Plasticity in metabolism of maternal androgens in avian embryos

Mothers can influence offspring phenotypes by transferring non-genetic information to the young, which provides them with a flexible tool to adjust the developmental trajectory of the young in fluctuating environments. Mothers can differentially deposit their resources in the same reproductive attempt in relation to the offspring position in the sibling hierarchy. However, whether embryos from different positions can be plastic in their response to the maternal signals, potentially leading to a mother-offspring conflict, is yet unclear. We used Rock pigeons (Columba livia), that lay two egg clutches where maternal androgen levels in second laid eggs at oviposition are higher than in first laid eggs, and investigated the plasticity of embryonic metabolism of maternal androgens. We experimentally elevated androstenedione and testosterone levels in first eggs to that present in second eggs and measured the change in androgen levels and its main metabolites (etiocholanolone and conjugated testosterone) after 3.5 days of incubation. We found that eggs with increased androgens show a different degree of androgen metabolism depending either on the egg laying sequence or initial androgen levels or both. Our findings indicate that embryos have certain plasticity in response to maternal androgen levels depending on maternal signals.

Early conversion of maternal androgens affects the embryo already in the first week of development

Maternal androgen exposure has potent effects on offspring development. As substantial levels of maternal androgens are deposited in avian egg yolks, avian eggs are frequently used to study maternal effects, with a strong focus on post-natal development. However, the underlying pathways are largely unknown. Since the hormones are taken up during the embryonic phase, and these are rapidly metabolized by avian embryos into metabolites such as etiocholanolone, we studied the effects of yolk androgens (testosterone and androstenedione) and their metabolite etiocholanolone during the first few days of embryonic development. As embryonic heart rate is often used as an indicator of embryonic development, we measured the heart rate from day 3 to day 6 of incubation by using a shell-less culture technique in rock pigeon eggs (Columba livia). Increased androgen exposure increased heart rate, and increased etiocholanolone mimicked this effect, albeit in a small sample size. This indicates that exposure to maternal androgens increases embryonic overall metabolism which may account for the developmental outcomes found in previous studies such as increased growth. Moreover, etiocholanolone is likely to be an important metabolite in a non-genomic pathway underlying the androgen-mediated maternal effect.

Embryonic heart rate is affected by yolk androgens and egg laying sequence, and correlates with embryonic tissue growth: A study in rock pigeons

Maternal androgen exposure can have crucial effects on offspring development. Bird eggs are frequently used for studying these effects and virtually all research in this field has focused on post-hatching offspring traits. Yet, much of the yolk, in which the maternal hormones are deposited, is consumed during the embryonic phase. Here, we studied the effects of yolk androgens during this prenatal period. As there is evidence that androgens stimulate post-hatching traits such as increased growth, we measured heart rate throughout incubation as a proxy for prenatal metabolism. Rock pigeons (Columba livia) typically lay 2-egg clutches with yolk androgen levels in second-laid eggs being consistently higher than in first-laid eggs. We investigated whether embryonic heart rate was higher in second- than first-laid eggs. Additionally, we increased yolk androgen levels (testosterone and androstenedione) with the mean difference between those in first- and second-laid eggs, to investigate whether the effects of androgens are egg sequence dependent. As expected, embryonic heart rate predicted body embryo organ- and body mass, and body dimensions, with body mass being significantly higher in second- than first-laid eggs. Androgen treated first-laid eggs increased heart rate to that of second-laid control eggs only temporally, yet it had an overall positive effect on embryo body dimensions but not on tissue mass. Our findings indicate that embryos from different egg laying sequence differed in heart rate and prenatal development outcomes but this can only partially be explained by their difference in maternal androgen levels.

Is maternal thyroid hormone deposition subject to a trade-off between self and egg because of iodine? An experimental study in rock pigeon

Maternal hormones constitute a key signalling pathway for mothers to shape offspring phenotype and fitness. Thyroid hormones (THs; triiodothyronine, T₃; and thyroxine, T₄) are metabolic hormones known to play crucial roles in embryonic development and survival in all vertebrates. During early developmental stages, embryos exclusively rely on exposure to maternal THs, and maternal hypothyroidism can cause severe embryonic maldevelopment. The TH molecule includes iodine, an element that cannot be synthesised by the organism. Therefore, TH production may become costly when environmental iodine availability is low. This may yield a trade-off for breeding females between allocating the hormones to self or to their eggs, potentially to the extent that it even influences the number of laid eggs. In this study, we investigated whether low dietary iodine may limit TH production and transfer to the eggs in a captive population of rock pigeons (Columba livia). We provided breeding females with an iodine-restricted (I−) diet or iodine-supplemented (I+) diet and measured the resulting circulating and yolk iodine and TH concentrations and the number of eggs laid. Our iodine-restricted diet successfully decreased both circulating and yolk iodine concentrations compared with the supplemented diet, but not circulating or yolk THs. This indicates that mothers may not be able to independently regulate hormone exposure for self and their embryos. However, egg production was clearly reduced in the I− group, with fewer females laying eggs. This result shows that restricted availability of iodine does induce a cost in terms of egg production. Whether females reduced egg production to preserve THs for themselves or to prevent embryos from exposure to low iodine and/or THs is as yet unclear.

Summary: Restricted dietary iodine in captive rock pigeons reduces egg production in some females, thus inducing a trade-off between offspring quality and offspring quantity.

An examination of the influence of prenatal sex hormones on handedness: Literature review and amniotic fluid data

Competing theories have posited roles for foetal androgen exposure in the development of human handedness. However, due to practical and ethical considerations, few studies have used hormonal measures to examine this possibility. The current paper reviews this literature and reveals a generally inconsistent pattern of results. We also present data from a longitudinal study of prenatal sex hormone exposure and subsequent handedness. More specifically, we examine correlations between testosterone and estradiol measured from second trimester amniotic fluid and hand preference (Dutch language version of the Edinburgh Handedness Inventory) and hand skill asymmetry (pegboard task) measured at 15 years of age. Prenatal sex hormone exposure was not associated with the direction of hand preference in either males or females. However, in females, high levels of prenatal testosterone were associated with weaker lateralisation of hand skill, and high levels of prenatal estradiol were associated with weaker hand preference. In addition, high levels of prenatal testosterone were associated with increased task duration (i.e., slow hand speed) for the right and left hands of males. The pattern of results observed here is not entirely consistent with any of the main theories linking sex hormones with handedness, suggesting that an association between these variables may be more complex than initially thought.

Testing the short-and long-term effects of elevated prenatal exposure to different forms of thyroid hormones

Maternal thyroid hormones (THs) are known to be crucial in embryonic development in humans, but their influence on other, especially wild, animals remains poorly understood. So far, the studies that experimentally investigated the consequences of maternal THs focused on short-term effects, while early organisational effects with long-term consequences, as shown for other prenatal hormones, could also be expected. In this study, we aimed at investigating both the short- and long-term effects of prenatal THs in a bird species, the Japanese quail Coturnix japonica. We experimentally elevated yolk TH content (the prohormone T₄, and its active metabolite T₃, as well as a combination of both hormones). We analysed hatching success, embryonic development, offspring growth and oxidative stress as well as their potential organisational effects on reproduction, moult and oxidative stress in adulthood. We found that eggs injected with T₄ had a higher hatching success compared with control eggs, suggesting conversion of T₄ into T₃ by the embryo. We detected no evidence for other short-term or long-term effects of yolk THs. These results suggest that yolk THs are important in the embryonic stage of precocial birds, but other short- and long-term consequences remain unclear. Research on maternal THs will greatly benefit from studies investigating how embryos use and respond to this maternal signalling. Long-term studies on prenatal THs in other taxa in the wild are needed for a better understanding of this hormone-mediated maternal pathway.

Ejaculate testosterone levels affect maternal investment in red junglefowl (Gallus gallus gallus)

Substantial concentrations of testosterone are not only present in a male’s circulation, but also in its ejaculate. Surprisingly, the regulation of ejaculate T and its effects on females and their offspring, potentially a cryptic paternal trait, are not known. We found lower circulating and higher ejaculate T concentrations in subordinate red junglefowl (Gallus gallus gallus) males compared to dominant males, suggestive of an adaptive trade-off in T allocation to circulation and their ejaculate. Subsequently, we artificially inseminated females with either testosterone enriched (TE) or control ejaculates (CE) in a cross-over design. TE females produced heavier eggs than CE females. Offspring growth and tonic immobility were affected in a sex-specific way by TE. TE sons were heavier with shorter TI duration than CE sons, and TE daughters were lighter than CE daughters but daughters did not differ in TI score. However, the chicks competitiveness was not influenced by the TE nor CE. This indicates a previously unknown function of ejaculate testosterone as well as a new form of interaction between a cryptic paternal trait and a maternal effect that may be widespread in the animal kingdom.

Maternal steroids in egg yolk as a pathway to translate predation risk to offspring: experiments with great tits

Exposure of mothers to risk of predation can induce phenotypic changes in offspring as shown in several species. We previously found that cross-fostered great tit (Parus major) chicks of females exposed to increased predation risk were smaller and lighter, but had faster wing growth than control cross-fostered chicks, possibly improving predator-escaping abilities. Here we examined the possible role of maternal steroids deposited in eggs as an underlying mechanism. We collected eggs from female great tits under either experimentally increased predation risk (PRED) or control treatments (CON) and analyzed the concentration of testosterone, androstenedione, and progesterone in the yolks. PRED eggs contained lower levels of testosterone than CON eggs, but levels of androstenedione and progesterone did not differ. The smaller size and mass of chicks found in the previous study may thus be explained by the lower testosterone concentrations, since yolk testosterone is known to boost growth and development. Alternatively, testosterone may act as a modulator of differential investment into morphological traits, rather than a simple growth enhancer, explaining lower body mass in conjunction with the accelerated wing growth. This could possibly occur concurrently with other hormones such as corticosterone.

Age-related Purkinje cell death is steroid dependent: RORα haplo-insufficiency impairs plasma and cerebellar steroids and Purkinje cell survival

A major problem of ageing is progressive impairment of neuronal function and ultimately cell death. Since sex steroids are neuroprotective, their decrease with age may underlie age-related neuronal degeneration. To test this, we examined Purkinje cell numbers, plasma sex steroids and cerebellar neurosteroid concentrations during normal ageing (wild-type mice, WT), in our model of precocious ageing (Rora(+/sg), heterozygous staggerer mice in which expression of the neuroprotective factor RORα is disrupted) and after long-term hormone insufficiency (WT post-gonadectomy). During normal ageing (WT), circulating sex steroids declined prior to or in parallel with Purkinje cell loss, which began at 18 months of age. Although Purkinje cell death was advanced in WT long-term steroid deficiency, this premature neuronal loss did not begin until 9 months, indicating that vulnerability to sex steroid deficiency is a phenomenon of ageing Purkinje neurons. In precocious ageing (Rora(+/sg)), circulating sex steroids decreased prematurely, in conjunction with marked Purkinje cell death from 9 months. Although Rora(+/sg) Purkinje cells are vulnerable through their RORα haplo-insufficiency, it is only as they age (after 9 months) that sex steroid failure becomes critical. Finally, cerebellar neurosteroids did not decrease with age in either genotype or gender; but were profoundly reduced by 3 months in male Rora(+/sg) cerebella, which may contribute to the fragility of their Purkinje neurons. These data suggest that ageing Purkinje cells are maintained by circulating sex steroids, rather than local neurosteroids, and that in Rora(+/sg) their age-related death is advanced by premature sex steroid loss induced by RORα haplo-insufficiency.

Variation of a carotenoid-based trait in relation to oxidative stress and endocrine status during the breeding season in the Eurasian kestrel: a multi-factorial study

Carotenoid-based skin colorations vary seasonally in many bird species and are thought to be honest sexually selected signals. In order to provide more insight in the potential signal function and underlying mechanisms of such colorations we here quantified patterns of variation of leg coloration in adult male and female Eurasian kestrels (Falco tinnunculus tinnunculus) over the breeding season, and evaluated the relationship between coloration and levels of carotenoids, androgens and estrogens, oxidative damage and plasma non-enzymatic antioxidant capacity. We studied both reproducing wild and non-reproducing captive birds to test for the effect of diet and breeding effort. Males were more colored than females only during mating, and independently of diet, suggesting that leg-color is a sexually selected trait. Seasonal variation in leg color was associated with circulating carotenoids, but concentrations of these molecules were not related to antioxidant capacity, body condition or oxidative damage. These results indicate that carotenoid-based colorations may not be an honest signal of health status in this species. Production of carotenoid rich eggs coincided with low levels of circulating carotenoids in females, indicating that carotenoids might be a limited resource for laying female kestrels. Finally, young rearing males had higher levels of oxidative damage than females, and wild birds of both sexes had higher levels of these parameters than captive birds. These results may indicate that parental effort and physical activity are costly, independently from hormonal status. Since androgens did not explain carotenoid variation we suggest that multiple interacting factors can regulate carotenoid levels along the season.

An unstable social environment affects sex ratio in guinea pigs: an adaptive maternal effect?

Abstract

Evolutionary theory suggests that offspring sex should be adjusted to environmental conditions in order to maximize future reproductive success. In several animal taxa environmental factors indeed affect the secondary sex ratio. In humans, changes in the sex ratio at birth have been associated with population stressors like war, environmental disasters or economic strife during pregnancy. Here we compared litter sex ratios of female guinea pigs, exposed experimentally to a stable and an unstable social environment. In the latter group composition was changed every three days. Under unstable social conditions the sex ratio was significantly more biased towards daughters than in the stable social situation. This finding was consistent among four independent experiments, conducted independently from each other. Life expectancy can be dramatically reduced under conditions of social instability. Hence mothers in such conditions should bias their investment towards the sex that reaches sexual maturity first, which is the female sex in this species. Thus, to shift the offspring sex ratio towards more daughters under conditions of social instability may represent a maternal strategy to maximize future reproductive success.

The Ins and Outs of Intracellular Peptides and Antigen Presentation by MHC Class I Molecules

MHC class I molecules present small intracellular generated fragments to the outside surveying immune system. This is the result of a series of biochemical processes involving biosynthesis, degradation, translocation, intracellular transport, diffusion, and many more. Critical intermediates and end products of this cascade of events are peptides. The peptides are generated by the proteasome, degraded by peptidases unless transported into the ER where another peptidase and MHC class I molecules are waiting. Unless peptides bind to MHC class I molecules, they are released from the ER and enter the cytoplasm by a system resembling the ERAD pathway in many aspects. The cycle of peptides over the ER membrane with the proteasome at the input site and peptidases or MHC class I molecules on the output site are central in the MHC class I antigen presentation pathway and this review.

Influence of comfort and social stimuli on a comfort movement and a display derived from it

We tested the extent of emancipation of the Whistle-shake display of male shelducks, Tadorna tadorna, from causal factors controlling its presumed evolutionary precursor, the Body-shake, a comfort movement. Both motor patterns show similarities in form and alternate in a yearly rhythm. First, in an artificial rain experiment, we analysed the influence of a stimulus controlling comfort movements. In spring almost exclusively Whistle-shakes were induced even in the absence of social stimuli, while in summer the birds performed predominantly Body-shakes. This is probably related to the seasonal production of sex hormones. Second, we tested the influence of social stimuli by confronting shelducks with displaying and non-displaying males. During moult when males almost exclusively perform Body-shakes we found no influence of social stimulation on the frequency or form of shakes. However, an effect of social stimulation was found in a period when the drakes gradually replace the Body-shake by the Whistle-shake which is after moult in autumn. In this experiment, birds were confronted with either an empty cage (control), artificial rain, a non-displaying or a displaying male conspecific. In relation to the control situation the birds performed more Whistle-shakes when stimulated socially and more Body-shakes when stimulated with artificial rain. The extent to which both shaking patterns share the same causal factors is discussed. Copyright 1998 The Association for the Study of Animal Behaviour.

On the Ontogeny of Display Behaviour in the Black-Headed Gull: I. the Gradual Emergence of the Adult Forms

Abstract

The central question in this paper concerns the mechanism by which displays develop their species-specific sterotyped form. To this end the ontogeny of display behaviour in the black-headed gull was studied in birds kept and raised in aviaries. First it was analyzed whether the complete adult form of the display is present early in the chick, or whether it develops gradually over a longer period. Second, to answer the question whether experience with incomplete forms of display is necessary to develop the complete form, the normal display development was manipulated. The descriptive part of the results deals with a detailed quantitative description of the form development in 3 agonistic displays: choking, oblique and forward, and with the development of the begging display. Data are presented concerning: ontogenetic changes in frequency of several form elements of postures and vocalizations, changes in their combinations, changes in duration of displays and changes in the context and sequences of displays. The development of these displays is summarized in Fig. 2. Choking develops from crouching via incomplete forms of choking. First a choking-like vocalization develops in which the notes are more and more placed in a stereotyped rhythm. This call is performed by young chicks first during crouching, but later almost exclusively during choking-like postures in which the bill points downwards. During ontogeny, these bill-down postures are increasingly performed with extended carpal joints, with a tilted body, with head movements, and while standing and not hiding, in this order. From the age of two weeks on, the young regularly extend the neck upwards while choking. These postures are increasingly performed with a particular modification of the choking-like vocalization, which gradually develops to the adult long-call, typical for the adult oblique display. Two independent sound sources in the vocal apparatus of the gull seem to be involved in the development of the long-call. Early oblique-like postures are often performed with the bill held down, as in choking. Older young perform oblique-like postures more often with bill positions typical for adults, but with the neck held vertically. Young older than 10 weeks often perform normal adult oblique postures. By this time the duration of the postures has increased considerably. The sequence of the oblique with the forward, which is typical for adults, is also regularly seen from this time onwards. Begging consists of a rapid up-and-down movement of the head; during every downward movement a specific call is uttered. In very young chicks, this display is hardly distinguishable from the alert posture. During ontogeny, the duration of the pumping bouts increases, while the form changes. This change in form is due to the fact that the incomplete form of the forward becomes integrated in the begging display, while this posture also changes in form. This integration of the alert and the incomplete form of the forward is reflected by the fact that in the begging display both the call typical for the alert posture, and the call typical for the incomplete forward posture, is given. The forward develops by a gradual lowering of the head and a more pronounced extension of the neck, while the carpal joints are increasingly raised. Adult gulls often perform the posture with an upward pointed bill and a bent in the neck. In accord with data from the literature it was found that under special conditions the young chick is capable of performing adult display. As a rule, however, before the tenth week of age, young gulls show incomplete forms of display. All species-specific adult displays, postures as well as vocalizations, develop gradually towards a more complete and pronounced form. This process takes place by addition of new form elements to already existing combinations, and by changes in the form of some elements. A display can also be built upon elements of another display. Nevertheless, the development of the form of the displays cannot be attributed to an increasing amount of coordination of the different elements; only the expression of the complete pattern increases with age. For the occurrence of one form element of an adult display in the precursor display of the chick is positively correlated with the occurrence of other such elements in that incomplete display. This already is the case early in ontogeny, before the complete form is regularly performed. The adult displays are not completely fixed in their form, and the variability in some displays of adults is of the same order as in the chick. The second part of this paper deals with an experiment in which the normal course of the development of display behaviour was manipulated. Since the displays seem to develop during social interactions with parents (the begging display) or with intruders on the territory (the agonistic displays), young birds are raised in small groups with conspecifics of the same age. Agonistic- and begging interactions in these groups are rare an the development of display behaviour, both in frequency and in form, is retarded. Based on the frequency of overt aggression and on the plumage of the birds, it is likely that in these birds also the development of agonistic behaviour and the testosterone production is retarded. At the age of 15 weeks, these birds are confronted with unknown conspecifics. Although the frequency of the displays increases during the test, the form of the postures still appears to be incomplete; the birds do not seem to be able to perform complete display postures at an age at which normally raised birds perform complete display very often. Based on the data of both the descriptive study as well as the experiment, possible mechanisms for the development of display behaviour arc discussed. The main items are: "imitation"; "matching" on the basis of feedback of the performed display via reactions of conspecifics or proprioceptive information and development of motivational factors including testosterone. This discussion lays the basis for a series of experiments which will be reported separately.