Chronic changes in developmental oxygen have little effect on mitochondria and tracheal density in the endothermic moth Manduca sexta.

Oxygen availability during development is known to impact the development of insect respiratory and metabolic systems. Drosophila adult tracheal density exhibits developmental plasticity in response to hypoxic or hyperoxic oxygen levels during larval development. Respiratory systems of insects with higher aerobic demands, such as those that are facultative endotherms, may be even more responsive to oxygen levels above or below normoxia during development. The moth Manduca sexta is a large endothermic flying insect that serves as a good study system to start answering questions about developmental plasticity. In this study, we examined the effect of developmental oxygen levels (hypoxia: 10% oxygen, and hyperoxia: 30% oxygen) on the respiratory and metabolic phenotype of adult moths, focusing on morphological and physiological cellular and intercellular changes in phenotype. Mitochondrial respiration rate in permeabilized and isolated flight muscle was measured in adults. We found that permeabilized flight muscle fibers from the hypoxic group had increased mitochondrial oxygen consumption, but this was not replicated in isolated flight muscle mitochondria. Morphological changes in the trachea were examined using confocal imaging. We used transmission electron microscopy to quantify muscle and mitochondrial density in the flight muscle. The respiratory morphology was not significantly different between developmental oxygen groups. These results suggest that the developing M. sexta trachea and mitochondrial respiration have limited developmental plasticity when faced with rearing at 10% or 30% oxygen.

Altering developmental oxygen exposure influences thermoregulation and flight performance of Manduca sexta.

Endothermic, flying insects are capable of some of the highest recorded metabolic rates. This high aerobic demand is made possible by the insect's tracheal system, which supplies the flight muscles with oxygen. Many studies focus on metabolic responses to acute changes in oxygen to test the limits of the insect flight metabolic system, with some flying insects exhibiting oxygen limitation in flight metabolism. These acute studies do not account for possible changes induced by developmental phenotypic plasticity in response to chronic changes in oxygen levels. The endothermic moth Manduca sexta is a model organism that is easy to raise and exhibits a high thorax temperature during flight (∼40°C). In this study, we examined the effects of developmental oxygen exposure during the larval, pupal and adult stages on the adult moth's aerobic performance. We measured flight critical oxygen partial pressure (Pcrit-), thorax temperature and thermoregulating metabolic rate to understand the extent of developmental plasticity as well as effects of developmental oxygen levels on endothermic capacity. We found that developing in hypoxia (10% oxygen) decreased thermoregulating thorax temperature when compared with moths raised in normoxia or hyperoxia (30% oxygen), when moths were warming up in atmospheres with 21-30% oxygen. In addition, moths raised in hypoxia had lower critical oxygen levels when flying. These results suggest that chronic developmental exposure to hypoxia affects the adult metabolic phenotype and potentially has implications for thermoregulatory and flight behavior.

Thyroid hormone manipulation influences development of endothermy and hatching in white leghorn chickens (Gallus gallus).

Chickens experience rapid change in their physiology and metabolism during hatching. We propose that thyroid hormones play a major role in regulating the developmental changes associated with attaining endothermy. To better understand the role thyroid hormones play in hatch timing and development of thermogenic capacity and metabolic rate we manipulated plasma thyroid hormone levels in chicken embryos beginning at 80% development (day 17 of a 21-day incubation) with either a single dose of triiodo-L-thyronine (T3) or the thyroperoxidase inhibitor methimazole (MMI). Manipulation of thyroid hormones altered the timing of hatching, accelerating hatching under hyperthyroid conditions, and prolonging hatching with hypothyroid conditions. Effect sizes comparisons of morphological variables between treatment groups revealed larger heart and body masses in hyperthyroid 1-day post hatch animals. Thyroid hormone manipulation influenced the thermal neutral zone for O2 consumption and body temperature during gradual cooling from 35 to 15 °C of externally pipped embryos and 1-day post hatch chicks. Hyperthyroid EP animals had a wider thermal neutral zone during cooling when compared to control animals. At the temperatures tested, the hypothyroid animals did not exhibit a thermal neutral zone. Similar differences between treatments in the breadth of the thermal neutral zone carried through to 1-day post hatch chickens. These findings suggest that thyroid manipulations influence the timing and development of the animal's thermogenic response to cooling.

Changes in growth and developmental timing in Manduca sexta when exposed to altered oxygen levels.

The effect of chronic oxygen exposure on growth and development of insects is an active field of research. It seeks to unravel the triggers and limitations to molting and growth across many insect groups, although even now there are gaps in our knowledge and inconsistencies that need to be addressed. The oxygen dependent induction of molting (ODIM) hypothesis states that the impetus for molting is triggered by the development of hypoxic tissue due to the rapid increase in mass coupled with the fixed nature of tracheal systems between molts. In this study, we raised Manduca sexta in three chronic oxygen treatments (10, 21, & 30% O2). We measured the mass of these insects throughout their larval development and as adults. We found that both hyperoxia and hypoxia had marked effects on size and developmental times. Hyperoxia exposure resulted in increased mass throughout development and into adulthood while increasing developmental times. Hypoxia also increased developmental time and decreased mass of adult moths. We show that pupation is a critical window for exposure to altered oxygen levels. This suggests that oxygen may play a role in affecting the timing of eclosion at the end of pupation.

Manipulating plasma thyroid hormone levels alters development of endothermy and ventilation in nestling red-winged blackbirds.

Thyroid hormones are key regulators of development and metabolism in vertebrates. During the nestling period, young of altricial species transition from an ectothermic phenotype to an endothermic phenotype. Red-winged blackbirds are an altricial species that exhibit an increase in plasma 3,3', 5-triiodo-L-thyronine (T3) levels during the first 5 days post-hatch (dph), begin to develop endothermic metabolic responses by 7 dph, and fledge within 10 days of hatching. We propose that thyroid hormones play an important role in regulating development of endothermy during the nestling period in altricial birds. To better understand the effects of thyroid hormones on endothermic metabolic development in an altricial species, we treated nestling red-winged blackbirds on 2, 3, and 5 dph with either methimazole (MMI) to induce hypothyroidism or supplemental T3 to induce hyperthyroidism. We then measured on 5, 7, and 9 dph morphology and whole animal O2 consumption ( V ˙ o 2 ) and ventilation in the thermal neutral zone and during gradual cooling. Treatment of nestlings with MMI resulted in lower plasma T3 levels on 5 dph that recovered by 7 dph, while supplementing with T3 did not affect plasma T3 levels on 5, 7 and 9 dph. Treatment with MMI resulted in smaller nestlings with smaller hearts and structural characters such as wing chord and femur length, but larger lungs and kidneys. Treatment with T3 produced smaller nestlings with smaller body masses and shorter femur and tarsus lengths. The development of V ˙ o 2 and ventilation endothermic responses to gradual cooling in MMI treated nestlings were delayed when compared with control nestlings. In 9 dph nestlings, hypothyroidism resulted in alterations in the responses of ventilation frequency and tidal volume to cooling when compared with the control nestlings. Supplemental T3 had no effect on the development of V ˙ o 2 and ventilation in the thermal neutral zone or in response to cooling. Our data suggest plasma thyroid hormone levels play an active role in the systemic development of endothermic capacity and the development of ventilatory control. In the nestling avian, multiple systems develop in concert to produce an endothermic phenotype, but reduced thyroid hormone delays maturation of endothermic capacity.

Developing chicken cardiac muscle mitochondria are resistant to variations in incubation oxygen levels.

Background: Chronic exposure to hypoxia during vertebrate development can produce abnormal cardiovascular morphology and function. The aim of this study was to examine cardiac mitochondria function in an avian model, the chicken, in response to embryonic development under hypoxic (15% O2), normoxic (21% O2), or hyperoxic (40% O2) incubation conditions. Methods: Chicken embryos were incubated in hypoxia, normoxia, or hyperoxia beginning on day 5 of incubation through hatching. Cardiac mitochondria oxygen flux and reactive oxygen species production were measured in permeabilized cardiac fibers from externally pipped and 1-day post hatchlings. Results: Altering oxygen during development had a large effect on body and heart masses of externally pipped embryos and 1-day old hatchlings. Hypoxic animals had smaller body masses and absolute heart masses, but proportionally similar sized hearts compared to normoxic animals during external pipping. Hyperoxic animals were larger with larger hearts than normoxic animals during external pipping. Mitochondrial oxygen flux in permeabilized cardiac muscle fibers revealed limited effects of developing under altered oxygen conditions, with only oxygen flux through cytochrome oxidase being lower in hypoxic hearts compared with hyperoxic hearts. Oxygen flux in leak and oxidative phosphorylation states were not affected by developmental oxygen levels. Mitochondrial reactive oxygen species production under leak and oxidative phosphorylation states studied did not differ between any developmental oxygen treatment. Conclusions: These results suggest that cardiac mitochondria function of the developing chicken is not altered by developing in ovo under different oxygen levels.

Manipulating plasma thyroid hormone levels at hatching alters development of endothermy and ventilation in Pekin duck (Anas platyrhynchos domestica).

At hatching in precocial birds, there are rapid physiological and metabolic phenotypic changes associated with attaining endothermy. During the transition to ex ovo life, thyroid hormone levels naturally increase, peaking at hatching, and then decline. To better understand the role of the natural increase in thyroid hormone at hatching in regulating the developmental trajectory of the Pekin duck's endothermic phenotype, we examined development of O2 consumption (V̇O2 ) and ventilation (frequency, tidal volume and minute ventilation) while inhibiting the developmental increase in thyroid hormones that occurs at hatching via administration of the thyroid-peroxidase inhibitor methimazole (MMI) or accelerating the developmental increase via triiodothyronine (T3) supplementation. Animals were dosed only on day 24 of a 28-day incubation period and studied on incubation day 25, during external pipping (EP) and 1 day post-hatching (dph). On day 25, there was an increase in V̇O2  in the hyperthyroid treatment compared with the other two treatments. During the EP stage, there was a significant effect of thyroid status on V̇O2 , with hyperthyroid V̇O2  being highest and hypothyroid V̇O2  the lowest. By 1 dph, the supplemented T3 and control animals had similar V̇O2  responses to cooling with comparable thermal neutral zones followed by increased V̇O2 Hypothyroid 1 dph hatchlings had a lower resting V̇O2  that did not increase to the same extent as the supplemented T3 and control animals during cooling. During EP, inhibiting the rise in T3 resulted in embryos with lower ventilation frequency and tidal volume than control and supplemented T3 embryos. At 1 dph, ventilation frequency of all animals increased during cooling, but tidal volume only increased in supplemented T3 and control hatchlings. Our data support the role of the late incubation increase in T3 in regulating the systemic development of endothermic metabolic capacity and associated control of ventilation occurring at hatching of the Pekin duck.

Physiological determinants of the internesting interval in sea turtles: a novel 'water-limitation' hypothesis.

The internesting interval separates successive clutches of sea turtle eggs, and its duration varies both among and within species. Here, we review the potential physiological limits to this interval, and develop the hypothesis that desalination capacity limits the internesting interval owing to the requirement for water deposition in eggs. Sea turtles deposit 1-4 kg of water per clutch in egg albumen; for most species, this represents about 2% of adult body mass. We calculate how quickly turtles can recover this water by estimating maximal salt excretion rates, metabolic water production and urinary losses. From this water balance perspective, the 'water-limitation' hypothesis is plausible for green turtles but not for leatherbacks. Some plasma biochemistry studies indicate dehydration in sea turtles during the nesting season, although this is not a universal finding and these data have rarely been collected during the internesting interval itself. There is mixed support for a trade-off between clutch size and the length of the interval. We conclude that the 'water-limitation' hypothesis is plausible for most sea turtle species, but requires direct experimentation.

Sarcoplasmic reticulum Ca2+-ATPase (SERCA) activity during the transition to endothermy in an altricial bird.

Sarco(endo)plasmic reticulum Ca2+-ATPase (SERCA) is a transmembrane pump critical to muscle calcium cycling during contraction, and SERCA has also been proposed as the basis for a non-shivering thermogenesis mechanism in birds. Despite its potential importance to both shivering and non-shivering thermogenesis, the activity of this transporter has rarely been studied in altricial birds, and never during the developmental transition from ectothermy to endothermy. Here, we describe SERCA activity in the pectoralis muscle and heart ventricle of red-winged blackbird (Agelaius phoeniceus) nestlings, fledglings and adults. Additionally, using a diet manipulation, we tested the hypothesis that muscle SERCA activity is affected by dietary fatty acid composition, as has been shown in some previous studies. In blackbird hearts, SERCA activity increased throughout development and into adulthood, conspicuously jumping higher just prior to fledging. In pectoralis muscle, SERCA activity increased throughout the nestling period, but then declined after fledging, an effect we attribute to remodeling of the muscle from a primarily heat-generating organ to a primarily force-generating organ. SERCA activity of the pectoralis muscle was correlated with the proportion of linoleic acid in muscle phospholipids when including all ages in the control group. However, in diet-manipulated birds, there was no consistent relationship between SERCA activity and muscle membrane fatty acid composition at any tested age (5-9 days old). It is unclear whether SERCA might be affected by developmental changes in fatty acid composition at younger ages.

Scaling of cardiac morphology is interrupted by birth in the developing sheep Ovis aries.

Scaling of the heart across development can reveal the degree to which variation in cardiac morphology depends on body mass. In this study, we assessed the scaling of heart mass, left and right ventricular masses, and ventricular mass ratio, as a function of eviscerated body mass across fetal and postnatal development in Horro sheep Ovis aries (~50-fold body mass range; N = 21). Whole hearts were extracted from carcasses, cleaned, dissected into chambers and weighed. We found a biphasic relationship when heart mass was scaled against body mass, with a conspicuous 'breakpoint' around the time of birth, manifest not by a change in the scaling exponent (slope), but rather a jump in the elevation. Fetal heart mass (g) increased with eviscerated body mass (Mb , kg) according to the power equation 4.90 Mb0.88 ± 0.26 (± 95%CI) , whereas postnatal heart mass increased according to 10.0 Mb0.88 ± 0.10 . While the fetal and postnatal scaling exponents are identical (0.88) and reveal a clear dependence of heart mass on body mass, only the postnatal exponent is significantly less than 1.0, indicating the postnatal heart becomes a smaller component of body mass as the body grows, which is a pattern found frequently with postnatal cardiac development among mammals. The rapid doubling in heart mass around the time of birth is independent of any increase in body mass and is consistent with the normalization of wall stress in response to abrupt changes in volume loading and pressure loading at parturition. We discuss variation in scaling patterns of heart mass across development among mammals, and suggest that the variation results from a complex interplay between hard-wired genetics and epigenetic influences.

Thyroid hormone manipulation influences development of cardiovascular regulation in embryonic Pekin duck, Anas platyrhynchos domestica.

Thyroid hormones are key regulators of avian metabolism and may play a significant role in development at hatching. To better understand the role of thyroid hormones in avian development, we examined autonomic control of heart rate and blood pressure while manipulating thyroid hormone levels in the late stage embryonic Pekin duck (Anas platyrhynchos domestica). Thyroid hormone levels were manipulated on day 24 of a 28-day incubation period with the thyroperoxidase inhibitor methimazole (MMI), triiodothyronine (T3), or saline. On day 25 of incubation, autonomic tone on cardiovascular function was studied by injections of cholinergic and adrenergic receptor antagonists. Embryos from all treatment groups expressed a cholinergic and ß-adrenergic tone on heart rate at this age. Cholinergic blockade with atropine produced a larger change in heart rate in the hyperthyroid animals compared with euthyroid animals. In response to ß-adrenergic blockade, hyperthyroid conditions produced a larger decrease in heart rate compared with euthyroid animals, with no change in mean arterial blood pressure. In response to α-adrenergic blockade, mean arterial blood pressure decreased in the euthyroid animals and more developed hyperthyroid animals. Collectively, the data indicate that elevated levels of T3 can influence maturation of cholinergic and adrenergic receptor-mediated cardiovascular regulation in developing Pekin ducks near the end of incubation.

Rapid embryonic accretion of docosahexaenoic acid (DHA) in the brain of an altricial bird with an aquatic-based maternal diet.

Docosahexaenoic acid (DHA) is an important and abundant fatty acid moiety in vertebrate brains. We measured brain phospholipid composition during development in red-winged blackbirds (Agelaius phoeniceus), an altricial species that breeds in aquatic habitats. We also manipulated diet by feeding nestlings fish oil or sunflower oil. Finally, we assessed selective uptake of yolk by comparing the yolk fatty acid composition of freshly laid eggs and day-old hatchlings. Relative to other altricial species, blackbirds achieved high DHA in brain phospholipids (20% of phospholipid fatty acids in day-old hatchlings). This was not a result of selective uptake from the yolk, but rather a consequence of a high proportion of DHA in the yolk (2.5% of total lipids) at laying. Our dietary study confirmed that nestling brains are sensitive to fatty acid supply. Red-winged blackbirds may be able to advance cognitive development relative to other altricial species owing to their aquatic maternal diet.

Comparative physiology of the ductus arteriosus among vertebrates.

The ductus arteriosus is typically viewed as a mammalian fetal blood vessel providing a right-to-left shunt of right ventricular outflow away from the lungs and to the systemic circuit, that must close at birth. This review provides a wider comparative examination of the ductus arteriosus in lungfish, reptiles, birds, and mammals. The ductus arteriosus evolved with the lung in the ancestors of the lungfish as a connection between the pulmonary arteries and dorsal aorta. During embryonic development, reptiles, birds, and mammals all possess either one or two paired ductus arteriosi that provide a fetal shunt of blood away from the lungs. Differences in the fetal circulatory arrangement are seen between these groups and this influences the importance of the ductus arteriosus as an embryonic shunt. The ductus arteriosus from lungfish and tetrapod vertebrates is an oxygen sensitive blood vessel, with shared conserved pathways involved in oxygen sensing. By expanding studies into more comparative models such as lungfish or developing birds a better understanding of the physiology of the ductus arteriosus can be developed.

The membrane pacemaker hypothesis: novel tests during the ontogeny of endothermy.

The 'membrane pacemaker' hypothesis proposes a biochemical explanation for among-species variation in resting metabolism, based on the positive correlation between membrane docosahexaenoic acid (DHA) and metabolic rate. We tested this hypothesis using a novel model, altricial red-winged blackbird nestlings, predicting that the proportion of DHA in muscle and liver membranes should increase with the increasing metabolic rate of the nestling as it develops endothermy. We also used a dietary manipulation, supplementing the natural diet with fish oil (high DHA) or sunflower oil (high linoleic acid) to alter membrane composition and then assessed metabolic rate. In support of the membrane pacemaker hypothesis, DHA proportions increased in membranes from pectoralis muscle, muscle mitochondria and liver during post-hatch development. By contrast, elevated dietary DHA had no effect on resting metabolic rate, despite causing significant changes to membrane lipid composition. During cold challenges, higher metabolic rates were achieved by birds that had lower DHA and higher linoleic acid in membrane phospholipids. Given the mixed support for this hypothesis, we conclude that correlations between membrane DHA and metabolic rate are likely spurious, and should be attributed to a still-unidentified confounding variable.

Ontogeny of skeletal and cardiac muscle mitochondria oxygen fluxes in two breeds of chicken.

From its earliest days of domestication, the domestic chicken (Gallus gallus domesticus) has been selectively bred for specific traits. Decades of genetic selection have resulted in significant dissimilarities in metabolism and growth between breeds, in particular fast-growing broilers and highly productive layers. A chicken develops the capacity to elevate metabolism in response to decreases in ambient temperature upon hatching, including well-developed methods of regulating thermogenesis. However, a differential timing between incipient endothermic capacities of broiler and layer strains exists. Although both broiler and layer chicks show the hallmark rapid attainment of endothermic capacity of precocial birds, endothermic capacity of broilers matures faster than that of layers. Here we characterized changes in morphology and mitochondria physiology during the developmental transition as the animals become endothermic. Changes in body mass occurred at a faster rate in broilers, with hatching embryos showing significant increases over embryonic body mass, while layers did not exhibit significant differences in mass until after hatch. Heart and liver both exhibited rapid growth upon hatching that occurred with little change in body mass in both breeds. Skeletal and cardiac mitochondrial respiration capacity in broilers increased from the embryonic stage through hatching. Oxidative phosphorylation was more tightly coupled to ATP production in broilers than layer muscles during external pipping. By selecting for faster growth and higher meat yield, the physiological transition from ectothermy to endothermy was also affected: differences in whole-animal, tissue, and organelle responses are evident in these two divergent breeds of chicken.

Development of endothermy in birds: patterns and mechanisms.

Endothermy is a conspicuous and important adaptation in birds. Even though juvenile and adult birds are endothermic and maintain a constant, high body temperature by means of internal heat production, they begin life expressing an ectothermic phenotype. Depending on where a species falls along a continuum of maturity at hatching, from precocial to altricial, they begin to express endothermic traits either close to the time of hatching or as nestlings over a period of 1-3 weeks. Developing endothermy requires attaining a high basal metabolic rate and associated aerobic scope to produce sufficient internal heat, insulation to retain the internally produced heat, and a thermostat that "turns on" heat production in response to cooling ambient temperatures. To support the high metabolic costs of endothermy, the animal must have the capacity to deliver sufficient oxygen and nutrients to the heat-generating tissues. In this review, we examine the development of physiological and morphological traits that are required for endothermy and discuss their potential to limit the development of endothermy. These include ventilatory and cardiovascular function, contribution of visceral organ masses, membrane lipid composition, substrate supply pathways, and skeletal muscle physiology. The developmental trajectories of each of these systems in precocial and altricial species can have significant effects on the development of an endothermic phenotype.

Thermal acclimation in American alligators: Effects of temperature regime on growth rate, mitochondrial function, and membrane composition.

We investigated the ability of juvenile American alligators (Alligator mississippiensis) to acclimate to temperature with respect to growth rate. We hypothesized that alligators would acclimate to cold temperature by increasing the metabolic capacity of skeletal muscles and the heart. Additionally, we hypothesized that lipid membranes in the thigh muscle and liver would respond to low temperature, either to maintain fluidity (via increased unsaturation) or to maintain enzyme reaction rates (via increased docosahexaenoic acid). Alligators were assigned to one of 3 temperature regimes beginning at 9 mo of age: constant warm (30°C), constant cold (20°C), and daily cycling for 12h at each temperature. Growth rate over the following 7 mo was highest in the cycling group, which we suggest occurred via high digestive function or feeding activity during warm periods and energy-saving during cold periods. The warm group also grew faster than the cold group. Heart and liver masses were proportional to body mass, while kidney was proportionately larger in the cold group compared to the warm animals. Whole-animal metabolic rate was higher in the warm and cycling groups compared to the cold group - even when controlling for body mass - when assayed at 30°C, but not at 20°C. Mitochondrial oxidative phosphorylation capacity in permeabilized fibers of thigh muscle and heart did not differ among treatments. Membrane fatty acid composition of the brain was largely unaffected by temperature treatment, but adjustments were made in the phospholipid headgroup composition that are consistent with homeoviscous adaptation. Thigh muscle cell membranes had elevated polyunsaturated fatty acids in the cold group relative to the cycling group, but this was not the case for thigh muscle mitochondrial membranes. Liver mitochondria from cold alligators had elevated docosahexaenoic acid, which might be important for maintenance of reaction rates of membrane-bound enzymes.

Breathing while altricial: the ontogeny of ventilatory chemosensitivity in red-winged blackbird (Agelaius phoeniceus) nestlings.

Altricial bird species, like red-winged blackbirds, hatch at an immature state of functional maturity with limited aerobic capacity and no endothermic capacity. Over the next 10-12 days in the nest, red-winged blackbirds develop increased metabolic capacity before fledging. Although ontogeny of respiration has been described in precocial birds, ontogeny of ventilatory chemosensitivity is unknown in altricial species. Here we examined developmental changes in chemosensitivity of tidal volume (Vt), breathing frequency (ƒ), minute ventilation (V̇e), and whole animal oxygen consumption (V̇o2) from hatching to just before fledging in red-winged blackbirds on days 1, 3, 5, 7, and 9 posthatching (dph) in response to hypercapnia (2 and 4% CO2) and hypoxia (15 and 10% O2). Under control conditions, there was a developmental increase in V̇e with age due to increased Vt Hypercapnic and hypoxic chemosensitivities were present as early as 1 dph. In response to hypoxia, 1, 3, and 9 dph nestlings increased V̇e at 10% O2, by increasing ƒ with some change in Vt in younger animals. In contrast to early neonatal altricial mammals, the hypoxic response of nestling red-winged blackbirds was not biphasic. In response to hypercapnia, 3 dph nestlings increased V̇e by increasing both ƒ and Vt From 5 dph on, the hypercapnic increase in V̇e was accounted for by increased Vt and not ƒ. Chemosensitivity to O2 and CO2 matures early in nestling red-winged blackbirds, well before the ability to increase V̇o2 in response to cooling, and thus does not represent a limitation to the development of endothermy.

Developmental plasticity of mitochondrial function in American alligators, Alligator mississippiensis.

The effect of hypoxia on cellular metabolism is well documented in adult vertebrates, but information is entirely lacking for embryonic organisms. The effect of hypoxia on embryonic physiology is particularly interesting, as metabolic responses during development may have life-long consequences, due to developmental plasticity. To this end, we investigated the effects of chronic developmental hypoxia on cardiac mitochondrial function in embryonic and juvenile American alligators (Alligator mississippiensis). Alligator eggs were incubated in 21% or 10% oxygen from 20 to 90% of embryonic development. Embryos were either harvested at 90% development or allowed to hatch and then reared in 21% oxygen for 3 yr. Ventricular mitochondria were isolated from embryonic/juvenile alligator hearts. Mitochondrial respiration and enzymatic activities of electron transport chain complexes were measured with a microrespirometer and spectrophotometer, respectively. Developmental hypoxia induced growth restriction and increased relative heart mass, and this phenotype persisted into juvenile life. Embryonic mitochondrial function was not affected by developmental hypoxia, but at the juvenile life stage, animals from hypoxic incubations had lower levels of Leak respiration and higher respiratory control ratios, which is indicative of enhanced mitochondrial efficiency. Our results suggest developmental hypoxia can have life-long consequences for alligator morphology and metabolic function. Further investigations are necessary to reveal the adaptive significance of the enhanced mitochondrial efficiency in the hypoxic phenotype.

Post-hatching development of mitochondrial function, organ mass and metabolic rate in two ectotherms, the American alligator (Alligator mississippiensis) and the common snapping turtle (Chelydra serpentina).

The ontogeny of endothermy in birds is associated with disproportionate growth of thermogenic organs and increased mitochondrial oxidative capacity. However, no similar study has been made of the development of these traits in ectotherms. For comparison, we therefore investigated the metabolism, growth and muscle mitochondrial function in hatchlings of a turtle and a crocodilian, two ectotherms that never develop endothermy. Metabolic rate did not increase substantially in either species by 30 days post-hatching. Yolk-free body mass and heart mass did not change through 30 days in alligators and heart mass was a constant proportion of body mass, even after 1 year. Yolk-free body mass and liver mass grew 36% and 27%, respectively, in turtles during the first 30 days post-hatch. The mass-specific oxidative phosphorylation capacity of mitochondria, assessed using permeabilized muscle fibers, increased by a non-significant 47% in alligator thigh and a non-significant 50% in turtle thigh over 30 days, but did not increase in the heart. This developmental trajectory of mitochondrial function is slower and shallower than that previously observed in ducks, which demonstrate a 90% increase in mass-specific oxidative phosphorylation capacity in thigh muscles over just a few days, a 60% increase in mass-specific oxidative phosphorylation capacity of the heart over a few days, and disproportionate growth of the heart and other organs. Our data thus support the hypothesis that these developmental changes in ducks represent mechanistic drivers for attaining endothermy.