Egg antimicrobials, embryo sex and chick phenotype in the yellow-legged gull

New insights into the relationships between egg maternal components: the interplays between albumen steroid hormones, proteins and eggshell protoporphyrin

Recent studies have shown that the egg yolk maternal components, which are a mixture of substances that can affect the developing embryo, do not act separately but are interconnected and co-adapted. Surprisingly, no study to date has focused on the associations between maternally derived albumen steroids and albumen and eggshell compounds with pleiotropic effects. Eggshell pigment protoporphyrin (PROTO IX) should provide primary antimicrobial protection for eggs, but as a proven pro-oxidant, it may compromise female fitness. Abundant albumen proteins ovotransferrin (OVOTR) and lysozyme (LSM) have been shown to have antimicrobial, antioxidant, immunoregulatory and growth-regulatory roles. To investigate associations between albumen steroids and OVOTR, LSM and eggshell cuticle PROTO IX, we used chicken eggs with differently pigmented eggshells. We found that albumen steroid hormones were strongly intercorrelated. In addition, we revealed that albumen LSM and testosterone (T) were positively associated, while a negative association was found between albumen LSM and pregnenolone (P5). Eggshell cuticle PROTO IX was negatively associated with the concentration of albumen 17α-hydroxypregnenolone (17-OHP5). Finally, of all the hormones tested, only the concentration of albumen 17-OHP5 correlated negatively with egg volume and varied with eggshell colour and chicken breed. Although experimental evidence for the effect of maternal albumen steroids on avian developing embryo is still scarce, our study is the first to highlight co-variation and potential co-adjustment of maternally derived albumen steroids, proteins and eggshell cuticle pigment suggesting similar allocation mechanisms known for yolk maternal compounds with the potential to influence the avian embryo and offspring phenotype.

Temperature-induced changes in egg white antimicrobial concentrations during pre-incubation do not influence bacterial trans-shell penetration but do affect hatchling phenotype in Mallards

Microbiome formation and assemblage are essential processes influencing proper embryonal and early-life development in neonates. In birds, transmission of microbes from the outer environment into the egg’s interior has been found to shape embryo viability and hatchling phenotype. However, microbial transmission may be affected by egg-white antimicrobial proteins (AMPs), whose concentration and antimicrobial action are temperature-modulated. As both partial incubation and clutch covering with nest-lining feathers during the pre-incubation period can significantly alter temperature conditions acting on eggs, we experimentally investigated the effects of these behavioural mechanisms on concentrations of both the primary and most abundant egg-white AMPs (lysozyme and avidin) using mallard (Anas platyrhychos) eggs. In addition, we assessed whether concentrations of egg-white AMPs altered the probability and intensity of bacterial trans-shell penetration, thereby affecting hatchling morphological traits in vivo. We observed higher concentrations of lysozyme in partially incubated eggs. Clutch covering with nest-lining feathers had no effect on egg-white AMP concentration and we observed no association between concentration of egg-white lysozyme and avidin with either the probability or intensity of bacterial trans-shell penetration. The higher egg-white lysozyme concentration was associated with decreased scaled body mass index of hatchlings. These outcomes demonstrate that incubation prior to clutch completion in precocial birds can alter concentrations of particular egg-white AMPs, though with no effect on bacterial transmission into the egg in vivo. Furthermore, a higher egg white lysozyme concentration compromised hatchling body condition, suggesting a potential growth-regulating role of lysozyme during embryogenesis in precocial birds.

Different incubation patterns affect selective antimicrobial properties of the egg interior: experimental evidence from eggs of precocial and altricial birds

Avian eggs contend with omnipresent microorganisms entering the egg interior, where they affect embryo viability and hatchling phenotype. The incubation behaviour and deposition of egg white antimicrobial proteins (AMPs) vary highly across the avian altricial-precocial spectrum. Experimental evidence of how these alterations in avian reproductive strategies affect the antimicrobial properties of the precocial and altricial egg interior is lacking, however. Here, we tested the egg white antimicrobial activity in eggs of two representative model species, from each end of the avian altricial-precocial spectrum, against potentially pathogenic and beneficial probiotic microorganisms. Eggs were experimentally treated to mimic un-incubated eggs in the nest, partial incubation during the egg-laying period, the onset of full incubation and the increased deposition of two main egg white AMPs, lysozyme and ovotransferrin. We moreover assessed to what extent egg antimicrobial components, egg white pH and AMP concentrations varied as a result of different incubation patterns. Fully incubated precocial and altricial eggs decreased their antimicrobial activity against a potentially pathogenic microorganism, whereas partial incubation significantly enhanced the persistence of a beneficial probiotic microorganism in precocial eggs. These effects were most probably conditioned by temperature-dependent alterations in egg white pH and AMP concentrations. While lysozyme concentration and pH decreased in fully incubated precocial but not altricial eggs, egg white ovotransferrin increased along with the intensity of incubation in both precocial and altricial eggs. This study is the first to experimentally demonstrate that different incubation patterns may have selective antimicrobial potential mediated by species-specific effects on antimicrobial components in the egg white.

Life history shapes variation in egg composition in the blue tit Cyanistes caeruleus

Maternal investment directly shapes early developmental conditions and therefore has long-term fitness consequences for the offspring. In oviparous species prenatal maternal investment is fixed at the time of laying. To ensure the best survival chances for most of their offspring, females must equip their eggs with the resources required to perform well under various circumstances, yet the actual mechanisms remain unknown. Here we describe the blue tit egg albumen and yolk proteomes and evaluate their potential to mediate maternal effects. We show that variation in egg composition (proteins, lipids, carotenoids) primarily depends on laying order and female age. Egg proteomic profiles are mainly driven by laying order, and investment in the egg proteome is functionally biased among eggs. Our results suggest that maternal effects on egg composition result from both passive and active (partly compensatory) mechanisms, and that variation in egg composition creates diverse biochemical environments for embryonic development.

Maternal body size influences offspring immune configuration in an oviparous snake

Like most ectothermic vertebrates, keelback snakes (Tropidonophis mairii) do not exhibit parental care. Thus, offspring must possess an immune system capable of dealing with challenges such as pathogens, without assistance from an attendant parent. We know very little about immune system characteristics of neonatal reptiles, including the magnitude of heritability and other maternal influences. To identify sources of variation in circulating white blood cell (WBC) concentrations and differentials, we examined blood smears from 246 hatchling snakes and their field-caught mothers. WBC concentrations were lower in hatchlings than in adults, and hatchlings had more basophils and fewer azurophils than adults. A hatchling keelback's WBC differential was also influenced by its sex and body size. Although hatchling WBC measures exhibited negligible heritability, they were strongly influenced by maternal body size and parasite infection (but not by maternal body condition, relative clutch mass or time in captivity). Larger mothers produced offspring with more azurophils and fewer lymphocytes. The mechanisms and consequences of WBC variation are currently unknown, but if these maternal effects enhance offspring fitness, the impact of maternal body size on reproductive success may be greater than expected simply from allometric increases in the numbers and sizes of progeny.

Nest Material Shapes Eggs Bacterial Environment

Selective pressures imposed by pathogenic microorganisms to embryos have selected in hosts for a battery of antimicrobial lines of defenses that includes physical and chemical barriers. Due to the antimicrobial properties of volatile compounds of green plants and of chemicals of feather degrading bacteria, the use of aromatic plants and feathers for nest building has been suggested as one of these barriers. However, experimental evidence suggesting such effects is scarce in the literature. During two consecutive years, we explored experimentally the effects of these nest materials on loads of different groups of bacteria (mesophilic bacteria, Enterobacteriaceae, Staphylococcus and Enterococcus) of eggshells in nests of spotless starlings (Sturnus unicolor) at the beginning and at the end of the incubation period. This was also explored in artificial nests without incubation activity. We also experimentally increased bacterial density of eggs in natural and artificial nests and explored the effects of nest lining treatments on eggshell bacterial load. Support for the hypothetical antimicrobial function of nest materials was mainly detected for the year and location with larger average values of eggshell bacterial density. The beneficial effects of feathers and plants were more easily detected in artificial nests with no incubation activity, suggesting an active role of incubation against bacterial colonization of eggshells. Pigmented and unpigmented feathers reduced eggshell bacterial load in starling nests and artificial nest boxes. Results from artificial nests allowed us to discuss and discard alternative scenarios explaining the detected association, particularly those related to the possible sexual role of feathers and aromatic plants in starling nests. All these results considered together confirm the antimicrobial functionality mainly of feathers but also of plants used as nest materials, and highlight the importance of temporally and geographically environmental variation associated with risk of bacterial proliferation determining the strength of such effects. Because of costs associated to nest building, birds should adjust nest building effort to expected bacterial environments during incubation, a prediction that should be further explored.

Effects of experimentally increased in ovo lysozyme on egg hatchability, chicks complement activity, and phenotype in a precocial bird

In birds, spectrum of egg white proteins deposited into the egg during its formation are thought to be essential maternal effects. Particularly, egg white lysozyme (LSM), exhibiting great between and within species variability, is considered to be essential for developing avian embryos due to its physiological, antimicrobial, and innate immune defense functions. However, there have been few studies investigating effects of LSM on early post-hatching phenotype, despite its broad physiological and protective role during embryogenesis. Here, we test how experimentally increased concentrations of egg white LSM affect hatchability in Japanese quail (Coturnix japonica) and chick phenotype immediately after hatching (particularly body weight, tarsus length, plasma LSM concentration, and plasma complement activity). Chicks from eggs with increased LSM concentration displayed reduced tarsus length compared to chicks from control eggs while hatchability, body weight and plasma LSM concentration were unaffected. It is worth noting that no effect of increased in ovo lysozyme on eggs hatchability could be related to pathogen-free environment during artificial incubation of experimental eggs causing minimal pressure on embryo viability. While tangible in vivo mechanisms during avian embryogenesis remain to be tested, our study is the first to document experimentally that egg white LSM appears to have growth-regulation role during embryo development, with possible underlying phenotypic consequences in the early post-hatching period in precocial birds.

Laying date, incubation and egg breakage as determinants of bacterial load on bird eggshells: experimental evidence

Exploring factors guiding interactions of bacterial communities with animals has become of primary importance for ecologists and evolutionary biologists during the last years because of their likely central role in the evolution of animal life history traits. We explored the association between laying date and eggshell bacterial load (mesophilic bacteria, Enterobacteriaceae, Staphylococci, and Enterococci) in natural and artificial magpie (Pica pica) nests containing fresh commercial quail (Coturnix coturnix) eggs. We manipulated hygiene conditions by spilling egg contents on magpie and artificial nests and explored experimental effects during the breeding season. Egg breakage is a common outcome of brood parasitism by great spotted cuckoos (Clamator glandarius) on the nests of magpie, one of its main hosts. We found that the treatment increased eggshell bacterial load in artificial nests, but not in magpie nests with incubating females, which suggests that parental activity prevents the proliferation of bacteria on the eggshells in relation to egg breakage. Moreover, laying date was positively related to eggshell bacterial load in active magpie nests, but negatively in artificial nests. The results suggest that variation in parental characteristics of magpies rather than climatic variation during the breeding season explained the detected positive association. Because the eggshell bacterial load is a proxy of hatching success, the detected positive association between eggshell bacterial loads and laying date in natural, but not in artificial nests, suggests that the generalized negative association between laying date and avian breeding success can be, at least partially, explained by differential bacterial effects.

Shifts in bacterial communities of eggshells and antimicrobial activities in eggs during incubation in a ground-nesting passerine

Microbial invasion of egg contents is a cause of embryonic death. To counter infection risks, the embryo is protected physically by the eggshell and chemically by antimicrobial proteins. If microbial pressure drives embryo mortality, then females may have evolved, through natural selection, to adapt their immune investment into eggs. Although frequently hypothesized, this match between immune allocation and microorganisms has not been explored yet. To examine if correlations between microbes on eggs and immunity in eggs exist, we collected eggs from red-capped larks (Calandrella cinerea) and simultaneously examined their bacterial communities and antimicrobial components—pH, lysozyme and ovotransferrin—during natural incubation. Using molecular techniques, we find that bacterial communities are highly dynamic: bacterial abundance increases from the onset to late incubation, Shannon’s α-diversity index increases during early incubation stages, and β-diversity analysis shows that communities from 1 day-old clutches are phylogenetically more similar to each other than the older ones. Regarding the antimicrobials, we notice a decrease of pH and lysozyme concentration, while ovotransferrin concentration increases during incubation. Interestingly, we show that two eggs of the same clutch share equivalent immune protection, independent of clutch age. Lastly, our results provide limited evidence of significant correlation between antimicrobial compounds and bacterial communities. Our study examined simultaneously, for the first time in a wild bird, the dynamics of bacterial communities present on eggshells and of albumen-associated antimicrobial components during incubation and investigated their relationship. However, the link between microorganisms and immunity of eggs remains to be elucidated further. Identifying invading microbes and their roles in embryo mortality, as well as understanding the role of the eggshell microbiome, might be key to better understand avian strategies of immune maternal investment.

Maternally derived egg hormones, antibodies and antimicrobial proteins: common and different pathways of maternal effects in Japanese quail

Avian eggs contain a variety of maternally-derived substances that can influence the development and performance of offspring. The levels of these egg compounds vary in relation to environmental and genetic factors, but little is known about whether there are correlative links between maternal substances in the egg underlying common and different pathways of maternal effects. In the present study, we investigated genetically determined variability and mutually adjusted deposition of sex hormones (testosterone-T, androstenedione-A₄ and progesterone-P₄), antibodies (IgY) and antimicrobial proteins (lysozyme) in eggs of Japanese quail (Coturnix japonica). We used different genetic lines that were independently selected for yolk T concentrations, duration of tonic immobility and social reinstatement behaviour, since both selections for behavioural traits (fearfulness and social motivation, respectively) produced considerable correlative responses in yolk androgen levels. A higher selection potential was found for increased rather than decreased yolk T concentrations, suggesting that there is a physiological minimum in egg T levels. Line differences in yolk IgY concentrations were manifested within each selection experiment, but no consistent inter-line pattern between yolk IgY and T was revealed. On the other hand, a consistent inverse inter-line pattern was recorded between yolk IgY and P₄ in both selections for behavioural traits. In addition, selections for contrasting fearfulness and social motivation were associated with changes in albumen lysozyme concentrations and an inverse inter-line pattern between the deposition of yolk IgY and albumen lysozyme was found in lines selected for the level of social motivation. Thus, our results demonstrate genetically-driven changes in deposition of yolk T, P₄, antibodies and albumen lysozyme in the egg. This genetic variability can partially explain mutually adjusted maternal deposition of sex hormones and immune-competent molecules but the inconsistent pattern of inter-line differences across all selections indicates that there are other underlying mechanisms, which require further studies.

Effect of restricted preen-gland access on maternal self maintenance and reproductive investment in mallards

Background

As egg production and offspring care are costly, females should invest resources adaptively into their eggs to optimize current offspring quality and their own lifetime reproductive success. Parasite infections can influence maternal investment decisions due to their multiple negative physiological effects. The act of preening – applying oils with anti-microbial properties to feathers – is thought to be a means by which birds combat pathogens and parasites, but little is known of how preening during the reproductive period (and its expected disease-protecting effects) influences maternal investment decisions at the level of the egg.

Methodology/Principal Findings

Here, we experimentally prevented female mallards (Anas platyrhynchos) from accessing their preen gland during breeding and monitored female immunoresponsiveness (e.g., plasma lysozyme concentration) as well as some egg traits linked to offspring quality (e.g., egg mass, yolk carotenoid content, and albumen lysozyme levels). Females with no access to their preen gland showed an increase in plasma lysozyme level compared to control, normally preening females. In addition, preen-gland-restricted females laid significantly lighter eggs and deposited higher carotenoid concentrations in the yolk compared to control females. Albumen lysozyme activity did not differ significantly between eggs laid by females with or without preen gland access.

Conclusion/Significance

Our results establish a new link between an important avian self-maintenance behaviour and aspects of maternal health and reproduction. We suggest that higher yolk carotenoid levels in eggs laid by preen-gland-restricted females may serve to boost health of offspring that would hatch in a comparatively microbe-rich environment.