The nutritional significance of sterol metabolic constraints in the generalist grasshopper Schistocerca americana

Sterols are essential nutrients for grasshoppers, as well as all other insects, but metabolic constraints can limit which phytosterols support normal growth and development. In the current study, the generalist grasshopper Schistocerca americana was used to address two questions related to grasshopper sterol nutrition: (1) how does sterol quantity influence growth and survival, and (2) how do mixtures of suitable and unsuitable sterols at different concentrations influence growth and survival? Results from the first experiment indicated that this grasshopper species had a minimum sterol requirement of 0.05% dry weight; as sterol quantity increased above this concentration, however, survival and performance were not enhanced. Results from the second experiment revealed two novel aspects of sterol nutrition in grasshoppers: (1) when suitable sterols were limiting, most individuals could not use unsuitable sterols to meet the minimum sterol requirement (i.e. no sparing occurred), and (2) above a certain threshold, unsuitable sterols were deleterious even when suitable sterols were present at a concentration that alone permits normal growth and development. We discuss these physiological findings in terms of how sterol metabolic constraints in grasshoppers might influence foraging.     

Phytosterol structure and its impact on feeding behaviour in the generalist grasshopper Schistocerca americana

Abstract.Sixth-stadium nymphs of the grasshopper Schistocerca americana (Drury) (Orthoptera: Acrididae) were observed in a series of experiments designed to measure feeding behaviour in response to suitable and unsuitable phytosterols. In the first experiment, grasshoppers were presented with artificial diet that contained either sitosterol, a suitable phytosterol, or a spinach lipid extract which contained only unsuitable sterols as well as other spinach lipids. The diet with the spinach lipid extract, but not the sitosterol diet, evoked a deterrent response. To determine if the spinach sterols were responsible for the deterrent response, a second experiment was performed where the spinach lipid extract was separated into three lipid classes, including desmethyl sterols, dimethyl sterols and the remaining spinach lipids. Grasshoppers presented with artificial diet containing the desmethyl sterols (the end-product sterols in spinach) exhibited deterrent responses. Finally, feeding behaviour to a suite of different sterols, including cholesterol (suitable), stigmasterol (unsuitable), and lathosterol (unsuitable), was observed; these sterols were selected because they show variation in the position of double bonds. Grasshoppers presented with diets containing unsuitable sterols again exhibited deterrent responses. Overall, the deterrent effect was strongest when sterols with a double bond at position 22 were in the diet.     

Growth, development, and nutritional physiology of grasshoppers from subarctic and temperate regions

Despite the importance of developmental rate, growth rate, and size at maturity in the life history of poikliotherms, the trade-offs among these traits and selection pressures involved in the evolution of these traits are not well understood. This study compared these traits in a grasshopper, Melanoplus sanguinipes F. (Orthoptera: Acrididae), from two contrasting geographical regions, subarctic Alaska and temperate Idaho. The growing season in the interior of Alaska is about 80 d shorter than at low-elevation sites in Idaho. We hypothesized that the Alaskan grasshoppers would show more rapid growth and development than grasshoppers from Idaho, at the cost of greater sensitivity to food quality. On a diet of lettuce and wheat bran, grasshoppers from Alaska developed from egg hatch to adult more rapidly than those from Idaho at each of three different temperature regimes. Averaged over all temperature treatments, the weight of the Alaskan grasshoppers was about 5% less than that of the Idaho grasshoppers at the adult molt. Feeding and digestive efficiencies were determined for the final two instars using two meridic diets: one with a high concentration of nutrients and the other with the same formulation but diluted with cellulose. Alaskan grasshoppers again developed more rapidly, weighed less, and had faster growth rates than those from Idaho. Alaskan grasshoppers supported their more rapid growth by increasing postingestive efficiencies; that is, they had higher conversion rates of digested matter to biomass on the high-quality diet, greater assimilation of food on the low-quality diet, and greater efficiency of nitrogen assimilation or retention on both diets. There was no evidence that performance of Alaskan grasshoppers suffered any more than that of the Idaho grasshoppers on the low-quality diet.     

Predictions of species interactions from consumer-resource theory: experimental tests with grasshoppers and plants

We tested the ability of consumer-resource theory to predict direct and indirect interactions among species, using an experimental system of insect herbivores and herbaceous plants. Specifically, we examined interactions among three species of grasshoppers (Melanoplus femur-rubrum, Spharagemon collare, andPhoetaliotes nebrascensis; Orthoptera, Acrididae) and herbaceous plants in experimental field cages placed over existing fertilized or unfertilized vegetation in a Minnesota old field. For the conditions inside these cages, we addressed whether (1) grasshopper diet predicted the presence of competition among grasshopper species, and (2) direct effects of grasshoppers on plants produced indirect interactions among plants, grasshoppers and soil nitrogen. Overall,M. femur-rubrum ate a greater proportion of forbs in cages, while the other two species ate primarily grasses. As expected, a pair of grasshopper species competed if they had similar diets. However, there were important exceptions that could be explained from observed indirect effects, although alternative explanations were also possible. First, all three grasshopper species significantly shifted their diets in the presence of other species, and these shifts occurred most often when competition was expected or occurred. Second, the two grassfeeding species reduced the biomass of the dominant grass (Schizachyrium scoparium) and increased available soil nitrogen and biomass of forbs. This effect may explain why the grass-feedingP. nebrascensis had a positive effect on the forb-feedingM. femur-rubrum on unfertilized plots. Overall, we show that direct effects of consumers on resources can predict competition and other important indirect interactions within a community.     

Mixed artificial diets enhance the developmental and reproductive performance of the edible grasshopper, <Emphasis Type="Italic">Ruspolia differens</Emphasis> (Orthoptera: Tettigoniidae)

Diet mixing is a common feeding habit among polyphagous insect herbivores and is believed to be advantageous for performance-related factors like growth, survival and oviposition. However, relatively little is known about the influence of artificial diet or their mixtures on the performance of edible insects. We examined the effects of artificial diet mixtures on the developmental and reproductive performance (survival, developmental time, fresh adult weight and female fecundity) of an edible grasshopper, Ruspolia differens (Orthoptera: Tettigoniidae). We raised individuals from eggs and reared newly hatched nymphs to adult stage on six different dietary treatments consisting of a single diet, and mixtures of two, three, five, six and eight artificial diets. More diversified diets resulted in shorter development time and greater adult fresh weight and female fecundity compared to the single diet or less diversified diets. Even with slight diet diversification, survival to adult stage was greatly improved. Overall, these results highlight the potential of diet mixtures in achieving maximum adult weights and female fecundity and shortening development time, information which could be used when designing mass-rearing programs for this edible grasshopper.     

Distribution and dynamic state of sterols and steroids in the tissues of an insect, the roach Eurycotis floridana

The total concentrations of sterols in the tissues of the roach, Eurycotis floridana, reared under aseptic conditions and on semisynthetic diets, are similar to, but somewhat lower than, those of tissues of vertebrates. Total concentrations of tissue sterols are relatively independent of dietary concentration of sterols whether the diet contains 0.1% cholesterol as the sole sterol, or a "minimal cholesterol" mixture (0.1% cholestanol together with 0.005% cholesterol). Under the latter conditions the cholesterol is incorporated preferentially into most tissues and remains almost exclusively unesterified, while the cholesterol-sparing sterol is esterified to varying degree, depending upon the tissue. The turnover of tissue sterols has been studied. Cholesterol of the tissues of adult insects grown on a diet containing this sterol alone may be displaced by cholestanol fed as 5% of the total diet, initially at an appreciable rate but later much less rapidly. In growing insects that have received a diet containing cholestanol together with minimal cholesterol, the unesterified cholesterol turns over slowly in all tissues and immeasurably slowly in some. The unesterified sparing sterol, on the other hand, turns over at a much greater rate. The turnover of sterols during growth is accompanied by a shift of sterols from the unesterified to the esterified pool in all tissues. The fat body of the growing insect stores sterols (apparently as their esters) that have been displaced from other tissues. The fat body of the adult does not show evidence of sterol storage. Polar derivatives of sterols are present in minor amount in all tissues of the insect, most abundantly in the mid-intestine and gastric caeca. These compounds seem likely to be C(27) steroids.     

Dietary sterols/steroids and the generalist caterpillar Helicoverpa zea: Physiology,biochemistry and midgut gene expression

Sterols are essential nutrients for insects because, in contrast to mammals, no insect (or arthropod for that matter) can synthesize sterols de novo. Plant-feeding insects typically generate their sterols, commonly cholesterol, by metabolizing phytosterols. However, not all phytosterols are readily converted to cholesterol. In this study we examined, using artificial diets containing single sterols/steroids, how typical (cholesterol and stigmasterol) and atypical (cholestanol and cholestanone) sterols/steroids affect the performance of a generalist caterpillar (Helicoverpa zea). We also performed sterols/steroids analyses, using GC/MS techniques, to explore the metabolic fate of these different dietary sterols/steroids. Finally, we used a microarray approach to measure, and compare, midgut gene expression patterns that arise as a function of dietary sterols/steroids. In general, H. zea performed best on the cholesterol and stigmasterol diets, with cholesterol as the dominant tissue sterol on these two treatments. Compared to the cholesterol and stigmasterol diets, performance was reduced on the cholestanol and cholestanone diets; on these latter treatments stanols were the dominant tissue sterol. Finally, midgut gene expression patterns differed as a function of dietary sterol/steroid; using the cholesterol treatment as a reference, gene expression differences were smallest on stigmasterol, intermediate on cholestanol, and greatest on cholestanone. Inspection of our data revealed two broad insights. First, they identify a number of genes potentially involved in sterol/steroid metabolism and absorption. Second, they provide unique mechanistic insights into how variation in dietary sterol/steroid structure can affect insect herbivores.     

Sterol utilization in honey bees fed a synthetic diet: Analysis of prepupal sterols

The sterols of prepupal honey bees, Apis mellifera L., from brood reared by workers fed chemically-defined synthetic diets containing cholesterol, campesterol, sitosterol, stigmasterol, 24-methylenecholesterol, or no sterol over a 12-week period were isolated, identified, and quantified. The major sterol present in each prepupal sample was 24-methylenecholesterol, but significant levels of sitosterol and isofucosterol were also present in every case, as was a very small percentage of desmosterol (usually < 1%). This is the first report of isofucosterol being identified in the sterols of the honey bee. A considerably larger percentage of each dietary sterol was found in prepupae reared by workers fed that particular sterol in the diet. This was most dramatic in the case of the cholesterol diet in which case cholesterol content increased to as much as 17.2% of the prepupal sterols, whereas cholesterol had not exceeded 2.2% in samples from other diet regimens. However, stigmasterol comprised no more than 6.3% of the total sterols in any sample from prepupae fed the stigmasterol diet. The preponderance of 24-methylenecholesterol in all prepupae, regardless of the dietary sterol provided to the workers, as well as the lesser quantities of sitosterol and isofucosterol present in all samples, suggest a unique system of utilization and metabolism of these dietary sterols by the worker bees. Apparently they make available to the brood varying amounts of unchanged dietary sterol plus considerable and fairly constant portions of 24-methylenecholesterol, sitosterol, and isofucosterol drawn from their own sterol pools.     

Sterol/steroid metabolism and absorption in a generalist and specialist caterpillar: Effects of dietary sterol/steroid structure,mixture and ratio

Insects cannot synthesize sterols de novo, so they typically require a dietary source. Cholesterol is the dominant sterol in most insects, but because plants contain only small amounts of cholesterol, plant-feeding insects generate most of their cholesterol by metabolizing plant sterols. Plants almost always contain mixtures of different sterols, but some are not readily metabolized to cholesterol. Here we explore, in two separate experiments, how dietary phytosterols and phytosteroids, in different mixtures, ratios, and amounts, affect insect herbivore sterol/steroid metabolism and absorption; we use two caterpillars species – one a generalist (Heliothis virescens), the other a specialist (Manduca sexta). In our first experiment caterpillars were reared on two tobacco lines – one expressing a typical phystosterol profile, the other expressing high amounts/ratios of stanols and 3-ketosteroids. Caterpillars reared on the control tobacco contained mostly cholesterol, but those reared on the modified tobacco had reduced amounts of cholesterol, and lower total sterol/steroid body profiles. In our second experiment, caterpillars were reared on artificial diets containing known amounts of cholesterol, stigmasterol, cholestanol and/or cholestanone, either singly or in various combinations and ratios. Cholesterol and stigmasterol-reared moths were mostly cholesterol, while cholestanol-reared moths were mostly cholestanol. Moth tissue cholesterol concentration tended to decrease as the ratio of dietary cholestanol and/or cholestanone increased. In both moths cholestanone was metabolized into cholestanol and epicholestanol. Interestingly, M. sexta generated much more cholestanol than epicholestanol, while H. virescens did the opposite. Finally, total tissue steroid levels were significantly reduced in moths reared on diets containing very high levels of cholestanol. We discuss how dietary sterol/steroid structural differences are important with respect to sterol/steroid metabolism and uptake, including species-specific differences.     

Discriminating tastes: self‐selection of macronutrients in two populations of grasshoppers

Abstract The capacity to self‐select an optimal balance of macronutrients (protein and carbohydrate) is studied in two populations of Melanoplus sanguinipes F. (Orthoptera: Acrididae). One population derives from the subarctic (interior of Alaska) and the other from the temperate zone (Idaho, U.S.A.). Over the duration of the fourth and fifth stadia, Alaskan grasshoppers consistently self‐select a diet centred on a 0.90 ratio of protein : carbohydrate, whereas protein and carbohydrate intake by the Idaho grasshoppers is contingent on the particular food choices presented to them. When restricted to imbalanced diets, the Alaskan grasshoppers develop more rapidly than the Idaho grasshoppers, regardless of diet composition. The Idaho grasshoppers also have a greater amount of lipid than the Alaskan grasshoppers across all diets. Performance measures (body mass, survival, developmental times) are more sensitive to dietary imbalances in the Alaskan grasshoppers than in the Idaho grasshoppers. When fed diets with low, but balanced, proportions of protein and carbohydrate, grasshoppers of both populations are able to increase consumption to compensate for the low concentration of nutrients. The results suggest that demographic responses of insects to changes in host plant quality, such as may result from climate change, may differ among populations within a species.     

酵母甾醇转运蛋白研究进展

甾醇是一类广泛存在于生物体内的环戊烷骈多氢菲衍生物,其不仅是细胞膜的重要组成成分,还具有重要的生理和药理活性。随着合成生物学和代谢工程技术的发展,近些年来应用酵母细胞异源合成甾醇的研究不断深入。但由于甾醇是疏水性大分子,倾向于积累在酵母的膜结构中而引发细胞毒性,一定程度上限制了甾醇产量的进一步提升。因此,揭示酵母中甾醇转运机制,特别是与甾醇转运相关的转运蛋白的工作原理,有助于设计新的策略,解除酵母细胞工厂中的甾醇积累毒性、实现甾醇增产。酵母中甾醇转运主要通过蛋白质介导的非囊泡运输机制来完成,本文归纳了酵母中已报道的5类甾醇转运相关蛋白,即OSBP/ORPs家族蛋白、LAM家族蛋白、NPC样甾醇转运蛋白、ABC转运家族蛋白和CAP超家族蛋白,汇总了这些蛋白对细胞内甾醇梯度分布和稳态维持所起的重要作用。此外,本文还综述了甾醇转运蛋白在酵母细胞工厂中的应用现状。     

Grasshoppers cope with low host plant quality by compensatory feeding and food selection: N limitation challenged

The effect of low host plant nitrogen (N) content on herbivore performance has rarely been studied together with the herbivore's feeding behaviour. We explored this relationship with juvenile Omocestus viridulus (Orthoptera: Acrididae) grasshoppers using fertilized and unfertilized host grasses. Due to lower growth rates, grasshoppers reared on N-poor grasses exhibited slightly prolonged development and smaller adult size, while mortality was similar among the fertilizer treatments. This was found both in the laboratory and in outdoor cages under natural climatic conditions. A parallel analysis of feeding behaviour revealed that the grasshoppers counterbalance N shortage by compensatory feeding, and are capable of selectively feeding among grasses of contrasting nutritional quality when given a choice. This indicates a striking ability of O. viridulus to regulate nutrient intake in the face of imbalanced food sources. Although the species exploits a relatively very poor autotroph nutrient base in the wild, as underpinned by N analysis of natural host grasses and grasshopper tissue, our data suggest that natural food quality imposes no relevant constraint on the herbivore's performance. Our study thus challenges the importance of simple plant-mediated control of herbivore populations, such as N limitation, but supports the view that herbivores balance their intake of N and energy.     

A reappraisal of sterol biosynthesis and metabolism in aphids

Aphids of Schizaphis graminum (Rondani) (biotype C) reared on its host-plant, Sorghum bicolor (L.) Moench, sequestered campesterol, stigmasterol and sitosterol. Aphids reared for 72 hr on holidic diets supplemented with [4-¹⁴C]-sitosterol contained both [¹⁴C]-sitosterol and [¹⁴C]-cholesterol, indicating that these aphids are capable of dealkylation at C-24. When aphids were reared on artificial diets containing [2-¹⁴C]-mevalonic acid, no detectable amounts of radioactively labelled desmethyl sterols, nor metabolic intermediates in sterol synthesis (i.e. squalene, 2,3-oxidosqualene, 4,4-dimethyl and 4-monomethyl sterols) were found to accumulate in their tissues. The relevance of these findings to previous research suggesting the ability of aphids, via their symbiotes, to synthesize sterols is discussed.     

Utilization of Δ5,7 - and Δ8-sterols by larvae of Heliothis zea

Larvae from two populations of Heliothis zea were reared on artificial diets containing various sterols, which supported suboptimal growth, and their tissue sterols were characterized in order to determine how these dietary sterols are utilized by this insect. The sterols studied included Δ^5,7-sterols (7-dehydrocholesterol or ergosterol), Δ⁸-sterols (lanosterol and/or 24-dihydrolanosterol), and a Δ⁵-sterol (4,4-dimethylcholesterol). Although larvae did not develop on 4,4-dimethylcholesterol, those fed primarily Δ⁸-4,4,14-trimethylsterols developed to the third instar. When the latter sterols were spared with cholesterol, the larvae reached the sixth instar and contained 4,4,14-trimethylsterols as well as cholesterol in their tissues. When larvae were fed 7-dehydrocholesterol, <1% of the larvae from one population developed to the sixth instar and these larvae contained 7-dehydrocholesterol as their principal sterol. The other larvae successfully completed their larval stage when they were transferred from the diet containing 7-dehydrocholesterol (or no sterol) to a diet containing cholesterol within at least 9 days. The sterol composition of larvae transferred from a diet containing cholesterol to a diet containing 7-dehydrocholesterol, after they had reached 60% of their final weight, was 54% cholesterol and 46% 7-dehydrocholesterol. The major sterol isolated from the tissues of the larvae fed ergosterol was also 7-dehydrocholesterol. Therefore, although the larva of H. zea can dealkylate and saturate the side chain of the Δ^5,7,22-24β-methylsterol, it carries out little metabolism of the B ring of the nucleus. These studies demonstrate that, when Δ^5,7- or Δ⁸-sterols are the principal sterols in the diet of H. zea, they are absorbed and incorporated into its tissues, although they slow the rate of growth and may prevent complete development of the larva.     

Effects of diet on titratable acid-base excretion in grasshoppers

Despite the potential for diet to affect organismal acid-base status, especially in herbivores, little is known about the effects of diet on acid-base loading and excretion. We tested the effects of diet on acid-base loading and excretion in grasshoppers by (a) comparing the fecal acid-base content of 15 grasshopper species collected from the field and (b) comparing fecal acid-base excretion rates of Schistocerca americana grasshoppers fed vegetable diets that differed in their ashed and raw acid-base contents. The field experiments indicated that grass-feeding species excrete fairly neutral fecal pellets, while forb/mixed-feeding species vary widely in their fecal acid-base contents. In the laboratory experiment, acid-base excretion rates were positively correlated with dietary ashed base intake rates but were not correlated with the acid-base content of raw, unashed diet or feeding rate. These experiments suggest that some diets could strongly challenge the acid-base homeostasis of herbivores; in some grasshoppers, dietary acid-base loads could produce certainly lethal 1-unit changes in average body pH within 6 h if they were not excreted.     

The response of the grape berry moth (Lobesia botrana) to a dietary phytopathogenic fungus (Botrytis cinerea): the significance of fungus sterols

A Tortricidae (Lobesia botrana) has a mutualistic relationship with the fungus (Botrytis cinerea). In this study, we investigated the growth, survival, fecundity and amount of sterols and steroids in larvae of this vineyard pest reared on artificial diets containing mycelium (3%) or purified sterols (0.01%) of the phytopathogenic fungus. Two principal questions related to the physiological and biochemical basis of this mutualistic relationship were addressed: (1) how the fungus influences growth, survival, fecundity, sterol and steroid contents of the insect and (2) are fungal sterols involved in the biochemical basis of mutualism? The presence of fungus in the diet led to a decrease of total duration of larval development (mean gain 5.1-9.4 days compared to the total duration in control of 42.9 days), an increase in survival (mean gain 50-76.3%) and fecundity (gain of 94-102%). These positive effects of the fungus on the biology and physiology of the insect were directly correlated to the presence of fungal sterols in the diet. Fungal sterols are one of the biochemical basis of the mutualistic relationship between L. botrana and B. cinerea.     

Plant sterols: Diversity,biosynthesis, and physiological functions

Sterols, which are isoprenoid derivatives, are structural components of biological membranes. Special attention is now being given not only to their structure and function, but also to their regulatory roles in plants. Plant sterols have diverse composition; they exist as free sterols, sterol esters with higher fatty acids, sterol glycosides, and acylsterol glycosides, which are absent in animal cells. This diversity of types of phytosterols determines a wide spectrum of functions they play in plant life. Sterols are precursors of a group of plant hormones, the brassinosteroids, which regulate plant growth and development. Furthermore, sterols participate in transmembrane signal transduction by forming lipid microdomains. The predominant sterols in plants are β-sitosterol, campesterol, and stigmasterol. These sterols differ in the presence of a methyl or an ethyl group in the side chain at the 24th carbon atom and are named methylsterols or ethylsterols, respectively. The balance between 24-methylsterols and 24-ethylsterols is specific for individual plant species. The present review focuses on the key stages of plant sterol biosynthesis that determine the ratios between the different types of sterols, and the crosstalk between the sterol and sphingolipid pathways. The main enzymes involved in plant sterol biosynthesis are 3-hydroxy-3methylglutaryl-CoA reductase, C24-sterol methyltransferase, and C22-sterol desaturase. These enzymes are responsible for maintaining the optimal balance between sterols. Regulation of the ratios between the different types of sterols and sterols/sphingolipids can be of crucial importance in the responses of plants to stresses.     

Supplementation with sterols improves food quality of a ciliate for Daphnia magna

Experimental results provide evidence that trophic interactions between ciliates and Daphnia are constrained by the comparatively low food quality of ciliates. The dietary sterol content is a crucial factor in determining food quality for Daphnia. Ciliates, however, presumably do not synthesize sterols de novo. We hypothesized that ciliates are nutritionally inadequate because of their lack of sterols and tested this hypothesis in growth experiments with Daphnia magna and the ciliate Colpidium campylum. The lipid content of the ciliate was altered by allowing them to feed on fluorescently labeled albumin beads supplemented with different sterols. Ciliates that preyed upon a sterol-free diet (bacteria) did not contain any sterols, and growth of D. magna on these ciliates was poor. Supplementation of the ciliates' food source with different sterols led to the incorporation of the supplemented sterols into the ciliates' cells and to enhanced somatic growth of D. magna. Sterol limitation was thereby identified as the major constraint of ciliate food quality for Daphnia. Furthermore, by supplementation of sterols unsuitable for supporting Daphnia growth, we provide evidence that ciliates as intermediary grazers biochemically upgrade unsuitable dietary sterols to sterols appropriate to meet the physiological demands of Daphnia.     

Life history traits associated with body size covary along a latitudinal gradient in a generalist grasshopper

Animal body size often varies systematically along latitudinal gradients, where individuals are either larger or smaller with varying season length. This study examines ecotypic responses by the generalist grasshopper Melanoplus femurrubrum (Orthoptera: Acrididae) in body size and covarying, physiologically based life history traits along a latitudinal gradient with respect to seasonality and energetics. The latitudinal compensation hypothesis predicts that smaller body size occurs in colder sites when populations must compensate for time constraints due to short seasons. Shorter season length requires faster developmental and growth rates to complete life cycles in one season. Using a common garden experimental design under laboratory conditions, we examined how grasshopper body size, consumption, developmental time, growth rate and metabolism varied among populations collected along an extended latitudinal gradient. When reared at the same temperature in the lab, individuals from northern populations were smaller, developed more rapidly, and showed higher growth rates, as expected for adaptations to shorter and generally cooler growing seasons. Temperature-dependent, whole organism metabolic rate scaled positively with body size and was lower at northern sites, but mass-specific standard metabolic rate did not differ among sites. Total food consumption varied positively with body size, but northern populations exhibited a higher mass-specific consumption rate. Overall, compensatory life history responses corresponded with key predictions of the latitudinal compensation hypothesis in response to season length.     

Sterol composition of gonad,muscle and digestive gland of Pecten maximus from Málaga (South Spain)

Sterol composition and content and their seasonal variations over 18 months were investigated in adductor muscle, digestive gland and gonads of Pecten maximus. Sterols were isolated by Silicagel 60 thin layer chromatography and identified by gas chromatography/mass spectrometry. Eleven sterols were identified, with cholesterol, brassicasterol, 24-methylenecholesterol and 22-trans-dehydrocholesterol being the principal components. The same sterols were found in all three tissues independent of season. The relative amounts of each sterol present in each tissue differed. Total sterol levels in gonad and muscle were higher than in digestive gland. Statistically significant differences (P<0.05) were found between the concentrations of each of the sterols isolated from the gonad or muscle and digestive gland. The seasonal variations in the sterol content of the gonad seem be related to the reproductive cycle, while the sterol content of the digestive gland appears to be linked to diet, mainly diatoms or dinoflagellates. The muscle sterol content showed minor changes throughout the year.