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.

Ratio of RMR and MMR (RA) as a function of average RMR (μLCO2/g0.67/min) for all three species pooled (homogeneity-of-slopes P > 0.05 for individual species). The blue squares represent individuals showing CYCGE and the green circles individuals showing CONGE for all individuals at all test temperatures (CYCGE: y = 0.3639 − 0.0081*x; P = 0.0153; r2 = 0.0790, CONGE: y = 0.4796 − 0.0083*x; P = 0.00004; r2 = 0.1030).Research highlights▶ Factors influencing cyclic gas exchange in three Glossina species were examined. ▶ Temperature-induced increases in VCO2 were modulated by increased burst volume. ▶ Higher metabolic rate (MR) resulted in fewer individuals showing cyclic gas exchange. ▶ The best model for predicting pattern type variation across species was RMR. ▶ Low RA and low MR are fairly good indicators of cyclic gas exchange patterns.