Evolution of sex-biased maternal effects in birds: I. Sex-specific resource allocation among simultaneously growing oocytes

Dietary yolk supplements and rate of yolk deposition do not influence sex ratios in Japanese Quail

Birds appear to have the ability to bias offspring sex ratios in response to environmental and social conditions. The mechanisms responsible remain unknown, however one previous study indicated a link between rates of ovarian follicle growth and the sexes of resulting eggs. This could indicate that either follicles destined to retain male or female follicles grow at different rates, or the rate of ovarian follicle growth determines the sex chromosome that will be retained to influence the sex of the resulting offspring. We tested for evidence of both possibilities by staining yolk rings that are indicative of daily growth. First, we tested for a correlation between the number of yolk rings and the sexes of the resulting germinal discs collected from each egg, and second, we tested whether experimentally decreasing follicle growth rates with a dietary yolk supplement impacts the sexes of the resulting germinal discs. There was no significant correlation between the number of yolk rings and the sexes of the resulting embryos, and decreasing follicle growth rates did not impact sexes of resulting germinal discs. These results indicate that offspring sex is not related to the rate of ovarian follicle growth in quail.

Dietary yolk supplementation decreases rates of yolk deposition in Japanese quail

Generation of egg yolk by birds requires the synthesis and deposition of large amounts of protein and lipid, and is often accompanied by the incorporation of additional physiological mediators. While there has been much work examining the relative quantities of yolk components, as well as potential adaptive patterns of their allocation, we still do not have a full understanding of what controls yolk formation and composition. Once ovarian follicles are recruited into the preovulatory hierarchy, the yolk is deposited in concentric rings, with one ring deposited per day. Previous studies have shown that there is substantial interspecific and intraspecific variation in the number of rings in yolks, and thus the number of days it took those yolks to grow. We hypothesized that the ability to grow follicles to maturity quickly is limited by the availability of materials to make yolk precursors in the female, either in body reserves or in dietary access. To test this, we supplemented the diets of Japanese quail with hard-boiled chicken yolk and examined the influences of treatment and female body condition on follicle growth rates. Contrary to predictions, females with higher body condition indices produced yolks that grew more slowly, and yolks from supplemented birds grew more slowly than controls. These results indicate that females can modulate the rate of yolk incorporation into developing follicles, and that an energy balance that is too high may not be optimal for the fast growth of developing ovarian follicles.

Maternal care according to offspring sex and maternal physical condition in mantled howler monkeys (Alouatta palliata)

The Trivers-Willard hypothesis (TWH) posits that maternal care will be biased in favor of the sex that provides the greatest fitness returns per unit of investment, depending on maternal physical condition. Our aim was to examine the TWH in mantled howler monkeys living at Los Tuxtlas (Veracruz, Mexico). The biological attributes of mantled howler monkeys (Alouatta palliata) meet the assumptions of TWH better than those of other explanations, so we expected that females in better physical condition should bias maternal care toward sons, whereas mothers in worse physical condition should bias care toward daughters. Between December 2017 and March 2019, we studied mother-infant interactions in 20 dyads with focal-animal sampling and continuous recording (N = 204 h). We performed genetic analysis to determine offspring sex (N = 7 daughters and 13 sons) and measured C-peptide in urine samples of mothers to assess their physical condition (N = 46 samples). Mothers in better physical condition spent less time in contact with their sons but more time in contact with their daughters. For proximity behavior, mothers in better physical condition spent more time near their sons and less time near their daughters. These results suggest a bias in maternal care towards daughters, contrary to our predictions. In light of current models of maternal investment, our results support that mothers obtain higher fitness returns through daughters.

Fledgling sex-ratio is biased towards the helping sex in a Neotropical cooperative breeder, the brown-and-yellow marshbird (Pseudoleistes virescens)

In many cooperatively breeding species, helpers increase the breeding success of their parents. The repayment hypothesis predicts a skewed sex-ratio towards the helping sex at population level; at individual level bias would increase in broods attended by a smaller number of helpers. We studied a brown-and-yellow marshbird (Pseudoleistes virescens) population during 11 breeding seasons. We found that 90% of helpers were males and that they increased nestling survival, although this effect disappeared in presence of parasitic shiny cowbirds. Helpers sometimes helped at nests of adults other than their parents. Population sex-ratio of fledglings was highly skewed towards males (1.4:1). At individual level, male-biased sex-ratio of fledglings was more pronounced early in the season and increased with brood losses but was not affected by number of helpers. Marshbirds feed at communal areas so retaining helpers would not be costly. Therefore, a general skew towards males might be the best adaptive strategy.

Androgen and mineralocorticoid receptors are present on the germinal disc region in laying hens: Potential mediators of sex ratio adjustment in birds?

Female birds skew offspring sex ratios based on environmental and social stimuli; however, the mechanism mediating this phenomenon remains unknown. Growing evidence suggests that testosterone and corticosterone may influence meiosis, as they skew sex ratios when given immediately before chromosomal segregation. It is unclear if these hormones act on the germinal disc (GD) or through a downstream mediator. It is also unknown whether the GD contains receptors for these hormones. If testosterone and/or corticosterone act on the GD to skew sex ratios, then the GD should have receptors for them and that receptor levels should be higher in the GD regions compared to other follicular regions. Furthermore, fluctuations of receptor levels should occur near meiotic segregation. We collected ovarian follicles at 5 h pre-ovulation (just before meiotic segregation) and 20 h pre-ovulation (when sex chromosomes are arrested), and measured androgen receptor (AR) and mineralocorticoid receptor (MR) protein levels via Western blot. ARs and MRs were on the follicle in the GD and non-GD regions, and at 5 h and 20 h pre-ovulation. Both AR and MR protein levels were higher in the GD region than the non-GD region at both time points, but did not differ between time points. These results suggest that hen ovarian follicles have receptors for testosterone and corticosterone, and that the ability for testosterone to respond may be specifically higher in the GD-region, providing further support for the role of testosterone in the alteration of meiotic segregation.

Sex-specific effects of the in ovo environment on early-life phenotypes in eiders

Maternal effects affect offspring phenotype and fitness. However, the roles of offspring sex-specific sensitivity to maternal glucocorticoids and sex-biased maternal investment remain unclear. It is also uncertain whether telomere length (a marker associated with lifespan) depends on early growth in a sex-specific manner. We assessed whether maternal traits including corticosterone (CORT; the main avian glucocorticoid) and in ovo growth rate are sex-specifically related to offspring CORT exposure, relative telomere length (RTL) and body condition in eiders (Somateria mollissima). We measured feather CORT (fCORT), RTL and body condition of newly hatched ducklings, and growth rate in ovo was expressed as tarsus length at hatching per incubation duration. Maternal traits included baseline plasma CORT, RTL, body condition and breeding experience. We found that fCORT was negatively associated with growth rate in daughters, while it showed a positive association in sons. Lower offspring fCORT was associated with higher maternal baseline plasma CORT, and fCORT was higher in larger clutches and in those hatching later. The RTL of daughters was negatively associated with maternal RTL, whereas that of males was nearly independent of maternal RTL. Higher fCORT in ovo was associated with longer RTL at hatching in both sexes. Duckling body condition was mainly explained by egg weight, and sons had a slightly lower body condition. Our correlational results suggest that maternal effects may have heterogeneous and even diametrically opposed effects between the sexes during early development. Our findings also challenge the view that prenatal CORT exposure is invariably associated with shorter telomeres.

Multilevel and sex-specific selection on competitive traits in North American red squirrels

Individuals often interact more closely with some members of the population (e.g., offspring, siblings, or group members) than they do with other individuals. This structuring of interactions can lead to multilevel natural selection, where traits expressed at the group-level influence fitness alongside individual-level traits. Such multilevel selection can alter evolutionary trajectories, yet is rarely quantified in the wild, especially for species that do not interact in clearly demarcated groups. We quantified multilevel natural selection on two traits, postnatal growth rate and birth date, in a population of North American red squirrels (Tamiasciurus hudsonicus). The strongest level of selection was typically within-acoustic social neighborhoods (within 130 m of the nest), where growing faster and being born earlier than nearby litters was key, while selection on growth rate was also apparent both within-litters and within-study areas. Higher population densities increased the strength of selection for earlier breeding, but did not influence selection on growth rates. Females experienced especially strong selection on growth rate at the within-litter level, possibly linked to the biased bequeathal of the maternal territory to daughters. Our results demonstrate the importance of considering multilevel and sex-specific selection in wild species, including those that are territorial and sexually monomorphic.

Estimating Genetic and Maternal Effects Determining Variation in Immune Function of a Mixed-Mating Snail

Evolution of host defenses such as immune function requires heritable genetic variation in them. However, also non-genetic maternal effects can contribute to phenotypic variation, thus being an alternative target for natural selection. We investigated the role of individuals' genetic background and maternal effects in determining immune defense traits (phenoloxidase and antibacterial activity of hemolymph), as well as in survival and growth, in the simultaneously hermaphroditic snail Lymnaea stagnalis. We utilized the mixed mating system of this species by producing full-sib families in which each parental snail had produced offspring as both a dam and as a sire, and tested whether genetic background (family) and non-genetic maternal effects (dam nested within family) explain trait variation. Immune defense traits and growth were affected solely by individuals' genetic background. Survival of snails did not show family-level variation. Additionally, some snails were produced through self-fertilization. They showed reduced growth and survival suggesting recessive load or overdominance. Immune defense traits did not respond to inbreeding. Our results suggest that the variation in snail immune function and growth was due to genetic differences. Since immune traits did not respond to inbreeding, this variation is most likely due to additive or epistatic genetic variance.

Life history plasticity of a tropical seabird in response to El Niño anomalies during early life

Food shortage and other challenges associated with El Niño Southern Oscillation (ENSO) experienced early in life may have long-term impacts on life history traits, but these potential impacts remain virtually unexplored. By monitoring 2556 blue-footed boobies from 11 cohorts, we showed that birds facing warm water ENSO conditions (and probably low food availability) in the natal year were underweight at fledging, recruited earlier and bred less frequently, but showed no deficit in longevity or breeding success over the first 10 years. Life history impacts of ENSO were substantial when experienced in the prenatal year, the natal year, or the second year of life, and absent when experienced in the third year of life, implying that harsh conditions have greater effects when experienced earlier in life. Sexual differences in impacts depended on the age when warm water conditions were experienced: pre-natal and natal experience, respectively, induced early recruitment and influenced the relationship between age and laying date only in females, whereas second year experience reduced total breeding success only of males. Most surprising were positive transgenerational impacts in females: daughters of females that experienced ENSO conditions in their natal year showed improved breeding success. Developmental plasticity of boobies thus enables them to largely neutralize potential long-term impacts of harsh climatic conditions experienced early in life.

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.

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.

Cross-fostering eggs reveals that female collared flycatchers adjust clutch sex ratios according to parental ability to invest in offspring

Across animal taxa, reproductive success is generally more variable and more strongly dependent upon body condition for males than for females; in such cases, parents able to produce offspring in above-average condition are predicted to produce sons, whereas parents unable to produce offspring in good condition should produce daughters. We tested this hypothesis in the collared flycatcher (Ficedula albicollis) by cross-fostering eggs among nests and using the condition of foster young that parents raised to fledging as a functional measure of their ability to produce fit offspring. As predicted, females raising heavier-than-average foster fledglings with their social mate initially produced male-biased primary sex ratios, whereas those raising lighter-than-average foster fledglings produced female-biased primary sex ratios. Females also produced male-biased clutches when mated to males with large secondary sexual characters (wing patches), and tended to produce male-biased clutches earlier within breeding seasons relative to females breeding later. However, females did not adjust the sex of individuals within their clutches; sex was distributed randomly with respect to egg size, laying order and paternity. Future research investigating the proximate mechanisms linking ecological contexts and the quality of offspring parents are able to produce with primary sex-ratio variation could provide fundamental insight into the evolution of context-dependent sex-ratio adjustment.

The effects of incubation temperature on the sex of Japanese quail chicks

The effects of incubation temperature on the sex of Japanese quail chicks were investigated in this study. The study was conducted on Japanese quail. In all, 4500 eggs obtained from 2 generations were used. At the beginning of the study, a new flock was formed from available hatching eggs. Hatching eggs were gathered at 3 different ages (8 to 10 weeks, 16 to 18 weeks and 22 to 24 weeks of age) from the laying period in this flock. These eggs were exposed to 5 different incubation temperatures (36.7, 37.2, 37.7, 38.2, and 38.7°C). The hatching results were evaluated for each group. Chicks obtained from these temperature groups were reared separately to obtain quail for breeding. Eggs for incubation were gathered from these breeding quail when they were between 15 and 18 weeks of age. These eggs were placed in an incubator at a standard (37.7°C) temperature, separated by F(1)-generation temperature groups. The chicks in all groups were reared separately, and the sex of the chicks was determined at maturity. Statistical differences (P < 0.05) were found for the sex of the chicks in the third group (22 to 24 weeks) of the F(1) generation, compared with other groups. This result confirmed the hypothesis that different incubation temperatures for the first generation (at the embryo stage) might influence the sex of the next generation of chicks. Further studies are needed to investigate the effects of incubation temperature on chicks from different perspectives.

Maternal effects mediated by egg quality in the Yellow-legged Gull Larus michahellis in relation to laying order and embryo sex

BACKGROUND

Maternal effects mediated by egg size and quality may profoundly affect offspring development and performance, and mothers may adjust egg traits according to environmental or social influences. In avian species, context-dependency of maternal effects may result in variation in egg composition, as well as in differential patterns of covariation among selected egg components, according to, for example, position in the laying sequence or offspring sex. We investigated variation in major classes of egg yolk components (carotenoids, vitamins and steroid hormones) in relation to egg size, position in the laying sequence and embryo sex in clutches of the Yellow-legged Gull (Larus michahellis). We also investigated their covariation, to highlight mutual adjustments, maternal constraints or trade-offs in egg allocation.

RESULTS

Laying sequence-specific patterns of allocation emerged: concentration of carotenoids and vitamin E decreased, while concentrations of androgens increased. Vitamin A, estradiol and corticosterone did not show any change. There was no evidence of sex-specific allocation or covariation of yolk components. Concentrations of carotenoids and vitamins were positively correlated. Egg mass decreased along the laying sequence, and this decrease was negatively correlated with the mean concentrations of carotenoids in clutches, suggesting that nutritionally constrained females lay low quality clutches in terms of carotenoid content. Finally, clutches with smaller decline in antioxidants between first- and last-laid eggs had a larger increase in yolk corticosterone, suggesting that a smaller antioxidant depletion along the laying sequence may entail a cost for laying females in terms of increased stress levels.

CONCLUSIONS

Since some of the analyzed yolk components (e.g. testosterone and lutein) are known to exert sex-specific phenotypic effects on the progeny in this species, the lack of sex-specific egg allocation by mothers may either result from trade-offs between contrasting effects of different egg components on male and female offspring, or indicate that sex-specific traits are controlled primarily by mechanisms of sexual differentiation, including endogenous hormone production or metabolism of exogenous antioxidants, during embryonic development.

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.

Maternal effects and range expansion: a key factor in a dynamic process?

Species that depend on ephemeral habitat often evolve distinct dispersal strategies in which the propensity to disperse is closely integrated with a suite of morphological, behavioural and physiological traits that influence colonizing ability. These strategies are maintained by natural selection resulting from spatial and temporal variation in resource abundance and are particularly evident during range expansion. Yet the mechanisms that maintain close alignment of such strategies with resource availability, integrate suites of dispersal traits and generate variability in dispersal propensity are rarely known. Breeding females can influence offspring phenotype in response to changes in current environmental conditions, making maternal effects uniquely suited to bridge fluctuations in resource abundance in the maternal generation and variation in offspring dispersal ability. Western bluebirds' (Sialia mexicana) dependence on nest cavities--an ephemeral resource--has led to the evolution of two distinct dispersal phenotypes: aggressive males that disperse and non-aggressive males that remain philopatric and cooperate with their relatives. Over the last 40 years, western bluebirds rapidly expanded their geographical range, providing us with an opportunity to test, in newly established populations, the importance of maternal effects for generating variability in dispersal propensity. Here, I show that, under variable resource conditions, breeding females group offspring of different competitive ability in different positions in the egg-laying order and, consequently, produce aggressive males that are more likely to disperse when resources are low and non-aggressive philopatric males when resources are abundant. I then show experimentally that the association between resource availability and sex-specific birth order is robust across populations. Thus, this maternal effect enables close tracking of resource availability and may explain how variation in dispersal is generated in newly colonized populations. More generally, these results suggest that, as a key source of variation in colonizing phenotypes, maternal effects are of crucial importance for understanding the dynamics of range expansion.

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.

Maternal inheritance and rapid evolution of sexual size dimorphism: passive effects or active strategies?

Adaptive evolution is often strongly influenced by maternal inheritance that transfers the parental strategies across generations. The consequences of maternal effects for the offspring generation depend on the between-generation similarity in environments and on the evolved sensitivity of the offspring's ontogeny to maternal effects. When these factors differ between sons and daughters, maternal effects can influence the evolution of sexual dimorphism. The establishment of house finch populations across western Montana during the last 30 years was accompanied by rapid evolutionary change in sexual size dimorphism. Here I show that traits that changed the most across generations were most influenced by maternal effects in males but not females. Maternal effects differentially affected sons' and daughters' survival; greater maternal effects were commonly associated with higher survival of sons, especially when maternal and offspring environments were similar. Stronger maternal effects extended preselection phenotypic variance in morphological traits of males, thereby producing some locally adaptive phenotypes and lessening juvenile mortality. Thus, the observed sex-specific maternal effects and their contribution to the evolution of sexual size dimorphism are likely a passive consequence of the distinct sensitivity of sons and daughters to maternal adaptations to breeding in ecologically distinct parts of the house finch's expanding range.

Yolk androgens and embryo sex: maternal effects or confounding factors?

Maternal effects occur when offspring phenotype is affected by environmental factors experienced by the mother and, in egg-laying species, are often mediated via egg resources. There is currently great interest among behavioural ecologists in maternally allocated yolk androgens, especially their relationship with offspring sex and development. Such studies need embryonic tissue for sexing, however, requiring eggs to be incubated (usually for 3 days). Therefore, there are concerns about whether the androgen concentrations assayed reflect those allocated by the mother. In addition, studies showing sex biases in maternal allocation of androgens could be confounded if male and female embryos uptake or metabolise androgens at different rates. We ran a series of experiments using zebra finch (Taeniopygia guttata) eggs to address these potential confounding factors. First we showed, using eggs naturally incubated for up to 5 days, that eggs containing embryos had lower yolk androgen concentrations than eggs that had failed to form embryos. We then tested various hypotheses for this difference using controlled incubation treatments. Our results suggested that (a) embryo development causes the yolk to become progressively more diluted with albumin; and (b) between 3 and 5 days of incubation embryos start uptaking or metabolising androgens. Crucially, we found no decline in yolk androgen concentration at 3 days incubation, and no evidence for sex-specific rates of uptake or metabolism of androgens. This strongly suggests that yolk androgen levels up to 3 days incubation do reflect those allocated by the mother, and that studies of sex biased maternal allocation of yolk androgens are not confounded by sex differences in embryo development.

Adaptive genetic complementarity in mate choice coexists with selection for elaborate sexual traits

Choice of genetically unrelated mates is widely documented, yet it is not known how self-referential mate choice can co-occur with commonly observed directional selection on sexual displays. Across 10 breeding seasons in a wild bird population, we found strong fitness benefits of matings between genetically unrelated partners and show that self-referential choice of genetically unrelated mates alternates with sexual selection on elaborate plumage. Seasonal cycles of diminishing variation in ornamentation, caused by early pairing of the most elaborated males, and influx of increasingly genetically unrelated available mates caused by female-biased dispersal, lead to temporal fluctuations in the target of mate choice and enabled coexistence of directional selection for ornament elaboration with adaptive pairing of genetically unrelated partners.

Sex-specific sibling interactions and offspring fitness in vertebrates: patterns and implications for maternal sex ratios

Vertebrate sex ratios are notorious for their lack of fit to theoretical models, both with respect to the direction and the magnitude of the sex ratio adjustment. The reasons for this are likely to be linked to simplifying assumptions regarding vertebrate life histories. More specifically, if the sex ratio adjustment itself influences offspring fitness, due to sex-specific interactions among offspring, this could affect optimal sex ratios. A review of the literature suggests that sex-specific sibling interactions in vertebrates result from three major causes: (i) sex asymmetries in competitive ability, for example due to sexual dimorphism, (ii) sex-specific cooperation or helping, and (iii) sex asymmetries in non-competitive interactions, for example steroid leakage between fetuses. Incorporating sex-specific sibling interactions into a sex ratio model shows that they will affect maternal sex ratio strategies and, under some conditions, can repress other selection pressures for sex ratio adjustment. Furthermore, sex-specific interactions could also explain patterns of within-brood sex ratio (e.g. in relation to laying order). Failure to take sex-specific sibling interactions into account could partly explain the lack of sex ratio adjustment in accordance with theoretical expectations in vertebrates, and differences among taxa in sex-specific sibling interactions generate predictions for comparative and experimental studies.

Evolution of sex-biased maternal effects in birds: III. Adjustment of ovulation order can enable sex-specific allocation of hormones, carotenoids, and vitamins

Overlap in growth of offspring should constrain the opportunity for sex-biased maternal effects, yet sex-specific allocation of maternal resources among simultaneously growing ova is often observed in vertebrates. In birds, such allocation can be accomplished either by temporal clustering of ova that become the same sex, resulting in sex-biased egg-laying order, or by follicle-specific delivery of maternal resources. Two house finch populations at the northern and southern boundaries of the species range have opposite ovulation sequences of male and female eggs, and thus, in the absence of sex differences in ova growth or sex-specific maternal strategies, would be expected to have opposite sex-specific accumulation of maternal products. We found that the populations had strong and similar gradients of steroid distribution in relation to ovulation order, whereas distribution of carotenoids and vitamins correlated with each follicle's accumulation of steroids. In both populations, temporal bias in production of sons and daughters within a clutch enabled strongly sex-specific acquisition of maternal products, and oocytes of the same sex were highly interdependent in their accumulation of steroids. Moreover, in nests where the sex-bias in relation to ovulation order deviated from population-specific patterns, eggs had highly distinct concentrations of steroids, carotenoids and vitamins. These results and previous findings of sex-specific yolk partitioning among oocytes suggest that oocytes that become males and females are temporally or spatially clustered during their ovarian growth. We discuss the implication of these findings for the evolution of sex-specific maternal resource allocation.

Evolution of sex-biased maternal effects in birds: II. Contrasting sex-specific oocyte clustering in native and recently established populations

In species that produce broods of multiple offspring, parents need to partition resources among simultaneously growing neonates that often differ in growth requirements. In birds, multiple ovarian follicles develop inside the female at the same time, resulting in a trade-off of resources among them and potentially limiting maternal ability for sex-specific allocation. We compared resource acquisition among oocytes in relation to their future sex and ovulation order in two populations of house finches with contrasting sex-biased maternal strategies. In a native Arizona population, where mothers do not bias offspring sex in relation to ovulation order, the male and female oocytes did not show sex-specific trade-offs of resources during growth and there was no evidence for spatial or temporal segregation of male and female oocytes in the ovary. In contrast, in a recently established Montana population where mothers strongly bias offspring sex in relation to ovulation order, we found evidence for both intra-sexual trade-offs among male and female oocytes and sex-specific clustering of oocytes in the ovary. We discuss the importance of sex-specific resource competition among offspring for the evolution of sex-ratio adjustment and sex-specific maternal resource allocation.

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

Maternal modification of offspring sex in birds has strong fitness consequences, however the mechanisms by which female birds can bias sex of their progeny in close concordance with the environment of breeding are not known. In recently established populations of house finches (Carpodacus mexicanus), breeding females lay a sex-biased sequence of eggs when ambient temperature causes early onset of incubation. We studied the mechanisms behind close association of incubation and sex-determination strategies in this species and discovered that pre-ovulation oocytes that produce males and females differed strongly in the temporal patterns of proliferation and growth. In turn, sex-specific exposure of oocytes to maternal secretion of prolactin and androgens produced distinct accumulation of maternal steroids in oocyte yolks in relation to oocyte proliferation order. These findings suggest that sex difference in oocyte growth and egg-laying sequence is an adaptive outcome of hormonal constraints imposed by the overlap of early incubation and oogenesis in this population, and that the close integration of maternal incubation, oocytes' sex-determination and growth might be under control of the same hormonal mechanism. We further document that population establishment and the evolution of these maternal strategies is facilitated by their strong effects on female and offspring fitness in a recently established part of the species range.