How costly is clutch formation in the Audouin's Gull Larus audouinii?

During the Audouin's Gull's breeding season at the Ebro Delta in 1993, 24 fresh eggs from eight three-egg clutches (modal clutch-size) were collected at the peak of the laying period. Eggs were processed to obtain formalin-fixed yolks, which were halved and stained using the potassium dichromate method. Digitized images of the yolks were examined to assess the daily rates of yolk deposition. We used these data in combination with egg compositional analysis to build a model of energy demands during the formation of an average clutch in Audouin's Gull. To show how the different parameters of clutch formation affect the daily energy investment peak, we performed a simulation analysis in which the rapid yolk development (RYD) period, the follicle triggering interval (FTI), the laying interval (LI) and the albumen synthesis period (ASP) were allowed to vary simultaneously. In our sample, the mean RYD period was seven days with a range from six to eight days. There were no significant differences in yolk volume among eggs in a clutch, but albumen volume was significantly smaller in third eggs. According to our model the albumen synthesis of the a-egg coincides with the energy demand peak for clutch formation. This peak represents an increase by ca. 42% in female energy requirements. Values obtained from the simulation analysis showed that only the ASP of the a-egg and the RYD durations of the second and third follicles produced noticeable reductions in peak energy investment. We predict that in gulls, whose laying intervals seem to be kept constant, significant increases of the durations of the RYD periods of second and third eggs, or even significant reductions of yolk size of these eggs, may operate simultaneously to match the energy demands during clutch formation to the prevailing food conditions.     

Intraspecific variation in egg composition and yolk formation in the common guillemot (Uria aalge)

Common guillemot eggs were collected and analysed in two years on Skomer Island, Wales, to investigate the effects of environmental conditons during the pre-laying period on egg formation and composition. Pre-laying attendance at breeding colonies indicated more favourable environmental conditions in 1987 than in 1986. The size of eggs did not differ between years, but eggs were laid earlier in 1987 and the time taken for yolk deposition was also shorter in 1987. Two distinct effects of environmental conditons on yolk formation were indicated: first, on the timing of initiation of yolk deposition; and secondly, on the duration of the yolk deposition period. Egg composition also differed significantly between years with a higher proportion of albumen and a higher proportion of lipid and non-lipid solids in yolks in 1987. These results are discussed with respect to environmental conditions and the constraints on egg production.     

Nutritional needs for egg formation in the Shag Phalacrocorax aristotelis

Egg production does not impose a major food need in the Shag Phalacrocorax aristotelis because the eggs are small, are formed slowly and are laid at 3-day intervals. I used dye-dosing of females, laying time and, after fixing and staining the yolk, daily ring counts to estimate the amounts of protein and energy needed each day to produce a clutch of three eggs. Maximum daily nutrient needs during egg formation were only 1.15 g per day additional protein and 34 kJ per day energy. Yolk formation times of 40 eggs were 13.5 (±1.2 s.d.) days. Based on yolk-ring counts and laying dates of 15 eggs that contained dye, the lag period between yolk completion and laying was 3.1 ± 0.06 days. The distinct light and dark rings of the stained yolk resulted from differences in the transparency of the yolk spheres. In the dark rings, the spheres were relatively clear, so more depth of stained yolk could be seen than in the light rings, which reflected the incident light.     

Studies on the yolk granules of the silkworm,Bombyx mori L.: The morphology of diapause and non-diapause eggs during early developmental stages

Summary The morphological features during development of diapause and non-diapause eggs of the silkworm,Bombyx mori, were investigated by means of light and electron microscopy, with special reference to eggs up to 24 h after oviposition.The blastoderm and yolk cells began to be formed about 6 and 24 h after oviposition, respectively, in both the diapause and non-diapause eggs, indicating that the diapause and non-diapause eggs develop at similar rates at least until 24 h after oviposition.Specific changes in the distribution of yolk granules were observed during early development of the diapause egg. Its yolk granules gradually aggregated into clusters from the periphery toward the inside of the egg during the period of blastoderm formation. Aggregation of yolk granules was most noticeable about 12 h after oviposition and then they dispersed again before yolk cell formation. On the other hand, yolk granules of the non-diapause eggs remained dispersed during development.     

Yolk androgens reduce offspring survival

Females may favour some offspring over others by differential deposition of yolk hormones. In American kestrels (Falco sparverius), we found that yolks of eggs laid late in the sequence of a clutch had more testosterone (T) and androstenedione (A4) than yolks of first-laid eggs. To investigate the effects of these yolk androgens on nestling 'fitness', we injected both T and A4 into the yolks of first-laid eggs and compared their hatching time, nestling growth and nestling survival with those of first-laid eggs in which we injected vehicle as a control. Compared to controls, injection of T and A4 at a dose intended to increase their levels to those of later-laid eggs delayed hatching and reduced nestling growth and survival rates. Yolk androgen treatment of egg 1 had no effect on survival of siblings hatching from subsequently laid eggs. The adverse actions of yolk androgen treatment in the kestrel are in contrast to the favourable actions of yolk T treatment found previously in canaries (Serinus canaria). Additional studies are necessary in order to determine whether the deposition of yolk androgens is an adaptive form of parental favouritism or an adverse by-product of endocrine processes during egg formation. Despite its adaptive significance, such 'transgenerational' effects of steroid hormones may have helped to evolutionarily shape the hormonal mechanisms regulating reproduction.     

Egg formation and the pre-laying period of the Common guillemot Una aalge

The patterns of colony attendance of male and female Common guillemots, relative to calendar date and relative to the female's laying date during the three weeks prior to egg laying, are presented. The probability of a male being at the colony was consistently higher than that for females. Male attendance peaked in the three days before his mate laid: female attendance was lowest at this time. Examination of the yolk showed that the egg was formed over 14–15 days, with yolk deposition (of first eggs) taking 11–5 days, and a lag period (between the end of yolk deposition and laying) of 3–4 days. Yolk deposition occurred over a shorter period (9–3 days) in replacement eggs, and followed a different pattern from first eggs.     

Antigenic and electrophoretic variants of vitellogenin in Oncopeltus blood and their control by juvenile hormone

Antigenic analysis of adult female-specific blood and yolk proteins in Oncopeltus demonstrated an incomplete vitellogenin (A), which appears in the blood prior to yolk deposition and is later modified or joined by an antigenically complete molecule (AB). Vitellogenin AB is antigenically indistinguishable from the major yolk protein of mature eggs, though the electrophoretic mobilities of the two differ in 6% acrylamide gels. Vitellogenin A alone appears in the blood of adult females in which the corpora allata are known to be inactive, i.e., during starvation or photoperiodically induced diapause. Stimulation of these females with a juvenile hormone analog restores yolk deposition, and also induces the appearance of AB in the blood. While juvenile hormone is needed for the termination of diapause and the maturation of vitellogenin in this species, diapause begins with the vitellogenin-producing mechanism already partially assembled.     

Do group size and laying order influence maternal deposition of testosterone in smooth-billed ani eggs?

The avian egg contains maternal hormones that affect behavior, growth, morphology, and offspring survival. Evidence to date suggests that patterns of yolk androgen deposition could provide females with a means to manipulate sibling competition and, thereby, increase their fitness. We examined yolk testosterone (T) concentrations in eggs of the smooth-billed ani (Crotophaga ani) to understand patterns of androgen deposition in eggs of this plural-breeding joint-nesting cooperatively breeding species. We tested the hatching asynchrony adjustment hypothesis, which states that increases in yolk androgen levels over the laying sequence function to mitigate the disadvantage of being a later-hatched chick in species without adaptive brood reduction. We also investigated the effect of group size on yolk T deposition to test the hypothesis that females in multi-female groups could give a competitive edge to their own chicks by depositing higher T levels in their eggs. Predictions of the hatching asynchrony adjustment hypothesis were supported in both single- and multi-female groups as yolk testosterone levels increased from early- to late-laid eggs. This suggests that ani females can influence nestling competition and chick survival by within-clutch differential T allocation. Unexpectedly, we did not observe an effect of group size on yolk T deposition. Yolk testosterone concentrations may not be a mere reflection of a female's hormonal status as female plasma circulating levels of T did not vary in the same direction as yolk T levels. Results of this study therefore support the idea that females may adaptively manipulate chick behavior through hormonal deposition in eggs.     

Changes in teleost yolk proteins during oocyte maturation: correlation of yolk proteolysis with oocyte hydration

Yolk proteins of prematuration occytes and postmaturation eggs were compared by SDS gel electrophoresis in several teleosts, including freshwater species that produce demersal eggs, estuarine and marine species with demersal eggs, and marine species with pelagic eggs. In certain teleosts distinct changes in yolk protein banding patterns during oocyte maturation are suggestive of extensive secondary proteolysis of yolk proteins at this time; proteolysis is most pronounced in marine fishes with pelagic eggs. In many teleosts the oocyte swells by hydration during maturation; this hydration is also most pronounced in marine fishes with pelagic eggs. The extent of yolk proteolysis is well correlated with the extent of oocyte hydration during maturation.     

Variations in anuran embryogenesis: yolk-rich embryos of Hyperolius puncticulatus (Hyperoliidae)

Anuran development is usually described using model species, most notably Xenopus laevis and Rana pipiens. We describe the development of the East African Reed Frog, Hyperolius puncticulatus, a species displaying development that is highly divergent from the "classic" anuran developmental pattern. Although having small eggs, the eggs of H. puncticulatus are characterized by a large amount of yolk, and embryonic development is reminiscent of species with large eggs. The eggs are teleolecithal and the cleavage is holoblastic, with a "pseudo-meroblastic" pattern. Gastrulation proceeds primarily at the dorsal lip and is characterized by reduced embryonic cavities. Gastrulation ends with a thickened "embryonic mantle" that sits upon a large yolk mass and forms most of the tissues of the embryo. The embryonic axis curves across the yolk mass, instead of the typical lengthening of anuran embryos. The tadpole hatches with a large ventral yolk mass which is gradually absorbed. We hypothesize about the developmental mechanisms that underlie this unusual development, based on comparisons with other anuran and fish species. We suggest that this type of development is not unique to this species, but can be found in many species of different anuran taxonomic groups. Comparing the development of H. puncticulatus and similar species to what is known about the development of model species, such as X. laevis, shows us the variation in early anuran embryogenesis. Knowing the existing diversity is a prerequisite to understanding the evolution of early anuran development and the changes in patterning mechanisms in different lineages.     

Japanese quail selected for high plasma corticosterone response deposit high levels of corticosterone in their eggs

Poor habitat quality or body condition often correlates with high responsiveness of the hypothalamo-pituitary-adrenal (HPA) axis rather than with elevated baseline levels of glucocorticoids. We hypothesized that, for egg-laying vertebrates, high responsiveness of the HPA axis would correspond to high concentrations of corticosterone in yolk. We tested the prediction that Japanese quail (Coturnix coturnix japonica) selected for high plasma corticosterone response to brief immobilization (HS quail) would lay eggs with higher yolk corticosterone concentrations than birds selected for low response (LS quail). Quail from both lines were left undisturbed, outside of the stressors associated with daily management, before a first round of egg collection. In a second experiment, quail of both lines were experimentally stressed during the week before egg collection. In both cases we found quail from the HS line to lay eggs with significantly higher yolk corticosterone concentrations than quail of the LS line. After exposure to added experimental stressors, the line difference was more pronounced (increasing from 62% to 96%). There was no line difference in concentrations of yolk testosterone. Our results suggest that (1) genetic differences underly differences in the transfer of maternal corticosterone to yolk and (2) females may be able to control deposition of corticosterone into yolk through a mechanism independent of baseline corticosterone titers.     

Developmental fate of the yolk protein lipovitellin in embryos and larvae of winter flounder, Pleuronectes americanus.

The developmental fate of the vitellogenin-derived yolk protein, lipovitellin (Lv), was investigated in winter flounder embryos and yolk-sac larvae. Since Lv is present as only one major polypeptide in ovulated winter flounder eggs, unlike the multiple yolk polypeptides found in the mature eggs of most teleosts, this system is presented as a simpler model of yolk protein structure and utilization during teleostean development. Winter flounder Lv is cleaved during embryogenesis from a 94 kD polypeptide at fertilization to 67 kD and 26 kD polypeptides at hatching. The rate of this proteolytic processing is slow during early embryonic development, but enters a more rapid phase between days 8 and 12 post-fertilization in embryos reared at 4-5 degrees C, and approaches 50% completion at day 10. Lv processing is essentially complete 3 days before hatching; nevertheless, major degradation of the Lv peptide by the developing winter flounder does not occur until after hatching. The Stokes radius of Lv changes only moderately following processing, from 4.50 nm in unfertilized eggs to 4.19 nm in late embryos and newly hatched larvae, whereas the processed Lv retains its heat stability relative to other yolk polypeptides. Nearly 50% of its lipid content, however, is released from the Lv particle during embryogenesis, concomitant with cleavage of the Lv 94 kD polypeptide. Lv processing may thus render a portion of the yolk protein-associated lipid more accessible to the developing embryo, whereas other yolk components are retained for later use by the winter flounder larva. Alternately, removal of lipid may lead to proteolytic vulnerability of the Lv polypeptide. In either case, only a portion of the lipid moiety of the Lv particle appears to play a significant nutritive role for the embryo, whereas its protein component is reserved for larval use. J. Exp. Zool. 284:686-695, 1999.     

Yolk testosterone varies with sex in eggs of the lizard,Anolis carolinensis

In the green anole (Anolis carolinensis), a lizard with genotypic sex determination, yolk testosterone (T) concentration is greater in male-producing than female-producing eggs at oviposition, but the source and potential effects were not clear from previous studies. If yolk T levels are also sex-specific before eggs are laid, a period during which embryonic steroidogenesis is unlikely, it would strongly suggest that the difference in yolk T is maternally derived. We collected yolk samples from eggs shelling within the oviducts of anesthetized females, and then allowed these females to lay the eggs naturally. Eggs were incubated to hatching to determine sex morphologically, and yolk T concentrations were analyzed by radioimmunoassay. As is the case just after they are laid, yolk T is higher in male than female oviductal eggs. To our knowledge, this is the earliest sex difference reported for any yolk steroid. We suggest that maternally derived yolk T levels could influence sex by differentially affecting male- and female-inducing sperm, because fertilization occurs after yolk deposition and ovulation, while the egg is in the oviduct. Our results, together with those of an increasing number of studies, suggest that a relationship between hormones and vertebrate sex determination may be more widespread than generally appreciated.     

Reproductive allocation by the blow fly Lucilia sericata in response to protein limitation

Adult females of the anautogenous blow fly Lucilia sericata (Meigen) (Diptera: Calliphoridae) were fed standardized meals of liquidized liver, in quantities shown to range around the minimum required to initiate yolk deposition. Females from each feeding regime were dissected at daily intervals between 4 and 11 days of age; the number and stage of development of all oöcytes were recorded. Once an initial threshold quantity of protein was ingested, yolk deposition was initiated in all available oöcytes. Subsequently, one of two distinct developmental pathways was followed: arrested development in all oöcytes at an early stage of yolk deposition, or more extensive yolk deposition followed by progressive oösorbtion and the maturation of small batches of eggs. The proportion of females showing oösorbtion relative to arrested follicular development increased with increasing protein meal size, suggesting that the difference in response may be triggered by a second protein threshold, either side of which the arrested development or oösorbtion pathways are followed. The behaviour observed may reflect strategies to maximize reproductive output in this short‐lived, resource‐limited insect species. Flies that display arrested development may have sufficient protein to mature few if any complete eggs, but may subsequently be able to mature a full egg‐batch if they obtain further protein meals; this possibility is offset by the risk of death before finding such a meal. Flies that show oöcyte development and oösorbtion produce smaller egg batches more quickly and hence have a higher probability of achieving at least some reproductive output. By initiating yolk deposition in all oöcytes, female L. sericata retain the potential to adopt either developmental pathway, depending on subsequent protein intake.     

The Incorporation of Orally Administered Radioiodine Into Hens’ Eggs and Follicles

Incorporation of radioiodine into eggs and follicles was studied by oral administration of I131 to hens. With the single dose administration of I131 the shell, including the shell membranes, reached a maximal concentration of 0.017 % of the given amount per g of shell in eggs laid within one day after administration. The white reached a maximal concentration of 0.01 % per g in eggs laid about one day after administration. The subsequent reduction in concentration both in the shell and white from later laid eggs showed a rapid course. In the yolk, radioiodine began to appear in eggs laid about one day after administration. A maximal concentration of 2.78 % of the given amount per yolk was reached in eggs laid about five days after administration. The subsequent reduction in concentration could be followed in the yolk from eggs laid up to and including the ninth day after administration; then the concentration was about 1/50 of the maximal concentration. With continuous administration of I131 once daily for 16 days, the concentration in the shell and white was in the same amount as corresponding maximal concentration in single dose administration as long as the daily supply continued. Following cessation of administration, the concentration decreased in the shell and white with a course simliar to that obtained in the single dose administration. In the yolk, the concentration increased in eggs laid up to and including the ninth day after commencement of the continuous dosing; thereafter, the equilibrium remained as long as the administration continued. About 13 % of the daily given dose of I131 was recovered per yolk. After discontinuation of administration, an obvious concentration reduction was first obtained in the yolks from eggs laid five days later. A calculation of the results for the yolks from the single dose administration to be valid for the daily continuous dosing showed good agreement with the results obtained in the present multiple dose experiments. The follicular uptake of I131 in single dose administration was rapid, with the greatest uptake by the follicle which was in the most rapid phase of growth at the time of administration. During autoradiography of the follicles and eggs following single dose administration of I131, most of the radioiodine was recovered in the growth zone which corresponded to the follicle’s development during the first day after administration.     

Metabolic fates of yolk lipid and individual fatty acids during embryonic development of the coot and moorhen

There is currently little information regarding the metabolic fates of yolk lipid and individual fatty acids during embryonic development of free-living avian species. Here we report the pattern of lipid utilization during embryonic development of the coot (Fulica atra) and the moorhen (Gallinula chloropus), two related species producing precocial offspring from eggs with a distinctive fatty acid composition and with an incubation period similar to that of the chicken. By the time of hatching, the proportions of the initial yolk lipid that had been transferred to the embryo were 88.2% and 79.8% for the coot and moorhen respectively. During the whole incubation period, 42.9% and 40.0% of the initial yolk lipid of the coot and moorhen respectively were lost from the system due to oxidation for energy, equating to 47.8% and 50.0% respectively of the actual amount of lipid transferred over this time. Thus, the lipid received by the embryos of both species is partitioned almost equally between the alternative fates of energy metabolism and incorporation into tissue lipids. In the coot, this 50:50 split between oxidation and tissue formation was maintained during the hatching process. The proportions of arachidonic (20:4n-6) and docosahexaenoic (22:6n-3) in the yolk lipids of these species were 2.5-3.5 times higher than in eggs of domestic poultry. In contrast to the situation in the chicken, there was no preferential uptake of 22:6n-3 from the yolk during coot and moorhen development. The fatty acid compositions of the whole body lipids of the coot and moorhen hatchlings were almost identical to those of the initial yolks indicating that, unlike the chicken, these species display relatively little overall biomagnification of 20:4n-6 and 22:6n-6 during development. It is suggested that the yolk fatty acid profiles of the coot and moorhen are particularly well matched to the requirements of the embryo, reducing the need for selective uptake of 22:6n-3 and for the overall biomagnification of 22:6n-3 and 20:4n-6.     

Intra-Clutch Ratio of Yolk Progesterone Level Changes with Laying Date in Rockhopper Penguins: A Strategy to Influence Brood Reduction?

Hatching asynchrony in avian species generally leads to a size hierarchy among siblings, favouring the first-hatched chicks. Maternally deposited hormones affect the embryo and chick's physiology and behaviour. It has been observed that progesterone, a hormone present at higher levels than other steroid hormones in egg yolks, is negatively related to body mass in embryos, chicks and adults. A differential within-clutch progesterone deposition could therefore be linked to the size hierarchy between siblings and to the resulting brood reduction. We tested whether yolk progesterone levels differed between eggs according to future parental ability to feed the entire clutch in wild rockhopper penguins Eudyptes chrysocome. This species presents a unique reversed egg-size dimorphism and hatching asynchrony, with the larger second-laid egg (B-egg) hatching before the smaller first-laid egg (A-egg). Yolk progesterone levels increased only slightly with female body mass at laying. However, intra-clutch ratios were not related to female body mass. On the other hand, yolk progesterone levels increased significantly with the date of laying onset for A-eggs while they decreased for B-eggs. Early clutches therefore had proportionally more progesterone in the B-egg compared to the A-egg while late clutches had proportionally less progesterone in the B-egg. We propose that females could strategically regulate yolk progesterone deposition within clutches according to the expected food availability during chick growth, an adaptive strategy to adjust brood reduction to conditions. We also discuss these results, relating to yolk progesterone, in the broader context of other yolk steroids.     

Egg formation and the pre-laying period of Black-browed and Grey-headed Albatrosses Diomedea melanophris and D. chrysostoma at Bird Island, South Georgia

Egg composition and factors influencing egg formation were studied in Black-browed and Grey-headed Albatrosses Diomedea melanophris and D. chrysostoma at Bird Island, South Georgia. At nests where eggs were laid, females arrived 6–7 days after males, stayed one day during which 96% of observed copulations occurred, then departed to sea for c. 16 days in D. chrysostoma, c. 10 days in D. melanophris , returning c. two days before laying. Yolk deposition, however, lasted 21 and 20 days, and started 32 and 29 days before laying, in D. chrysostoma and D. melanophris respectively. Therefore, it is probably initiated by environmental factors not by copulation. Egg, albumen and yolk mass are significantly greater in D. chrysostoma but the proportionate composition of the species' eggs is nearly identical. Reduced differences in chick mass at hatching may reflect the longer incubation period in D. chrysostoma or relate to subsequent differences in chick growth rate.     

Ovarian follicle growth in the catfish Iheringichthys labrosus (Siluriformes: Pimelodidae)

The morphofunctional organisation of the female reproductive system, the oocyte growth and the follicular envelope ultrastructure were studied by the first time in the catfish Iheringichthys labrosus from Upper Paraná River basin, Southeastern Brazil, in order to contribute to the knowledge of the reproductive behaviour strategies of this species. As in other Neotropical freshwater siluriforms, the ovaries are of the cystovarian type, the oocytes develop in an asynchronous pattern and mature oocytes are released in clusters in the ovarian lumen, being transported through the oviduct to the urogenital papilla. During the primary growth, nuclear material is transported to the ooplasm, forming the yolk nucleus, where proliferate membranous organelles. The onset of the zona radiata formation occurs during the late perionucleolar stage with the deposition of the outer layer. At the vitellogenic stage, this envelope reaches 6.35+/-0.84microm of thickness, being constituted by three distinct layers crossed by pore-canals containing oocyte and follicular cells processes. Cytochemical analyses evidence neutral glycoproteins in cortical alveoli, yolk globules and zona radiata. Follicular cells with squamous shape during the primary growth acquire synthetic activity at the secondary growth, reaching 37.82+/-4.72mum in height at the mature vitellogenic follicles. These cells accumulate sulphated polysaccharides in large electron-lucent vesicles during the vitellogenic stage which are possibly secreted to form a mucous coat at the egg surface. These evidences suggest that I. labrosus may have adhesive eggs as also detected in other Neotropical freshwater Siluriformes.     

Yolk androgen deposition without an energetic cost for female rockhopper penguins: a compensatory strategy to accelerate brood reduction?

Whether androgen deposition in eggs is physiologically costly for female birds has remained a crucial but unsolved question, despite a broad use of this assumption in functional studies. We tested whether females depositing high androgen concentrations experienced higher mass losses than females depositing low androgen concentrations. Analysing female body mass change during egg formation in rockhopper penguins (Eudyptes chrysocome chrysocome), we observed no energetic cost to androgen deposition. Nevertheless, lighter females laid eggs with higher yolk androgen concentrations. This relationship existed only for the second-laid egg (B-egg), but not for the first-laid egg (A-egg). Since the B-egg is usually the first to hatch and the only one to produce a fledging chick, we hypothesize that differential yolk androgen deposition may be an adaptive strategy for females to affect brood reduction.