The energetics of embryonic growth and development. I. Oxygen consumption, biomass growth, and heat production.

A quantitative phenomenological model to describe the relationships between biomass growth rate, oxygen consumption, and heat production in developing embryos has been developed and tested using a wide range of experimental data. The model employs generalized material and energy balances, principles of enzyme kinetics, and an overall metabolic model scheme based on known biochemical principles. The phosphorylation concentration ratio of ATP and ADP occurs naturally and becomes a significant parameter in the analysis. The model is applied to the growth of Escherichia coli, Oryzias latipes, chick spinal cord, and whole chicken eggs. Excellent agreement between the model and the experimental data is obtained. In a succeeding paper (Part II) environmental effects and growth efficiency are discussed.     

The effect of desiccation and low temperature on the viability of eggs and emerging larvae of the tick, Rhipicephalus (Boophilus) microplus (Canestrini) (Ixodidae)

Process-based population models need sound and comprehensive data on an animal's response to climatic factors if they are to function reliably under a wide range of climatic conditions. To this end, different aged egg masses of the livestock tick, Rhipicephalus (Boophilus) microplus, were either desiccated in atmospheres with saturation deficits of 5, 10, 15 or 20 mmHg at 20 or 26 degrees C, or chilled at temperatures of 5, 10 or 14 degrees C with a saturation deficit of 1 mmHg for varying periods. The survival rate of the eggs through to hatching was related to the initial age of the eggs, the severity of the treatments and the duration of exposure. We established a relationship between desiccation and weight loss of eggs and, secondarily between weight loss and mortality. Mortality increased with weight loss until it reached 100% when the weight loss was about 35%. Low temperatures were increasingly detrimental to eggs as they reduced from 14 to 5 degrees C. Freshly laid eggs were more susceptible to both low temperatures and desiccation than were older eggs. Larvae emerging from eggs that were stressed by either cold or desiccation lived for a shorter time under optimal conditions than did larvae from eggs incubated under optimal conditions. Larvae from eggs with the same hatching rate had the same viability, whether the stress was induced by desiccation or low temperatures. Models were developed to describe the dynamics of weight loss of eggs with desiccation, the accumulation of cold stress of the eggs, and their effects on egg survival and larval viability. These data provide a sound basis for the development of predictive models for use under field conditions, although the response of different aged eggs to low temperatures was too variable to allow us to develop an accurate model to describe that relationship. Field models will also need to take diurnal temperature fluctuations into account.     

Evaluation of the rate of basal oxygen consumption in the isolated frog skin and toad bladder.

In the study of active transport it is important to distinguish between oxygen consumption sustaining transepithelial transport and that responsible for other tissue functions (basal metabolism). Since amiloride blocks transepithelial active sodium transport and the associated oxygen consumption in the frog skin and toad bladder, we and others have employed this agent to evaluate the rate of basal metabolism. This technique has recently been criticized in a report that amiloride (and ouabain) increased oxygen consumption when no sodium was available for transport. We have been unable to corroborate these observations. With magnesium-Ringer as external bathing solutions, amiloride and ouabain failed to stimulate oxygen consumption. With sodium-Ringer as external bathing solution amiloride reduced oxygen consumption about 30%, to a level indistinguishable from that found on external substitution of magnesium-Ringer for sodium-Ringer. We conclude that the use of amiloride permits evaluation of the rate of basal metabolism with acceptable accuracy; a possible slight depressant effect of ouabain on basal metabolism remains to be investigated.     

ON THE RATE OF OXYGEN CONSUMPTION BY FERTILIZED AND UNFERTILIZED EGGS : V. COMPARISONS AND INTERPRETATION

1. The rate of oxygen consumption by eggs may not merely undergo no change at fertilization, as in the case of the starfish, but it decreases to about half in Chaetopterus and in Cumingia. 2. The absolute rate of oxygen consumption in mm.³ O₂ per hour per 10 mm.³ eggs differs widely in several species of unfertilized eggs. It is very low in the sea urchin, intermediary in Nereis, and high in Chaetopterus and Cumingia. The range for these eggs is approximately 0.4 to 3.1 mm.³ O₂ per hour per 10 mm.³ eggs at 21°C., in the ratio of about 1:8. 3. The absolute rates of oxygen consumption by the same fertilized eggs are much more nearly the same. They lie within the range 1.3 to 2.0 mm.³ O₂ per hour per 10 mm.³ eggs at 21°C., in the ratio of approximately 1:1.5. Within this same range lie the values obtained by a number of investigators using a variety of eggs of invertebrates from several phyla. Amoeba proteus and frog skin also are within this range (see Fig. 2). 4. The changes in rate of oxygen consumption at fertilization by the different species of eggs, differing both in direction and magnitude, appear to be such as to bring the rate, when development is initiated, to about the same rate, which is also the rate of other comparable normally growing cells. 5. The direction and magnitude of the change in rate at fertilization therefore appears in the cases cited to be primarily a function of the absolute rate of oxygen consumption by the unfertilized eggs, which are characterized in their peculiar inhibited condition, among other things, by a wide range of respiratory rates. 6. It is not to be supposed that this range of rates will apply at all universally to eggs, especially to eggs of extremes in proportional content of inert materials, such as large yolky eggs. Fish and amphibian eggs for example respire at a much lower rate per unit volume. The effect on surface: volume ratios attending extremes of cell size might also be expected to shift the absolute rate. 7. The absolute rate of oxygen consumption by the eggs of the alga Fucus vesiculosus is considerably higher than the rates of the animal eggs measured. It is of the same order of magnitude as the rates of several other small-celled algae, which respire at a greater rate per unit volume than most non-motile animal cells. 8. The comparatively high rates of oxygen consumption by the inhibited (unfertilized) eggs of Chaetopterus and Cumingia are not directly associated with nuclear or morphological activity of the cell since they continue at the high rate for hours after cessation of the brief initial nuclear activity, which takes place when the eggs are placed in sea water. 9. It is concluded that the rate of oxygen consumption is not necessarily and probably not generally the limiting factor which causes inhibition of the unfertilized egg. Increase in rate of oxygen consumption is not directly related to the initiation of development, in general, nor even necessarily concomitant. It is not improbable that the low rate of oxygen consumption is an immediate part of the cause of inhibition of the unfertilized sea urchin egg, but this is a special case. 10. This thesis, that the rate of oxygen consumption is not necessarily nor ordinarily the limiting factor in the inhibition of the unfertilized egg, and conversely that increase in the rate of oxygen consumption is not usually the essential feature of fertilization, is quite in agreement with the general relations between the rate of oxygen consumption on the one hand and anesthesia, growth, and development on the other in fertilized eggs and other organisms. 11. This conclusion is opposed to Loeb''s explanation of the essential feature of fertilization, as an increase in oxidation rate or more strictly to generalization of his hypothesis to include eggs other than those of the sea urchins (or of other similar special cases which may be discovered). It extends to fertilization (the initiation of development) his and Wasteney''s well established conclusion that "oxidation is not the independent variable in development." 12. It is suggested that the crux of the problem of fertilization lies in the nature of the inhibition of the unfertilized egg. Certain similarities between this condition, arrived at spontaneously in the case of the egg cell, and the condition of cells in narcosis or anesthesia are pointed out. 13. Although the rate of oxygen consumption by the unfertilized eggs of Chaetopterus and Cumingia cannot be regarded as the limiting factor which causes the inhibition of the eggs, in these and other cases with different absolute rates, it appears highly probable that the rate of oxygen consumption is in some way, at present obscure, tied up with or related to the condition of inhibition. This seems probable especially in view of the sharp change in rate which in most cases immediately attends cessation of the inhibition, but the relationship may be a non-causal one, as in narcosis. 14. It must be borne in mind that oxygen consumption is not necessarily a complete measure of oxidation, and that other measures such as of heat and metabolite production are necessary before the complete amount of oxidation is known. When these are completely worked out, if free energy relations are known, it is probable that more direct and inclusive relations may be found between oxidation, growth, development, and anesthesia. Generalization of Loeb''s hypothesis, using "oxidation" in the broad sense might then turn out to hold, with fertilization fitting into the general scheme, but there is no basis for it at the present time.     

Reproductive tradeoffs and yolk steroids in female leopard geckos, Eublepharis macularius

Life history theory predicts tradeoffs among reproductive traits, but the physiological mechanisms underlying such tradeoffs remain unclear. Here we examine reproductive tradeoffs and their association with yolk steroids in an oviparous lizard. Female leopard geckos lay two eggs in a clutch, produce multiple clutches in a breeding season, and reproduce for several years. We detected a significant tradeoff between egg size and the number of clutches laid by females during their first two breeding seasons. Total reproductive effort was strongly condition-dependent in the first season, but much less so in the second season. Although these and other tradeoffs were unmistakable, they were not associated with levels of androstenedione, oestradiol, or testosterone in egg yolk. Female condition and egg size, however, were inversely related to dihydrotestosterone (DHT) levels in egg yolk. Finally, steroid levels in egg yolk were not directly related to steroid levels in the maternal circulation when follicles were developing, indicating that steroid transfer to eggs is regulated. These findings suggest that maternal allocation of DHT could mitigate tradeoffs that lead to poor offspring quality (i.e. poor female condition) and small offspring size (i.e. small egg size).     

Evaluation of the rate of basal oxygen consumption in the isolated frog skin and toad bladder

In the study of active transport it is important to distinguish between oxygen consumption sustaining transepithelial transport and that responsible for other tissue functions (basal metabolism). Since amiloride blocks transepithelial active sodium transport and the associated oxygen consumption in the frog skin and toad bladder, we and others have employed this agent to evaluate the rate of basal metabolism. This technique has recently been criticized in a report that amiloride (and ouabain) increased oxygen consumption when no sodium was available for transport. We have been unable to corroborate these observations.With magnesium-Ringer as external bathing solutions, amiloride and ouabain failed to stimulate oxygen consumption. With sodium-Ringer as external bathing solution amiloride reduced oxygen consumption about 30%, to a level indistinguishable from that found on external substitution of magnesium-Ringer for sodium-Ringer. We conclude that the use of amiloride permits evaluation of the rate of basal metabolism with acceptable accuracy; a possible slight depressant effect of ouabain on basal metabolism remains to be investigated.     

Egg-size evolution in aquatic environments: does oxygen availability constrain size?

Selection against large eggs has been proposed for aquatic environments, putatively because large eggs should have more difficulty obtaining the required oxygen. Here, we use brown trout (Salmo trutta) eggs to provide an experimental test of this hypothesis. At high levels of dissolved oxygen (14 mg l(-1)), egg survival was high and independent of egg size. At low oxygen levels (2.3 mg l(-1)), survival decreased overall, and was higher for large-egged than small-egged siblings. Thus, contrary to conventional expectation, low oxygen levels selected for large rather than small eggs. A second experiment using Atlantic salmon (S. salar) eggs indicated that oxygen consumption increases relatively slowly with increasing egg mass (allometric constant = 0.44). The failure of the conventional 'bigger is worse during incubation' hypothesis may thus be due to the erroneous assumption that oxygen consumption increases at a greater rate with increasing egg mass than does the egg surface area that is available for oxygen diffusion. We also demonstrate, using data from Atlantic salmon, that nest-specific oxygen consumption decreases with increasing egg size, but that this effect is more pronounced for large than for small females. This may help to explain the positive correlation between adult body size and egg size observed in fishes that cluster their eggs.     

Nonequilibrium Thermodynamic Analysis of the Coupling between Active Sodium Transport and Oxygen Consumption

Sodium transport and oxygen consumption have been simultaneously studied in the short-circuited toad skin. A constant stoichiometric ratio was observed in each skin under control condition (NaCl-Ringer's solution bathing both sides of the skin) and after block of sodium transport by ouabain. During alterations of sodium transport by removal and addition of K to the internal solution the stoichiometric ratio is constant although having a value higher than that observed in other untreated skins. The coupling between active sodium transport and oxygen consumption was studied after a theoretical nonequilibrium thermodynamic model. Studies were made of the influence of Na chemical potential difference across the skin on the rates of Na transport and oxygen consumption. A linear relationship was observed between the rates of Na transport and oxygen consumption and the Na chemical potential difference. Assuming the Onsager relationship to be valid, the three phenomenological coefficients which describe the system were evaluated. Transient increases in the rate of sodium transport and oxygen consumption were observed after a transitory block of sodium transport by removal of Na from the external solution. Cyanide blocks completely the rate of oxygen consumption in less than 2 min and the short-circuit current measured after that time decays exponentially with time, suggesting a depletion of ATP from a single compartment.     

Oxygen consumption by developing and diapausing eggs of Eupholidoptera smyrnensis (Orthoptera: Tettigoniidae)

Oxygen consumption by eggs of the Mediterranean bush-cricket Eupholidoptera smyrnesis was manometrically measured at 18, 24, 30 and 33°C. The aim was to study changes in oxygen consumption during embryonic development, and to compare a facultative initial and a facultative penultimate diapause with subitaneous developing eggs and an obligatory final diapause. In developing eggs, oxygen consumption increased as the embryo grew, but remained constant during katatrepsis. A comparison of the different diapauses revealed that in the initial diapause, oxygen consumption was lower than that of the corresponding embryonic stage in subitaneous eggs; the temperature dependence of oxygen consumption was also low. In contrast, in penultimate and in final diapause, oxygen consumption remained at the same high level as in developing embryos of the same size, and the temperature dependence of oxygen consumption was high. The results suggest that it is energetically profitable to the embryo to spend the hot season in a facultative initial and/or penultimate diapause.     

A Laboratory Investigation of the Suspension,Transport, and Settling of Silver Carp Eggs Using Synthetic Surrogates

Asian carp eggs are semi-buoyant and must remain suspended in the water to survive, supported by the turbulence of the flow, until they hatch and develop the ability to swim. Analysis of the transport and dispersal patterns of Asian carp eggs will facilitate the development and implementation of control strategies to target the early life stages. Experimenting with Asian carp eggs is complicated due to practical issues of obtaining eggs in close proximity to experimental facilities and extensive handling of eggs tends to damage them. Herein, we describe laboratory experiments using styrene beads (4.85 mm diameter) as synthetic surrogate eggs to mimic the physical properties of water-hardened silver carp eggs. The first set of experiments was completed in a rectangular vertical column filled with salt water. The salinity of the water was adjusted in an iterative fashion to obtain a close approximation of the fall velocity of the styrene beads to the mean fall velocity of silver carp water-hardened eggs. The terminal fall velocity of synthetic eggs was measured using an image processing method. The second set of experiments was performed in a temperature-controlled recirculatory flume with a sediment bed. The flume was filled with salt water, and synthetic eggs were allowed to drift under different flow conditions. Drifting behavior, suspension conditions, and settling characteristics of synthetic eggs were observed. At high velocities, eggs were suspended and distributed through the water column. Eggs that touched the sediment bed were re-entrained by the flow. Eggs saltated when they touched the bed, especially at moderate velocities and with a relatively flat bed. At lower velocities, some settling of the eggs was observed. With lower velocities and a flat bed, eggs were trapped near the walls of the flume. When bedforms were present, eggs were trapped in the lee of the bedforms in addition to being trapped near the flume walls. Results of this research study provide insights about transport, suspension, and dispersion of silver carp eggs. The knowledge gained from this study is useful to characterize the critical hydrodynamic conditions of the flow at which surrogates for silver carp water-hardened eggs settle out of suspension, and provides insight into how eggs may interact with riverbed sediments and morphology.     

Combining genetic variation and phenotypic plasticity in tradeoff modelling

Tradeoffs lead to antagonistic relationships between phenotypic traits and are thought to be determined both genetically and environmentally. We present here an allocation model that distinguishes between the genetic and environmental components of variation in resource allocation. In this model we introduced plasticity of resource allocation which was considered to be an adaptive response to environmental variations. The results show that resource allocation plasticity is a key parameter for the existence of environmental (i.e. inter environments and intra genotype) correlations and is therefore necessary to detect environment-induced tradeoffs. We also investigated the impact of the resource allocation plasticity and other factors on genetic (i.e. inter genotypes) correlations. Our results show that resource allocation plasticity induces a masking effect of tradeoffs when studying genetic correlations and increases the masking effect of resource variation by making apparent correlations positive when negative correlations are expected. In addition, by simulating different sources of resource acquisition variation, we demonstrated that resource variation might have different effects on correlations according to the experimental design and the studied biological material. Further development of this model may be used to investigate the theoretical implications of tradeoffs in evolutionary biology and to improve design and interpretation of experimental studies.     

Transport of bacteria in porous media: II. A model for convective Transport and growth

A model is presented for the coupled processes of bacterial growth and convective transport of bacteria has been modeled using a fractional flow approach. The various mechanisms of bacteria retention can be incorporated into the model through selection of an appropriate shape of the fractional flow curve. Permeability reduction due to pore plugging by bacteria was simulated using the effective medium theory. In porous media, the rates of transport and growth of bacteria, the generation of metabolic products, and the consumption of nutrients are strongly coupled processes. Consequently, the set of governing conservation equations form a set of coupled, nonlinear partial differential equations that were solved numerically. Reasonably good agreement between the model and experimental data has been obtained indicating that the physical processes incorporated in the model are adequate. The model has been used to predict the in situ transport and growth of bacteria, nutrient consumption, and metabolite production. It can be particularly useful in simulating laboratory experiments and in scaling microbial-enhanced oil recovery or bioremediation processes to the field. (c) 1994 John Wiley & Sons, Inc.     

Sodium transport and oxygen consumption in toad bladder. A thermodynamic approach.

The relationship between active sodium transport and oxygen consumption was investigated in toad urinary bladder exposed to identical sodium-Ringer's solution at each surface, while controlling the transepithelial electrical potential difference delta phi. Rates of sodium transport and oxygen consumption were measured simultaneously, both in the short-circuited state (delta phi = 0) and when delta phi was varied. Under short-circuit conditions, when the rates of active sodium transport changed spontaneously or were depressed with amiloride, the ratio of active sodium transport to the estimated suprabasal oxygen consumption Na/O2 was constant for each tissue, but varied among different tissues. Only when delta phi was varied did the ratio Na+/O2 change with the rate of active sodium transport; under these circumstances dNa+/dO2 was constant but exceeded the ratio measured at short-circuit [(Na+/O2)delta phi = 0[. This suggests that coupling between transport and metabolism is incomplete. The results are analyzed according to the principles of nonequilibrium thermodynamics, and intepreted in terms of a simple model of the transepithelial sodium transport system.     

Activation of Na+-dependent transport at fertilization in the sea urchin: requirements of both an early event associated with exocytosis and a later event involving increased energy metabolism

Transport of glycine, phosphate, and thymidine has been studied in parallel in eggs and embryos of Strongylocentrotus purpuratus. Uptake of each of these substrates is shown to be sodium dependent and expressed at fertilization in this species. The data indicate that activation of unfertilized eggs by ammonia does not result in significant expression of any of these transport systems compared to fertilized eggs. Activation of unfertilized eggs by sperm or by ionophore A-23187 in seawater results in complete development of these transport systems. However, if eggs are activated by ionophore in sodium-free seawater or fertilized by sperm in complete seawater and transferred to sodium-free seawater, Na+-dependent transport does not develop. Ammonia reverses the inhibitory effect of sodium-free seawater on development of these transport systems. This reversal is sensitive to 2,4-dinitrophenol. The data suggest that transport of glycine, phosphate, and thymidine share a common mechanism of activation. Moreover, this activation requires both an early event (less than 5 min postinsemination) and a later event involving increased energy metabolism.     

Identifying and reducing AFLP genotyping error: an example of tradeoffs when comparing population structure in broadcast spawning versus brooding oysters

Phylogeographic inferences about gene flow are strengthened through comparison of co-distributed taxa, but also depend on adequate genomic sampling. Amplified fragment length polymorphisms (AFLPs) provide a rapid and inexpensive source of multilocus allele frequency data for making genomically robust inferences. Every AFLP study initially generates markers with a range of locus-specific genotyping error rates and applies criteria to select a subset for analysis. However, there has been very little empirical evaluation of the best tradeoff between culling all but the lowest-error loci to minimize overall genotyping error versus the potential for increasing population genetic signal by retaining more loci. Here, we used AFLPs to compare population structure in co-distributed broadcast spawning (Crassostrea virginica) and brooding (Ostrea equestris) oyster species. Using existing methods for almost entirely automated marker selection and scoring, genotyping error tradeoffs were evaluated by comparing results across a nested series of data sets with mean mismatch errors of 0, 1, 2, 3, 4 and >4%. Artifactual population structure was diagnosed in high-error data sets and we assessed the low-error point at which expected population substructure signal was lost. In both species, we identified substructure patterns deemed to be inaccurate at average mismatch error rates 2 and >4%. In the species comparison, the optimum data sets showed higher gene flow for the brooding oyster with more oceanic salinity tolerances. AFLP tradeoffs may differ among studies, but our results suggest that important signal may be lost in the pursuit of 'acceptable' error levels and our procedures provide a general method for empirically exploring these tradeoffs.     

Respiration of Aquatic and Terrestrial Amphibian Embryos

Respiratory constraints on the structure of single eggs andegg masses have affected the mode of amphibian reproductionin water and in air. Aquatic eggs generally require less oxygen,develop faster, and hatch earlier, but these characteristicsare related to small ovum size. A comparison of two speciesof aquatic and terrestrial breeding frogs with similarly sizedova shows no differences in hatching stage, maximum rate ofoxygen uptake, oxygen conductance of the egg capsule, or Po₂difference across the capsule. However, the aquatic speciesdevelops about 2.4 times faster and tolerates lower environmentalPo₂, suggesting adaptation for development in ephemeral water.Modelling of diffusive oxygen transport into a single aquaticegg shows that a large amount of jelly (or a boundary layer)around the capsule may not greatly restrict gas exchange, ifthe inner radius of the capsule is large. However, gelatinousegg masses that contain other embryos that compete for oxygenare therefore limited in size, unless the eggs are ventilatedby convection of water among them. Aquatic egg are often suspendedin masses above the substrate, promoting oxygen movement intothe mass from all directions. Terrestrial egg masses are morediffusion limited, because gravity and surface tension collapsethem, preventing convection between the eggs, and restrictingthe source for oxygen diffusion. Terrestrial embryos are oftenlarger than their aquatic counterparts and have higher demandsfor oxygen. Terrestrial conditions have selected for adaptationsthat reduce respiratory competition between embryos, for example,separating of embryos by large volumes of jelly or reducingthe number of eggs in a clutch. The size of foam nests is unlimited,because oxygen for each embryo is supplied directly from thefoam.     

The role of oxygen availability in the embryonation of Heterakis gallinarum eggs.

The importance of oxygen availability in the embryonation of the infective egg stages of the gastrointestinal nematode parasite Heterakis gallinarum was studied in the laboratory. Unembryonated H. gallinarum eggs were kept under either aerobic conditions by gassing with oxygen, or anaerobic conditions by gassing with the inert gas nitrogen, under a range of constant temperatures. Oxygenated eggs embryonated at a rate influenced by temperature. Conversely, eggs treated with nitrogen showed no embryonation although when these eggs were transferred from nitrogen to oxygen gas after 60 days of treatment, embryonation occurred. This demonstrated that oxygen is an essential requirement for H. gallinarum egg development, although undeveloped eggs remain viable, even after 60 days in low oxygen conditions. The effects of climate on the biology of free-living stages studied under constant laboratory conditions cannot be applied directly to the field where climatic factors exhibit daily cycles. The effect of fluctuating temperature on development was investigated by including an additional temperature group in which H. gallinarum eggs were kept under daily temperature cycles between 12 and 22°C. Cycles caused eggs to develop significantly earlier than those in the constant mean cycle temperature, 17°C, but significantly slower than those in constant 22°C suggesting that daily temperature cycles had an accelerating effect on H. gallinarum egg embryonation but did not accelerate to the higher temperature. These results suggest that daily fluctuations in temperature influence development of the free-living stages and so development cannot be accurately predicted on the basis of constant temperature culture.     

Oxygen consumption during embryonic development of the annual fish Nothobranchius korthausae with special reference to diapause

The oxygen consumption of Nothobranchius korthausae eggs in different developmental stages, including diapause II and III, was measured. Oxygen consumption increases exponentially during embryonic development. In diapause II and III there is a drop in oxygen consumption, which attains a minimal level in diapause II after 3 weeks and in diapause III after 2 weeks. During early development the embryos can escape from hypoxic stress by entering diapause I and II. During late embryogenesis embryos in diapause III can escape from hypoxic stress by hatching. We conclude that survival of annual fish embryos is enhanced during conditions of low oxygen concentration by reduced oxygen consumption rates during diapause.     

A theoretical model for oxygen transport in skeletal muscle under conditions of high oxygen demand.

Oxygen transport from capillaries to exercising skeletal muscle is studied by use of a Krogh-type cylinder model. The goal is to predict oxygen consumption under conditions of high demand, on the basis of a consideration of transport processes occurring at the microvascular level. Effects of the decline in oxygen content of blood flowing along capillaries, intravascular resistance to oxygen diffusion, and myoglobin-facilitated diffusion are included. Parameter values are based on human skeletal muscle. The dependence of oxygen consumption on oxygen demand, perfusion, and capillary density are examined. When demand is moderate, the tissue is well oxygenated and consumption is slightly less than demand. When demand is high, capillary oxygen content declines rapidly with axial distance and radial oxygen transport is limited by diffusion resistance within the capillary and the tissue. Under these conditions, much of the tissue is hypoxic, consumption is substantially less than demand, and consumption is strongly dependent on capillary density. Predicted consumption rates are comparable with experimentally observed maximal rates of oxygen consumption.     

Silencing of Maternal Heme-binding Protein Causes Embryonic Mitochondrial Dysfunction and Impairs Embryogenesis in the Blood Sucking Insect Rhodnius prolixus

The heme molecule is the prosthetic group of many hemeproteins involved in essential physiological processes, such as electron transfer, transport of gases, signal transduction, and gene expression modulation. However, heme is a pro-oxidant molecule capable of propagating reactions leading to the generation of reactive oxygen species. The blood-feeding insect Rhodnius prolixus releases enormous amounts of heme during host blood digestion in the midgut lumen when it is exposed to a physiological oxidative challenge. Additionally, this organism produces a hemolymphatic heme-binding protein (RHBP) that transports heme to pericardial cells for detoxification and to growing oocytes for yolk granules and as a source of heme for embryo development. Here, we show that silencing of RHBP expression in female fat bodies reduced total RHBP circulating in the hemolymph, promoting oxidative damage to hemolymphatic proteins. Moreover, RHBP knockdown did not cause reduction in oviposition but led to the production of heme-depleted eggs (white eggs). A lack of RHBP did not alter oocyte fecundation. However, produced white eggs were nonviable. Embryo development cellularization and vitellin yolk protein degradation, processes that normally occur in early stages of embryogenesis, were compromised in white eggs. Total cytochrome c content, cytochrome c oxidase activity, citrate synthase activity, and oxygen consumption, parameters that indicate mitochondrial function, were significantly reduced in white eggs compared with normal dark red eggs. Our results showed that reduction of heme transport from females to growing oocytes by RHBP leads to embryonic mitochondrial dysfunction and impaired embryogenesis.