Respiratory pattern transitions in three species of Glossina (Diptera, Glossinidae)

Glossina exhibit cyclic (^CYCGE) or continuous gas exchange (^CONGE) patterns at rest. However, the factors influencing the transition from one pattern to another are not well understood for these or other insect species. This study examines which factors could aid in predicting the presence or absence of ^CYCGE in adults of three Glossina species: G. palpalis, G. brevipalpis and G. austeni. We report the results of temperature effects on VCO₂, pattern type and the proportion of a population showing ^CYCGE, and the prediction of ^CYCGE versus ^CONGE in Glossina. First, we investigated the influence of temperature on VCO₂ and found significant elevation in resting metabolic rate (RMR) with higher temperature in all three species (P < 0.001). Temperature-induced increases in VCO₂ were modulated by increased burst volume and by cycle frequency, except in G. brevipalpis which only appeared to modulate burst volume. These results are largely in keeping with VCO₂ modulation reported for other Glossina species previously. Second, elevating temperature resulted in significantly reduced numbers of individuals showing ^CYCGE (P < 0.001 for all three species) contrary to previous reports for other Glossing species. Finally, we examined a range of variables as potential predictors of presence or absence of ^CYCGE in these three species. Using an information theoretic approach (Akaike weights) to select the best explanatory combination of variables which predicts likelihood of ^CYCGE, we found that results varied among species. When species were pooled, the simplest, best-fit model (ΔAIC < 2 from the best model, 44.4% probability of being the best model) for predicting pattern type variation was RMR. Overall these results suggest that RMR is a key variable driving pattern type and that elevated temperature reduces the number of individuals showing cyclic patterns through elevation of RMR in these species. This study supports the idea that an interaction between cellular metabolic demand, morphological features of the gas exchange system (e.g. tracheal and spiracular conductances), and CO₂ buffer capacity likely determine gas exchange pattern variation over short time-scales.     

Low repeatability of preferred body temperature in four species of Cordylid lizards: Temporal variation and implications for adaptive significance

Preferred body temperatures (T _sel) of ectotherms are important for ecological and evolutionary studies. In lizards, the measurement of T _sel is controversial for several reasons, generally related to hypotheses addressing how T _sel may evolve in the wild. Although seldom explicitly tested, evolutionary hypotheses of adaptation to local climate require that T _sel meets the conditions of natural selection, which include repeatability, heritability and a link to fitness. Here, we investigated repeatability (τ, intra-class correlation coefficient) of T _sel at several time-scales using four Cordylid species from heterogeneous thermal habitats. Although there was significant inter-individual variation within days (P < 0.005 in most cases), there was no significant inter-individual variation when calculated across several days (P > 0.05). Repeatability was low in all species investigated (from 0 to 0.482) when compared against other estimates of repeatability of T _sel in the literature. Irrespective of how T _sel was calculated, it showed inconsistent and variable temporal effects across species. Furthermore, repeatability of T _sel did not change with acclimation to laboratory conditions. These data have implications for understanding the evolution of thermoregulation in these and other ectotherms.     

Detecting phylogenetic signal in mutualistic interaction networks using a Markov process model

Ecological interaction networks, such as those describing the mutualistic interactions between plants and their pollinators or between plants and their frugivores, exhibit non‐random structural properties that cannot be explained by simple models of network formation. One factor affecting the formation and eventual structure of such a network is its evolutionary history. We argue that this, in many cases, is closely linked to the evolutionary histories of the species involved in the interactions. Indeed, empirical studies of interaction networks along with the phylogenies of the interacting species have demonstrated significant associations between phylogeny and network structure. To date, however, no generative model explaining the way in which the evolution of individual species affects the evolution of interaction networks has been proposed. We present a model describing the evolution of pairwise interactions as a branching Markov process, drawing on phylogenetic models of molecular evolution. Using knowledge of the phylogenies of the interacting species, our model yielded a significantly better fit to 21% of a set of plant–pollinator and plant–frugivore mutualistic networks. This highlights the importance, in a substantial minority of cases, of inheritance of interaction patterns without excluding the potential role of ecological novelties in forming the current network architecture. We suggest that our model can be used as a null model for controlling evolutionary signals when evaluating the role of other factors in shaping the emergence of ecological networks.     

Acclimation effects on critical and lethal thermal limits of workers of the Argentine ant, Linepithema humile

For the Argentine ant Linepithema humile, bioclimatic models often predict narrower optimal temperature ranges than those suggested by behavioural and physiological studies. Although water balance characteristics of workers of this species have been thoroughly studied, gaps exist in current understanding of its thermal limits. We investigated critical thermal minima and maxima and upper and lower lethal limits following acclimation to four temperatures (15, 20, 25, 30 degrees C; 12L:12D photoperiod) in adult workers of the Argentine ant, L. humile, collected from Stellenbosch, South Africa. At an ecologically relevant rate of temperature change of 0.05 degrees Cmin(-1), CTMax varied between 38 and 40 degrees C, and CTMin varied between 0 and 0.8 degrees C. In both cases the response to acclimation was weak. A significant time by exposure temperature interaction was found for upper and lower lethal limits, with a more pronounced effect of acclimation at longer exposure durations. Upper lethal limits varied between 37 and 44 degrees C, whilst lower lethal limits varied between -4 and -10.5 degrees C, with an acclimation effect more pronounced for upper than lower lethal limits. A thermal envelope for workers of the Argentine ant is provided, demonstrating that upper thermal limits do likely contribute to distributional limits, but that lower lethal limits and limits to activity likely do not, or at least for workers who are not exposed simultaneously to the demands of load carriage and successful foraging behaviour.     

Scaling of gas exchange cycle frequency in insects

Previously, it has been suggested that insect gas exchange cycle frequency (fC) is mass independent, making insects different from most other animals where periods typically scale as mass-0.25. However, the claim for insects is based on studies of only a few closely related taxa encompassing a relatively small size range. Moreover, it is not known whether the type of gas exchange pattern (discontinuous versus cyclic) influences the fC-mass scaling relationship. Here, we analyse a large database to examine interspecific fC-mass scaling. In addition, we investigate the effect of mode of gas exchange on the fC-scaling relationship using both conventional and phylogenetically independent approaches. Cycle frequency is scaled as mass(-0.280) (when accounting for phylogenetic non-independence and gas exchange pattern), which did not differ significantly from mass(-0.25). The slope of the fC-mass relationship was shallower with a significantly lower intercept for the species showing discontinuous gas exchange than for those showing the cyclic pattern, probably due to lower metabolic rates in the former. Insects therefore appear no different from other animals insofar as the scaling of gas exchange fC is concerned, although gas exchange fC may scale in distinct ways for different patterns.     

Creatine kinase and creatine kinase isoenzyme responses to heat stress

During this investigation the effects of heat acclimation and exercise on creatine kinase and creatine kinase BB isoenzyme responses in various tissues and serum of male Sprague-Dawley rats were ascertained. Forty rats were randomly divided into two groups of 20 rats each. One group was housed at 22+/-1 degrees C and the other at 33+/-1 degrees C. Each of the two groups were subdivided into two subgroups of ten rats each. One subgroup of each group was subjected to a programme of treadmill running of progressive intensity over a period of 6 weeks at the temperature at which it was housed while the other served as a resting control. At the end of the acclimation programme the rats were running at 23 m/min for 80 min. On the day of sacrifice all four subgroups were subjected to a discontinuous exercise protocol (10 min running alternated by a 2-min rest period; repeated three times) at 30+/-1 degrees C on a rodent treadmill at 23 m/min. The tissues investigated were kidney, heart and muscle. The rats were anaesthetized with pentobarbital sodium (6 mg/100 g body mass) injected intraperitoneally. The tissues were freeze-clamped and stored in liquid air until analysed. The body temperature of the four subgroups at the end of the experimental protocol were not significantly different. Acclimation at 33+/-1 degrees C resulted in significantly lower creatine kinase activity levels. Exercise at 30+/-1 degrees C also resulted in decreased creatine kinase activity levels in both acclimated groups. A similar trend was observed regarding creatine kinase BB isoenzyme activity levels, especially in kidney.     

Thermal biology,population fluctuations and implications of temperature extremes for the management of two globally significant insect pests

The link between environmental temperature, physiological processes and population fluctuations is a significant aspect of insect pest management. Here, we explore how thermal biology affects the population abundance of two globally significant pest fruit fly species, Ceratitis capitata (medfly) and C. rosa (Natal fruit fly), including irradiated individuals and those expressing a temperature sensitive lethal (tsl) mutation that are used in the sterile insect technique. Results show that upper and lower lethal temperatures are seldom encountered at the field sites, while critical minimum temperatures for activity and lower developmental thresholds are crossed more frequently. Estimates of abundance revealed that C. capitata are active year-round, but abundance declines markedly during winter. Temporal autocorrelation of average fortnightly trap captures and of development time, estimated from an integrated model to calculate available degree days, show similar seasonal lags suggesting that population increases in early spring occur after sufficient degree-days have accumulated. By contrast, population collapses coincide tightly with increasing frequency of low temperature events that fall below critical minimum temperatures for activity. Individuals of C. capitata expressing the tsl mutation show greater critical thermal maxima and greater longevity under field conditions than reference individuals. Taken together, this evidence suggests that low temperatures limit populations in the Western Cape, South Africa and likely do so elsewhere. Increasing temperature extremes and warming climates generally may extend the season over which these species are active, and could increase abundance. The sterile insect technique may prove profitable as climates change given that laboratory-reared tsl flies have an advantage under warmer conditions.     

The effects of amino acids on albumin synthesis by the isolated perfused rat liver

Albumin synthesis was measured in the isolated perfused rat liver by using the livers of both well-fed and starved rats. Starvation markedly decreased albumin synthesis. The livers from starved rats were unable to increase synthesis rates after the addition to the perfusates of single amino acids or the addition of both glucagon and tryptophan. Arginine, asparagine, isoleucine, leucine, lysine, methionine, phenylalanine, proline, threonine, tryptophan and valine, added together to ten times their normal peripheral blood concentrations, restored synthesis rates to normal. The plasma aminogram (i.e. the relative concentrations, of amino acids) was altered by depriving rats of protein for 48h. The use of blood from the deprived rats as perfusate, instead of normal blood, decreased albumin synthesis rates significantly by livers obtained from well-fed rats. The addition of single amino acids, including the non-metabolizable amino acid, alpha-aminoisobutyric acid, to the above mixture increased albumin synthesis rates to normal values. It is concluded that amino acids play an important role in the control of albumin synthesis and that more than one mechanism is probably involved.     

The acute hypoxic ventilatory response: testing the adaptive significance in human populations

The acute Hypoxic Ventilatory Response (HVR) is an important component of human hypoxia tolerance, hence presumably physiological adaptation to high altitude. We measured the isocapnic HVR (L min(-1) %(-1)) in two genetically divergent low altitude southern African populations. The HVR does not differ between African Xhosas (X) and Caucasians (C) (X:-0.34+/-0.36; C:-0.42+/-0.33; P > 0.34), but breathing patterns do. Among all Xhosa subjects, size-independent tidal volume was smaller (X: 0.75+/-0.20; C: 1.11+/-0.32 L; P < 0.01), breathing frequency higher (X: 22.2+/-5.7; C: 14.3+/-4.2 breaths min(-1); P < 0.01) and hypoxic oxygen saturation lower than among Caucasians (X: 78.4+/-4.7%; C: 81.7+/-4.7%; P < 0.05). The results remained significant if subjects from Xhosa and Caucasian groups were matched for gender, body mass index and menstrual cycle phase in the case of females. The latter also employed distinct breathing patterns between populations in normoxia. High repeatability (intra-class correlation coefficient) of the HVR in both populations (0.77-0.87) demonstrates that one of the prerequisites for natural selection, consistent between-individual variation, is met. Finally, we explore possible relationships between inter-population genetic distances and HVR differences among Xhosa, European, Aymara Amerindians, Tibetan and Chinese populations. Inter-population differences in the HVR are not attributable to genetic distance (Mantel Z-test, P = 0.59). The results of this study add novel support for the hypothesis that differences in the HVR, should they be found between other human populations, may reflect adaptation to hypoxia rather than genetic divergence through time.     

Metabolic rate variation in Glossina pallidipes (Diptera: Glossinidae): gender, ageing and repeatability

Despite the importance of metabolic rate in determining flight time of tsetse and in mediating the influence of abiotic variables on life history parameters (and hence abundance and distribution), metabolic rate measurements and their repeatability have not been widely assessed in these flies. We investigate age-related changes in standard metabolic rate (SMR) and its repeatability, using flow-through respirometry, for a variety of feeding, gender and pregnancy classes during early adult development in laboratory-reared individuals of the tsetse fly, Glossina pallidipes. Standard metabolic rate (144-635 microW) was generally within 22% of previous estimates, though lower than the values found using closed system respirometry. There was no significant difference between the genders, but metabolic rate increased consistently with age, probably owing to flight muscle development. Repeatability of metabolic rate was generally high (r=0.6-.09), but not in younger teneral adults and pregnant females (r approximately equal to 0.05-0.4). In these individuals, low repeatability values are a consequence of muscle or in utero larval development. Tsetse and other flies generally have a much higher metabolic rate, for a given size, than do other insect species investigated to date.