Seasonal Dynamics of Leaf Stoichiometry of Phragmites australis: A Case Study From Yangguan Wetland, Dunhuang, China

Leaf stoichiometry can enhance our understanding of leaf elements' (C, N and P) concentrations and their corresponding ratios in an ecosystem with seasonal environment changes. This study quantified the seasonal dynamics of leaf stoichiometry of P. australis from Yangguan wetland, Dunhuang, China as a case study example. The leaf C concentration (LC) of P. australis changed between seasons and was 392.26 (g×kg-1), 417.35 (g×kg-1) and 392.58 (g×kg-1) in spring, summer and autumn, respectively. Leaf N and P concentrations (LN and LP) were 23.49 (g×kg-1), and 17.54 (g×kg-1) and 5.86 (g×kg-1), and 1.00 (g×kg-1), 0.75 (g×kg-1) and 0.16 (g×kg-1), respectively, in the three seasons. The maximum (77.68) and the minimum values (17.00) of LC:LN were observed in the autumn and spring, respectively. Seasonal variations in LC:LP also showed a similar trend, with the greatest value of 3015.91 in autumn and the lowest value of 429.39 in spring. However, the highest (45.67) and the lowest values (24.18) of LN:LP were observed in autumn and summer, respectively, indicating that the growth of P. australis was mainly affected by P. Based on these results, it can be concluded that P. australis adopted a competition strategy during the early growth stage but took on a defense life strategy at the late growth stage to cope with various environments.

Host use and host shifts in Drosophila

Over a thousand Drosophila species have radiated onto a wide range of feeding and breeding sites. These radiations involve adaptations for locating, accepting, and growing in hosts with highly differing characteristics. In a number of species, owing to the availability of sequenced genomes, particular steps in host specialization and genes that control them, are being identified. Many cases of specialization involve the ability to detoxify some component of the host. Examples include Drosophila sechellia and the octanoic acid in Morinda citrifolia, alpha-amanitin in mycophagous drosophilids, and the alkaloids in cactophilic species. Owing to the known ecologies of many species for which genomes exist, the Drosophila model system provides an unprecedented opportunity to simultaneously examine the genes underlying HOST LOCATION, HOST ACCEPTANCE and HOST USE, the types of selection acting upon them and any coevolutionary interactions among the genes underlying these steps.

GRASP [Genomic Resource Access for Stoichioproteomics]: comparative explorations of the atomic content of 12 Drosophila proteomes

BACKGROUND

"Stoichioproteomics" relates the elemental composition of proteins and proteomes to variation in the physiological and ecological environment. To help harness and explore the wealth of hypotheses made possible under this framework, we introduce GRASP (http://www.graspdb.net), a public bioinformatic knowledgebase containing information on the frequencies of 20 amino acids and atomic composition of their side chains. GRASP integrates comparative protein composition data with annotation data from multiple public databases. Currently, GRASP includes information on proteins of 12 sequenced Drosophila (fruit fly) proteomes, which will be expanded to include increasingly diverse organisms over time. In this paper we illustrate the potential of GRASP for testing stoichioproteomic hypotheses by conducting an exploratory investigation into the composition of 12 Drosophila proteomes, testing the prediction that protein atomic content is associated with species ecology and with protein expression levels.

RESULTS

Elements varied predictably along multivariate axes. Species were broadly similar, with the D. willistoni proteome a clear outlier. As expected, individual protein atomic content within proteomes was influenced by protein function and amino acid biochemistry. Evolution in elemental composition across the phylogeny followed less predictable patterns, but was associated with broad ecological variation in diet. Using expression data available for D. melanogaster, we found evidence consistent with selection for efficient usage of elements within the proteome: as expected, nitrogen content was reduced in highly expressed proteins in most tissues, most strongly in the gut, where nutrients are assimilated, and least strongly in the germline.

CONCLUSIONS

The patterns identified here using GRASP provide a foundation on which to base future research into the evolution of atomic composition in Drosophila and other taxa.

The nutritional content of prey affects the foraging of a generalist arthropod predator

While foraging theory predicts that predatory responses should be determined by the energy content and size of prey, it is becoming increasingly clear that carnivores regulate their intake of specific nutrients. We tested the hypothesis that prey nutrient composition and predator nutritional history affects foraging intensity, consumption, and prey selection by the wolf spider, Pardosa milvina. By altering the rearing environment for fruit flies, Drosophila melanogaster, we produced high quality flies containing more nitrogen and protein and less lipid than low quality fruit flies. In one experiment, we quantified the proportion of flies taken and consumption across a range of densities of either high or low quality flies and, in a second experiment, we determined the prey capture and consumption of spiders that had been maintained on contrasting diets prior to testing. In both cases, the proportion of prey captured declined with increasing prey density, which characterizes the Type II functional response that is typical of wolf spiders. Spiders with similar nutritional histories killed similar numbers of each prey type but consumed more of the low quality prey. Spiders provided high quality prey in the weeks prior to testing killed more prey than those on the low quality diet but there was no effect of prior diet on consumption. In the third experiment, spiders were maintained on contrasting diets for three weeks and then allowed to select from a mixture of high and low quality prey. Interestingly, feeding history affected prey preferences: spiders that had been on a low quality diet showed no preference but those on the high quality diet selected high quality flies from the mixture. Our results suggest that, even when prey size and species identity are controlled, the nutritional experience of the predator as well as the specific content of the prey shapes predator-prey interactions.

"Cost" of virginity in wild Drosophila melanogaster females

Laboratory studies have revealed a significant "cost of mating" to Drosophila melanogaster females in the form of reduced longevity. The effect is attributable to nonsperm components of the ejaculate. Female D. melanogaster are known to mate up to six times in nature, and given that they do not typically remate daily, it raises the question as to the extent to which the longevity of wild mated females is reduced. Here I addressed this question by comparing the longevity of wild virgin females, collected as they emerged from rotting fruit, to the longevity of randomly collected mature females at the same site. Because the randomly collected females all were inseminated and were fully pigmented at the time of collection, they already were older than the virgins when the experiment began. Contrary to expectations from laboratory studies, the older, mated females lived significantly longer than the virgins. Rather than a "cost of mating," there appears to be a "cost of virginity" to female D. melanogaster in the wild.

Dietary protein and sugar differentially affect development and metabolic pools in ecologically diverse Drosophila

We examined the effects of 3 diets differing in their relative levels of sugar and protein on development and metabolic pools (protein, TG, and glycogen) among sets of isofemale lines of 2 ecologically distinct Drosophila species, D. melanogaster and D. mojavensis. Our high protein:sugar ratio diet contained 7.1% protein and 17.9% carbohydrate, the EPS diet was 4.3% protein and 21.2% carbohydrate, and the LPS was only 2.5% protein and 24.6% carbohydrate. Larvae of D. melanogaster, a generalist fruit breeder, were able to survive on all 3 diets, although all 3 metabolic pools responded with significant diet and diet × line interactions. Development was delayed by the diet with the most sugar relative to protein. The other species, D. mojavensis, a cactus breeder ecologically unaccustomed to encountering simple sugars, completely failed to survive when fed the diet with the highest sugar and showed very poor survival even with the diet with equal parts of protein and sugar. Furthermore, the D. mojavensis adult metabolic pools of protein, TG, and glycogen significantly differed from those of D. melanogaster adults fed the identical diet. Thus, considerable within- and between-species differences exist in how diets are metabolized. Given that the genomes of both of these Drosophila species have been sequenced, these differences and their genetic underpinnings hold promise for understanding human responses to nutrition and for developing strategies for dealing with metabolic disease.

Prior mating experience modulates the dispersal of Drosophila in males more than in females

Cues from both an animal's internal physiological state and its local environment may influence its decision to disperse. However, identifying and quantifying the causative factors underlying the initiation of dispersal is difficult in uncontrolled natural settings. In this study, we automatically monitored the movement of fruit flies and examined the influence of food availability, sex, and reproductive status on their dispersal between laboratory environments. In general, flies with mating experience behave as if they are hungrier than virgin flies, leaving at a greater rate when food is unavailable and staying longer when it is available. Males dispersed at a higher rate and were more active than females when food was unavailable, but tended to stay longer in environments containing food than did females. We found no significant relationship between weight and activity, suggesting the behavioral differences between males and females are caused by an intrinsic factor relating to the sex of a fly and not simply its body size. Finally, we observed a significant difference between the dispersal of the natural isolate used throughout this study and the widely-used laboratory strain, Canton-S, and show that the difference cannot be explained by allelic differences in the foraging gene.

Metabolic pools differ among ecologically diverse Drosophila species

Studies of the genetic mechanisms underlying metabolic storage have focused on a few model organisms. Although very fruitful, these studies have not allowed for the examination of mechanisms across a phylogenetic spectrum. The exploration of natural patterns of metabolic pool size variation across species will help us to better understand the genetics of metabolic adaptation. We examined the metabolic pools size (triglyceride, glycogen and protein) at two ages in 12 Drosophila species with distinctly different ecologies for which complete genome sequences (for 11 of the 12 species) are known. Overall, there were significant differences across species for all three pools, while age and sex appear to affect some metabolic pools more than others. After correcting for the phylogenetic relatedness of the species used, we observed no association between triglyceride and glycogen content. Although within species these two pools sometimes are correlated, at a larger phylogenetic scale control of triglyceride and glycogen contents may have been shaped independently by natural selection.

Biological stoichiometry: a chemical bridge between ecosystem ecology and evolutionary biology

The mission of the American Society of Naturalists is "to advance and diffuse knowledge of organic evolution and other broad biological principles so as to enhance the conceptual unification of the biological sciences." In this article, I argue that the area of biology least integrated with knowledge of organic evolution is the field of ecosystem ecology, as evidenced by a semiquantitative literature survey of use of terms in the scientific literature. I present an overview of recent theoretical developments and empirical findings in the emerging field of biological stoichiometry (the study of the balance of energy and multiple chemical elements in living systems). These developments hold some promise as a means to conceptually integrate ecosystem ecology, with its emphasis on flows and pools of energy and chemical elements, with evolutionary biology, with its emphasis on genetic fitness and the biochemical products of the genome. For example, recent evidence indicates that organismal C : P and N : P ratios have a major impact on biologically mediated flows of energy and phosphorus; in turn, variations among taxa in these ratios are connected to evolved differences in organismal growth rate because of the connection between growth rate and the need for increased allocation to P-rich ribosomal RNA. In this way, evolutionary change in growth-related traits, by altering organismal P requirements, has direct biogeochemical implications, while ecosystem conditions can constrain evolutionary acceleration of growth rates by imposing a direct P limitation on production of the needed biochemical machinery of growth. Thus, stoichiometric theory provides a broad biological principle that can interconvert the currencies and concerns of ecosystem ecology and evolutionary biology, facilitating integration of diverse fields of study and contributing to conceptual unification of the biological sciences.

Nutrient regulation in relation to diet breadth: a comparison ofHeliothissister species and a hybrid

We examined the nutritional correlates of diet breadth in insect herbivores by comparing patterns of diet selection, nutrient balancing, post-ingestive utilization and development in two sister species of caterpillar and a hybrid between them. One species, Heliothis virescens (HV) has a broad host range, feeding on plants in at least 14 families. The other, Heliothis subflexa (HS), is a specialist on plants in the genus Physalis(Solanaceae). Experiments using synthetic foods showed that when the caterpillars were allowed to mix their diet, the generalist self-selected a higher-protein diet whereas the specialist ate almost equal amounts of protein and carbohydrate, which accords with differences between the two species in the nutrient content of their natural diets. When confined to nutritionally imbalanced diets, the generalist showed a propensity to over-eat high protein:carbohydrate (P:C) diets to a greater degree than did the specialist and maintained higher rates of development. The generalist did not, however,over-eat low P:C diets to the same degree as the specialist. The hybrid selected a diet composition that was indistinguishable to that of its generalist father (HV), while its response to imbalanced diets was closely similar to that of the specialist mother (HS). The generalist converted ingested nutrient to growth with lower efficiency than did the specialist and the hybrid. Our findings imply that different behavioural and physiological traits linked to nutrient regulation are under genetic control and are explicable in terms of the different life-histories, feeding ecologies and presumed levels of nutritional heterogeneity in the environments of the two insects.

Biological stoichiometry of growth in Drosophila melanogaster

We examined the relationship between growth rate, C:N:P stoichiometry, and nucleic acid content in Drosophila melanogaster. The "Growth Rate Hypothesis" predicts that N and P contents per unit body mass will be high during ontogenetic stages characterized by rapid growth, reflecting the large requirement for P-rich ribosomal RNA during these periods. The ratio of RNA:DNA also is predicted to change with changes in growth rate. Growth is rapid in early D. melanogaster larvae, slowing considerably just prior to pupation. As predicted, a positive relationship was found between growth rate and N and P content, but not C. Thus, body C:P and N:P ratios declined with increasing growth rate. The relationship between RNA content and growth rate also was positive. Additionally, the fraction of total body P contributed by ribosomal RNA increased with increasing growth rate.

N : P ratios in terrestrial plants: variation and functional significance

Nitrogen (N) and phosphorus (P) availability limit plant growth in most terrestrial ecosystems. This review examines how variation in the relative availability of N and P, as reflected by N : P ratios of plant biomass, influences vegetation composition and functioning. Plastic responses of plants to N and P supply cause up to 50-fold variation in biomass N : P ratios, associated with differences in root allocation, nutrient uptake, biomass turnover and reproductive output. Optimal N : P ratios - those of plants whose growth is equally limited by N and P - depend on species, growth rate, plant age and plant parts. At vegetation level, N : P ratios <10 and >20 often (not always) correspond to N- and P-limited biomass production, as shown by short-term fertilization experiments; however long-term effects of fertilization or effects on individual species can be different. N : P ratios are on average higher in graminoids than in forbs, and in stress-tolerant species compared with ruderals; they correlate negatively with the maximal relative growth rates of species and with their N-indicator values. At vegetation level, N : P ratios often correlate negatively with biomass production; high N : P ratios promote graminoids and stress tolerators relative to other species, whereas relationships with species richness are not consistent. N : P ratios are influenced by global change, increased atmospheric N deposition, and conservation managment. Contents Summary 243 I Introduction 244 II Variability of N : P ratios in response to nutrient  supply 244 III Critical N : P ratios as indicators of nutrient  limitation 248 IV Interspecific variation in N : P ratios 252 V Vegetation properties in relation to N : P ratios 255 VI Implications of N : P ratios for human impacts  on ecosystems 258 VII Conclusions 259 Acknowledgements 259 References 260.