicks from immunologically superior fathers can bear the cost of prenatal exposure to high androgen levels. To test this hypothesis, we paired roosters from two selection lines, one up- and one down-selected for natural antibodies (NAbs), with hens from a control line. We measured yolk testosterone and androstenedione levels, and we injected the treatment group of eggs of each female with testosterone suspended in sesame oil and the control group with sesame oil only. We then measured hatching success and growth, and characterized the humoral and cellular immune responses using three different challenges: a phyto-hemagglutinin, a lipopolysaccharide and a sheep red blood cell challenge. We found that the hatching success, body mass, initial levels of natural antibodies and the chicks’ immunological responses to the three different challenges and development were affected neither by paternal immunocompetence nor by treatment. These results do not support the hypothesis that chicks from low-NAb line fathers are more sensitive to testosterone exposure during embryonic development than chicks from high-NAb line fathers.

Summary: Our study shows that there were no effects of paternal natural antibody line, increased embryonic testosterone exposure or the interaction of both on immunocompetence of chicks.

The relationship between male social status, ejaculate and circulating testosterone concentration and female yolk androgen transfer in red junglefowl (Gallus gallus)

Several studies show that avian females prefer males based on their secondary sexual ornaments and dominance status. We tested in red junglefowl (Gallus gallus) how comb size affected the result of fighting and how the dominance status related to testosterone concentrations in their circulation and ejaculates. We subsequently tested how social status was related to female reproductive investment, including yolk hormone transfer. We found that after a fight 1) winners increased plasma T and decreased ejaculates T whereas losers' T remained unchanged, and 2) plasma T of winners was higher but ejaculates T was lower than those of losers. We argued those are consistent with the different reproductive strategies of dominant and subordinate males. Furthermore, in line with offspring sex-dependent growth patterns females transferred significantly more androstenedione to female than male embryos when mated with winners, while doing the opposite when mated with losers. We concluded therefore that female reproductive investment was affected by both partner quality and embryo sex. The results indicate that male quality influences sex-specific maternal investment, which could be mediated by ejaculate testosterone concentration.

Between- and Within-Individual Variation of Maternal Thyroid Hormone Deposition in Wild Great Tits (Parus major)

Maternal hormones are often considered a mediator of anticipatory maternal effects; namely, mothers adjust maternal hormone transfer to prepare the offspring for the anticipated environment. The flexibility for mothers to adjust hormone transfer is therefore a prerequisite for such anticipatory maternal effects. Nevertheless, previous studies have focused only on the average differences of maternal hormone transfer between groups and neglected the substantial individual variation, despite the fact that individual plasticity in maternal hormone transfer is actually the central assumption. In this study, we studied the between- and within-individual variation of maternal thyroid hormones (THs) in egg yolk of wild great tits (Parus major) and estimated the individual plasticity of maternal yolk THs across environmental temperature, clutch initiation dates, and egg laying order using linear mixed effects models. Interestingly, our models provide statistical evidence that the two main THs-the main biologically active hormone T3 and T4, which is mostly considered a prohormone-exhibited different variation patterns. Yolk T3 showed significant between-individual variation on the average levels, in line with its previously reported moderate heritability. Yolk T4, however, showed significant between-clutch variation in the pattern over the laying sequence, suggesting a great within-individual plasticity. Our findings suggest that the role and function of the hormone within the endocrine axis likely influences its flexibility to respond to environmental change. Whether the flexibility of T4 deposition brings a fitness advantage should be examined along with its potential effects on offspring, which remain to be further investigated.

Seasonal shifts in sex ratios are mediated by maternal effects and fluctuating incubation temperatures

Sex-specific maternal effects can be adaptive sources of phenotypic plasticity. Reptiles with temperature-dependent sex determination (TSD) are a powerful system to investigate such maternal effects because offspring phenotype, including sex, can be sensitive to maternal influences such as oestrogens and incubation temperatures.In red-eared slider turtles (Trachemys scripta), concentrations of maternally derived oestrogens and incubation temperatures increase across the nesting season; we wanted to determine if sex ratios shift in a seasonally concordant manner, creating the potential for sex-specific maternal effects, and to define the sex ratio reaction norms under fluctuating temperatures across the nesting season.Eggs from early and late season clutches were incubated under a range of thermally fluctuating temperatures, maternally derived oestradiol concentrations were quantified via radioimmunoassay, and hatchling sex was identified. We found that late season eggs had higher maternal oestrogen concentrations and were more likely to produce female hatchlings. The sex ratio reaction norm curves systematically varied with season, such that with even a slight increase in temperature (0.5°C), late season eggs produced up to 49% more females than early season eggs.We found a seasonal shift in sex ratios which creates the potential for sex-specific phenotypic matches across the nesting season driven by maternal effects. We also describe, for the first time, systematic variation in the sex ratio reaction norm curve within a single population in a species with TSD.

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.

Evolutionary ecology. Cycles of species replacement emerge from locally induced maternal effects on offspring behavior in a passerine bird

An important question in ecology is how mechanistic processes occurring among individuals drive large-scale patterns of community formation and change. Here we show that in two species of bluebirds, cycles of replacement of one by the other emerge as an indirect consequence of maternal influence on offspring behavior in response to local resource availability. Sampling across broad temporal and spatial scales, we found that western bluebirds, the more competitive species, bias the birth order of offspring by sex in a way that influences offspring aggression and dispersal, setting the stage for rapid increases in population density that ultimately result in the replacement of their sister species. Our results provide insight into how predictable community dynamics can occur despite the contingency of local behavioral interactions.

Maternal effects in quail and zebra finches: Behavior and hormones

Maternal effects are influences of parents on offspring phenotype occurring through pathways other than inherited DNA. In birds, two important routes for such transmission are parental behavior and non-DNA egg constituents such as yolk hormones. Offspring traits subject to parental effects include behavior and endocrine function. Research from the Adkins-Regan lab has used three avian species to investigate maternal effects related to hormones and behavior. Experiments with chickens and Japanese quail have shown that maternal sex steroids can influence sex determination to produce biased offspring sex ratios. Because all birds have a ZZ/ZW chromosomal sex determining system in which the female parent determines the sex of the offspring, these results raise the possibility that maternal steroids can influence the outcome of sex chromosome meiosis. Learning has been shown to influence egg investment by female quail in ways that are likely to alter offspring phenotype. In quail, embryonic and exogenous sex steroids have well established and long-lasting effects on sexual differentiation of behavior during a critical period in ovo, but elevated yolk testosterone has long-term effects on behavior that do not seem to be occurring through an alteration in sexual differentiation. In biparental zebra finches, removal of mothers alters not only later behavior, but also the adult response of the hypothalamic-pituitary-adrenal (HPA) axis to an environmental stressor, as indicated by plasma corticosterone. Birds raised only by fathers have lower levels of mRNA for both glucocorticoid receptors in several brain regions as adults. These studies add to the evidence that one generation influences the behavioral or endocrine phenotype of the next through routes other than transmission of DNA. Additional research will be required to understand the adaptive significance of these effects.

The role of steroid hormones in the adjustment of primary sex ratio in birds: compiling the pieces of the puzzle

There is ample evidence that birds have the ability to adjust their offsprings' sex ratios before fertilization occurs. Recent work has focused on pinpointing when during the process of oocyte maturation adjustment of sex ratio takes place. Additionally, there is growing support for the idea that there is hormonal control over the process of adjustment of sex ratio in birds. Whether steroid hormones represent direct mediators of the process, however, remains unclear. This review outlines the precise points during maturation of ovarian follicles during which adjustment of primary sex ratios could potentially occur, compiles the evidence for hormonal involvement in the process of primary adjustment of sex ratio, and discusses potential hormonal targets during maturation and fertilization of oocytes where hormones may trigger adjustment of sex ratio in birds.

"Homeostatic hitchhiking": a mechanism for the evolutionary retention of complex adaptations

The complexity of organismal organization channels and accommodates novel genomic and developmental modifications. Here, I extend this perspective to suggest that emergent processes that dominate homeostasis-co-option, re-use, and recombination of accumulated elements-can create configurations and dependencies among these elements that strongly reduce the number of evolutionary steps needed for the evolution of precise novel adaptations. Evolutionary retention and environmental matching of such configurations are further facilitated when they include elements of homeostasis that are responsive to particular environmental cues. I apply this perspective to the study of evolution of sex-biased egg-laying in birds, a phenomenon that combines precision, complexity, context-dependency, and reversibility. I show that homeostatic hitchhiking can overcome the main difficulty in the evolution of this adaptation-the perceived necessity of de novo co-evolution of oogenesis, sex-determination, and order of ovulation in each environmental context-something that would require unrealistic expectations of evolutionary rates and population sizes and is not a desirable outcome for a process that needs to retain substantial environmental sensitivity. First, I explain the rationale behind the homeostatic-hitchhiking hypothesis and outline its predictions specifically for studies of sex-bias in order of egg-laying. Second, I show that a combination of self-regulatory and emergent processes and ubiquitous re-use of conserved growth factors make oogenesis particularly amendable to homeostatic hitchhiking. Third, I review empirical evidence for this mechanism in the rapid evolution of adaptive sex-biased order of egg-laying that accompanied colonization of North America by the house finch (Carpodacus mexicanus).

Between-female variation in house sparrow yolk testosterone concentration is negatively associated with CYP19A1 (aromatase) mRNA expression in ovarian follicles

Maternally-derived yolk androgens influence the development and long-term phenotype of offspring in oviparous species. Between-female variation in the amounts of these yolk androgens has been associated with a number of social and environmental factors, suggesting that the variation is adaptive, but the mechanisms behind it are unknown. Using two different approaches, we tested the hypothesis that variation in yolk androgen levels across individuals is associated with variation in their capacity to synthesize androgens. First, we injected female house sparrows with exogenous gonadotropin-releasing hormone (GnRH) to maximally stimulate ovarian steroidogenesis. Second, we collected pre-ovulatory follicle tissue and quantified the mRNA expression of four key enzymes of the steroid synthesis pathway: steroidogenic acute regulatory protein (StAR), cytochrome P450-side chain cleavage enzyme (CYP11A1), 17β-hydroxysteroid dehydrogenase (HSD17B1), and aromatase (CYP19A1). Thirty minutes after GnRH injection, androgen concentrations in both the plasma and in the yolks of pre-ovulatory follicles were significantly elevated compared to controls. However, this measure of steroidogenic capacity did not explain variation in yolk testosterone levels, although physiological differences between house sparrows and more widely studied poultry models were revealed by this approach. Steroidogenic enzyme mRNA levels were detectable in all samples and were significantly lower in the most mature pre-ovulatory follicles. Of the four measured genes, CYP19A1 expression exhibited a significant negative relationship with yolk testosterone concentrations in laid eggs, revealing a key mechanism for between-female variation in yolk testosterone. Furthermore, this suggests that any factors which alter the expression of CYP19A1 within an individual female could have dramatic effects on offspring phenotype.

Elevated testosterone during meiotic segregation stimulates laying hens to produce more sons than daughters

Biases in avian sex ratios have been documented in relation to a variety of social and environmental conditions. Previous studies suggest that treatment with hormones can stimulate females to manipulate offspring sex, and that this effect occurs before ovulation. For example, acute and chronic treatments with testosterone stimulated significant skews towards male offspring. Hormones may act by influencing which sex chromosome is donated by the heterogametic female bird into the oocyte. However, it is difficult to pinpoint when effects of testosterone on offspring sex occurred in previous experiments because testosterone treatments were given either chronically over the entire period of follicular development or many hours before the critical period of chromosome segregation. We injected laying hens with testosterone injections 5 h prior to ovulation to target this critical period and quantified the sexes of the subsequently ovulated eggs. We hypothesized that an injection of testosterone coincident with segregation of sex chromosomes would stimulate hens to produce more male than female offspring. As hypothesized, hens injected with testosterone produced a significant bias towards male offspring compared to controls, nearly 70%. These results suggest that acute testosterone elevation during meiotic segregation may mediate skews in avian primary sex ratios.

Adaptive sex allocation in relation to hatching synchrony and offspring quality in house wrens

Increased variance in the reproductive success of males relative to females favors mothers that optimally allocate sons and daughters to maximize their fitness return. In altricial songbirds, one influence on the fitness prospects of offspring arises through the order in which nestlings hatch from their eggs, which affects individual mass and size before nest leaving. In house wrens (Troglodytes aedon), the influence of hatching order depends on the degree of hatching synchrony, with greater variation in nestling mass and size within broods hatching asynchronously than in those hatching synchronously. Early-hatching nestlings in asynchronous broods were heavier and larger than their later-hatching siblings and nestlings in synchronous broods. The effect of hatching order was also sex specific, as the mass of males in asynchronous broods was more strongly influenced by hatching order than the mass of females, with increased variation in the mass of males relative to that of females. As predicted, mothers hatching their eggs asynchronously biased first-laid, first-hatching eggs toward sons and late-laid, late-hatching eggs toward daughters, whereas females hatching their eggs synchronously distributed the sexes randomly among the eggs of their clutch. We conclude that females allocate the sex of their offspring among the eggs of their clutch in a manner that maximizes their own fitness.

Social instability in laying quail: consequences on yolk steroids and offspring's phenotype

Individual phenotypic characteristics of many species are influenced by non-genetic maternal effects. Female birds can influence the development of their offspring before birth via the yolk steroid content of their eggs. We investigated this prenatal maternal effect by analysing the influence of laying females' social environment on their eggs' hormonal content and on their offspring's development. Social instability was applied to groups of laying Japanese quail females. We evaluated the impact of this procedure on laying females, on yolk steroid levels and on the general development of chicks. Agonistic interactions were more frequent between females kept in an unstable social environment (unstable females) than between females kept in a stable social environment (stable females). Testosterone concentrations were higher in unstable females' eggs than in those of stable females. Unstable females' chicks hatched later and developed more slowly during their first weeks of life than those of stable females. The emotional reactivity of unstable females' chicks was higher than that of stable females' chicks. In conclusion, our study showed that social instability applied to laying females affected, in a non-genetic way, their offspring's development, thus stressing the fact that females' living conditions during laying can have transgenerational effects.

No evidence for selective follicle abortion underlying primary sex ratio adjustment in pigeons

Primary sex ratio adjustment in birds has been extensively studied, yet the underlying physiological mechanisms are far from understood. Avian females are the heterogametic sex (ZW), and the future sex of the offspring is determined at chromosome segregation during meiosis I, shortly before the oocyte is ovulated. Assuming that the mother can detect the sex of the developing oocyte before ovulation, it has been suggested that a follicle of the un-preferred sex could selectively be induced to become atretic and regress instead of being ovulated (selective follicle abortion). This potential mechanism has been proposed to underlie biased primary sex ratios in birds, including the homing pigeon (Columba livia domestica), which produces a modal clutch size of two eggs. However, without replacement by an additional, already mature follicle, abortion of a preovulatory follicle would most likely result in either reduced clutch sizes or laying gaps, since a not-yet-recruited follicle still needed to undergo the whole maturation phase. In the current study we killed female pigeons, which were adjusting embryo sex of first eggs according to change in body mass. We examined ovaries for signs of follicle abortion but did not find any supporting evidence. All females produced one or two mature follicles but only two out of the 56 experimental birds produced an additional third mature follicle. Therefore, our results do not corroborate the hypothesis that pigeon mothers manipulate primary offspring sex by selectively aborting follicles of the un-preferred sex.

Testosterone has a long-term effect on primary sex ratio of first eggs in pigeons--in search of a mechanism

Despite accumulating evidence that birds, in which females are the heterogametic sex, are able to manipulate primary offspring sex ratio, the underlying mechanism remains elusive. Steroid hormones, which govern female reproduction and are also accumulated by the developing follicle could potentially affect primary sex ratio by differential follicle development in relation to future sex and meiotic drive, or by sex specific influence on oocyte abortion or fertilization. So far, experimental results on the involvement of maternal testosterone (T) in offspring sex manipulation are ambiguous. To investigate the effect of T on primary sex ratio and elucidate underlying mechanisms, we elevated circulating T levels in female homing pigeons (Columba livia). During the course of the experiment females produced three clutches--before and during T implantation, and one year after implant removal. Intriguingly, first eggs, but not second eggs of T females were significantly male biased relative to sham-implanted controls. One year after cessation of the treatment the male bias was still present, indicating long-term effects on female reproductive physiology. T treatment did not affect maternal body condition, nor was body condition correlated with offspring sex ratio. Our data on timing of oviposition, lack of infertile eggs, and yolk weight indicate a possible role for sex specific follicle abortion, perhaps in combination with meiotic drive. However, despite T treatment elevating maternal plasma levels, egg yolk T concentrations did not differ between treatment groups and did not vary with embryo sex, suggesting that yolk T is not involved in meiotic drive.

Evolutionary significance of phenotypic accommodation in novel environments: an empirical test of the Baldwin effect

When faced with changing environments, organisms rapidly mount physiological and behavioural responses, accommodating new environmental inputs in their functioning. The ubiquity of this process contrasts with our ignorance of its evolutionary significance: whereas within-generation accommodation of novel external inputs has clear fitness consequences, current evolutionary theory cannot easily link functional importance and inheritance of novel accommodations. One hundred and twelve years ago, J. M. Baldwin, H. F. Osborn and C. L. Morgan proposed a process (later termed the Baldwin effect) by which non-heritable developmental accommodation of novel inputs, which makes an organism fit in its current environment, can become internalized in a lineage and affect the course of evolution. The defining features of this process are initial overproduction of random (with respect to fitness) developmental variation, followed by within-generation accommodation of a subset of this variation by developmental or functional systems ('organic selection'), ensuring the organism's fit and survival. Subsequent natural selection sorts among resultant developmental variants, which, if recurrent and consistently favoured, can be inherited when existing genetic variance includes developmental components of individual modifications or when the ability to accommodate novel inputs is itself heritable. Here, I show that this process is consistent with the origin of novel adaptations during colonization of North America by the house finch. The induction of developmental variation by novel environments of this species's expanding range was followed by homeostatic channelling, phenotypic accommodation and directional cross-generational transfer of a subset of induced developmental outcomes favoured by natural selection. These results emphasize three principal points. First, contemporary novel adaptations result mostly from reorganization of existing structures that shape newly expressed variation, giving natural selection an appearance of a creative force. Second, evolutionary innovations and maintenance of adaptations are different processes. Third, both the Baldwin and parental effects are probably a transient state in an evolutionary cycle connecting initial phenotypic retention of adaptive changes and their eventual genetic determination and, thus, the origin of adaptation and evolutionary change.

Evolution of "determinants" in sex-determination: a novel hypothesis for the origin of environmental contingencies in avian sex-bias

Sex-determination is commonly categorized as either "genetic" or "environmental"-a classification that obscures the origin of this dichotomy and the evolution of sex-determining factors. The current focus on static outcomes of sex-determination provides little insight into the dynamic developmental processes by which some mechanisms acquire the role of sex determinants. Systems that combine "genetic" pathways of sex-determination (i.e., sex chromosomes) with "environmental" pathways (e.g., epigenetically induced segregation distortion) provide an opportunity to examine the evolutionary relationships between the two classes of processes and, ultimately, illuminate the evolution of sex-determining systems. Taxa with sex chromosomes typically undergo an evolutionary reduction in size of one of the sex chromosomes due to suppressed recombination, resulting in pronounced dimorphism of the sex chromosomes, and setting the stage for emergence of epigenetic compensatory mechanisms regulating meiotic segregation of heteromorphic sex chromosomes. Here we propose that these dispersed and redundant regulatory mechanisms enable environmental contingency in genetic sex-determination in birds and account for frequently documented context-dependence in avian sex-determination. We examine the evolution of directionality in such sex-determination as a result of exposure of epigenetic regulators of meiosis to natural selection and identify a central role of hormones in integrating female reproductive homeostasis, resource allocation to oocytes, and offspring sex. This approach clarifies the evolutionary relationship between sex-specific molecular genetic mechanisms of sex-determination and non-sex-specific epigenetic regulators of meiosis and demonstrates that both can determine sex. Our perspective shows how non-sex-specific mechanisms can acquire sex-determining function and, by establishing the explicit link between physiological integration of oogenesis and sex-determination, opens new avenues to the studies of adaptive sex-bias and sex-specific resource allocation in species with genetic sex-determination.

Maternal effects as generators of evolutionary change: a reassessment

Despite being a center of debate in biology for centuries, the connection between the generation of novel adaptive variation and its inheritance remains a contentious issue. In evolutionary and behavioral ecology, assigning natural and sexual selection a creative and anticipatory role unmasks the need to explicitly consider the link between a trait's functional importance and its inheritance and results in confusion about selection as an adaptive modifier of development versus selection as a passive filter of already produced forms. In developmental genetics, an emphasis on regulatory versus coding aspects of molecular evolutionary change overlooks the fundamental question of the origination of an inherited developmental toolkit and assignment of its regulatory functions. Because maternal effects, by definition, combine developmental induction of functionally important changes and their inheritance, they bridge the origin and evolution of organismal adaptability, at least on short time scales. The explosion of empirical studies of maternal effects raises a question--are maternal effects ubiquitous but short-term adjusters and fine-tuners of an evolved form with only secondary importance for evolutionary change? Or are they a particularly clear example of a stage in a continuum of inheritance systems that accumulates, internalizes, and passes on the most consistent and adaptive organism-environment interactions? Here I place recent empirical studies of avian maternal effects into the evolutionary framework of variation, selection, and inheritance to examine whether maternal effects provide a window into evolutionary processes.

Review. Meiotic drive and sex determination: molecular and cytological mechanisms of sex ratio adjustment in birds

Differences in relative fitness of male and female offspring across ecological and social environments should favour the evolution of sex-determining mechanisms that enable adjustment of brood sex ratio to the context of breeding. Despite the expectation that genetic sex determination should not produce consistent bias in primary sex ratios, extensive and adaptive modifications of offspring sex ratio in relation to social and physiological conditions during reproduction are often documented. Such discordance emphasizes the need for empirical investigation of the proximate mechanisms for modifying primary sex ratios, and suggests epigenetic effects on sex-determining mechanisms as the most likely candidates. Birds, in particular, are thought to have an unusually direct opportunity to modify offspring sex ratio because avian females are heterogametic and because the sex-determining division in avian meiosis occurs prior to ovulation and fertilization. However, despite evidence of strong epigenetic effects on sex determination in pre-ovulatory avian oocytes, the mechanisms behind such effects remain elusive. Our review of molecular and cytological mechanisms of avian meiosis uncovers a multitude of potential targets for selection on biased segregation of sex chromosomes, which may reflect the diversity of mechanisms and levels on which such selection operates in birds. Our findings indicate that pronounced differences between sex chromosomes in size, shape, size of protein bodies, alignment at the meiotic plate, microtubule attachment and epigenetic markings should commonly produce biased segregation of sex chromosomes as the default state, with secondary evolution of compensatory mechanisms necessary to maintain unbiased meiosis. We suggest that it is the epigenetic effects that modify such compensatory mechanisms that enable context-dependent and precise adjustment of primary sex ratio in birds. Furthermore, we highlight the features of avian meiosis that can be influenced by maternal hormones in response to environmental stimuli and may account for the precise and adaptive patterns of offspring sex ratio adjustment observed in some species.

Hormone-mediated maternal effects in birds: mechanisms matter but what do we know of them?

Over the past decade, birds have proven to be excellent models to study hormone-mediated maternal effects in an evolutionary framework. Almost all these studies focus on the function of maternal steroid hormones for offspring development, but lack of knowledge about the underlying mechanisms hampers further progress. We discuss several hypotheses concerning these mechanisms, point out their relevance for ecological and evolutionary interpretations, and review the relevant data. We first examine whether maternal hormones can accumulate in the egg independently of changes in hormone concentrations in the maternal circulation. This is important for Darwinian selection and female physiological trade-offs, and possible mechanisms for hormone accumulation in the egg, which may differ among hormones, are reviewed. Although independent regulation of plasma and yolk concentrations of hormones is conceivable, the data are as yet inconclusive for ovarian hormones. Next, we discuss embryonic utilization of maternal steroids, since enzyme and receptor systems in the embryo may have coevolved with maternal effect mechanisms in the mother. We consider dose-response relationships and action pathways of androgens and argue that these considerations may help to explain the apparent lack of interference of maternal steroids with sexual differentiation. Finally, we discuss mechanisms underlying the pleiotropic actions of maternal steroids, since linked effects may influence the coevolution of parent and offspring traits, owing to their role in the mediation of physiological trade-offs. Possible mechanisms here are interactions with other hormonal systems in the embryo. We urge endocrinologists to embark on suggested mechanistic studies and behavioural ecologists to adjust their interpretations to accommodate the current knowledge of mechanisms.

Evolution of sex-biased maternal effects in birds. IV. Intra-ovarian growth dynamics can link sex determination and sex-specific acquisition of resources

The evolutionary importance of maternal effects is determined by the interplay of maternal adaptations and strategies, offspring susceptibility to these strategies, and the similarity of selection pressures between the two generations. Interaction among these components, especially in species where males and females differ in the costs and requirements of growth, limits inference about the evolution of maternal strategies from their expression in the offspring phenotype alone. As an alternative approach, we examine divergence in the proximate mechanisms underlying maternal effects across three house finch populations with contrasting patterns of sex allocation: an ancestral population that shows no sex-biased ovulation, and two recently established populations at the northern and southern boundaries of the species range that have opposite sequences of ovulation of male and female eggs. For each population, we examined how oocyte acquisition of hormones, carotenoids and vitamins was affected by oocyte growth and overlap with the same and opposite sexes. Our results suggest that sex-specific acquisition of maternal resources and sex determination of oocytes are linked in this system. We report that acquisition of testosterone by oocytes that become males was not related to growth duration, but instead covaried with temporal exposure to steroids and overlap with other male oocytes. In female oocytes, testosterone acquisition increased with the duration of growth and overlap with male oocytes, but decreased with overlap with female oocytes. By contrast, acquisition of carotenoids and vitamins was mostly determined by organism-wide partitioning among oocytes and oocyte-specific patterns of testosterone accumulation, and these effects did not differ between the sexes. These results provide important insights into three unresolved phenomena in the evolution of maternal effects - (i) the evolution of sex-specific maternal allocation in species with simultaneously developing neonates of both sexes; (ii) the link between sex determination and sex-specific acquisition of maternal products; and (iii) the evolution of context-dependent modulation of maternal effects.

Environmental induction and phenotypic retention of adaptive maternal effects

BACKGROUND

The origin of complex adaptations is one of the most controversial questions in biology. Environmental induction of novel phenotypes, where phenotypic retention of adaptive developmental variation is enabled by organismal complexity and homeostasis, can be a starting point in the evolution of some adaptations, but empirical examples are rare. Comparisons of populations that differ in historical recurrence of environmental induction can offer insight into its evolutionary significance, and recent colonization of North America by the house finch (Carpodacus mexicanus) provides such an opportunity.

RESULTS

In both native (southern Arizona) and newly established (northern Montana, 18 generations) populations, breeding female finches exhibit the same complex adaptation - a sex-bias in ovulation sequence - in response to population-specific environmental stimulus of differing recurrence. We document that, in the new population, the adaptation is induced by a novel environment during females' first breeding and is subsequently retained across breeding attempts. In the native population, first-breeding females expressed a precise adaptive response to a recurrent environmental stimulus without environmental induction. We document strong selection on environmental cue recognition in both populations and find that rearrangement of the same proximate mechanism - clustering of oocytes that become males and females - can enable an adaptive response to distinct environmental stimuli.

CONCLUSION

The results show that developmental plasticity induced by novel environmental conditions confers significant fitness advantages to both maternal and offspring generations and might play an important role not only in the successful establishment of this invasive species across the widest ecological range of extant birds, but also can link environmental induction and genetic inheritance in the evolution of novel adaptations.

Heritability of dispersal rate and other life history traits in the Glanville fritillary butterfly

Knowing the variances and heritabilities (h(2)) of life history traits in populations living under natural conditions is necessary for a mechanistic understanding of respective evolutionary processes. I estimated heritabilities of several life history traits, including dispersal rate, body mass, age at first reproduction, egg mass, clutch size and lifetime reproductive success, in the Glanville fritillary butterfly (Melitaea cinxia) using parent-offspring regression. Experiments were conducted under field conditions in a large population cage (32 x 26 m). Heritability estimates ranged from zero to almost one and several were significantly different from zero. Body size for both sexes, female age at first reproduction and egg weight were all moderately to highly heritable, whereas heritabilities were low or non-existent in clutch size and lifetime egg production. Heritability estimates for dispersal rate varied between the sexes, so that dispersal was heritable from mother to her female offspring only. This finding is consistent with previous results showing that the F1 female but not male offspring of females that naturally established new populations in the field are significantly more dispersive than butterflies in old populations.

Context-dependent development of sexual ornamentation: implications for a trade-off between current and future breeding efforts

Allocation of resources into the development of sexual displays is determined by a trade-off between the competing demands of current reproduction and self-maintenance. When reproduction overlaps with acquisition of sexual ornamentation, such as in birds with a yearly post-breeding moult, such a trade-off can be expressed in elaboration of sexual traits used in subsequent matings. In turn, selection for elaboration of sexual ornaments should favour resolution of this trade-off through a modification of the ornaments' development, resulting in variable and life history-dependent development of sexual displays. Here we examined a novel hypothesis that the trade-off between current reproduction and development of sexual ornamentation in the house finch (Carpodacus mexicanus) can be mediated by the shared effects of prolactin - a pituitary hormone that regulates both parental care and moult in this species. We compared developmental variation in sexual ornamentation between breeding, nonbreeding, and juvenile males and examined the relative contribution of residual levels of prolactin and individual condition during moult to the acquisition of sexual ornamentation. Males that invested heavily in parental care entered post-breeding moult in lower condition and later in the season, but their higher plasma prolactin was associated with shorter and more intense moult ultimately resulting in equal or greater elaboration of sexual ornamentation compared with nonparental males. Elaboration of sexual ornamentation of nonparental males that entered moult in greater condition, but with lower prolactin, was produced by longer and earlier moult and by lesser overlap in moult between sexual ornaments. Ornamentation of juvenile males that acquire sexual ornamentation for the first time was closely associated with physiological condition during moult. We discuss the implications of such context-dependent ontogenies of sexual ornamentation and resulting differences in condition-dependence of sexual traits across life history stages on the evolution of female preference for elaborated sexual displays.

Selection for rapid embryo development correlates with embryo exposure to maternal androgens among passerine birds

Greater offspring predation favors evolution of faster development among species. We hypothesized that greater offspring predation exerts selection on mothers to increase levels of anabolic androgens in egg yolks to achieve faster development. Here, we tested whether (1) concentrations of yolk androgens in passerine species were associated with offspring predation and (2) embryo and nestling development rates were associated with yolk androgen concentrations. We examined three androgens that increase in potency along the synthesis pathway: androstenedione (A(4)) to testosterone (T) to 5 alpha -dihydrotestosterone (5 alpha -DHT). Concentrations of none of these steroids were related to clutch size; only A(4) was allometrically related to egg volume. Species that experience greater predation showed higher yolk concentrations of T and 5 alpha -DHT. Higher concentrations of T and particularly 5 alpha -DHT were strongly correlated with faster development during the embryo period and less so during the nestling period. Development rates were most strongly correlated with 5 alpha -DHT, suggesting that potency increases along the androgen synthesis pathway and that effects are mediated by the androgen receptor pathway. These results are consistent with the hypothesis that selection for faster development by time-dependent offspring mortality may be achieved epigenetically by varying embryo exposure to maternal anabolic steroids.

Sexual versus individual differentiation: the controversial role of avian maternal hormones

Avian embryos are exposed not only to endogenous sex steroids, which are produced by their gonads and have a key role in sexual differentiation, but also to maternal steroids transferred into the egg yolk, which can modulate the development of individual differences in behavior. Studies of maternal hormones have primarily focused on ultimate questions (evolutionary trade-offs, functional significance), whereas proximate mechanistic questions have been largely ignored. A central problem that must be addressed is how exposure to maternal hormones affects the individual phenotype without interfering with sexual differentiation. Separate effects could result from the action of different hormones, at different doses or at different times, on different targets.