Intrapopulational variation in the standard metabolic rate of insects:repeatability, thermal dependence and sensitivity (Q10) of oxygen consumption in a cricket

Abstract

SUMMARYStudies focusing on physiological variation among individuals, and its possible evolutionary consequences, are scarce. A trait can only be a target of natural selection if it is consistent over time, that is, a trait must be repeatable. In ectotherms it has been suggested that standard metabolic rate(MR) is related to Darwinian fitness, since it reflects energy usage and expenditure. The metabolic rate of the cricket Hophlosphyrum griseuswas determined at three ambient temperatures. Repeatability of MR was estimated by product–moment correlation on residuals of body mass, as well as the thermal sensitivity of MR on an individual basis (individual Q10). The MR of H. griseus was significantly repeatable(r=0.53) and highly dependent on ambient temperature, and its sensitivity (Q10) was dependent on the temperature range. Our estimation of MR repeatability was high in comparison to published studies in vertebrates. Ours is the second report of repeatability (i.e. consistency over time of an individual's performance ranking within a population) of any aspect of energy metabolism in an insect, and also the first study to report significant repeatability of MR.Individual Q10 values revealed important interindividual variation, which reflects the existence of intrapopulational variability in the thermal sensitivity of MR. In addition, individual Q10 values were negatively correlated between temperature ranges. This means that crickets having low Q10 at low temperatures, presented high Q10 at high temperatures, and vice versa. Our results suggest that MR could be of selective value in insects, showing consistency over time and intrapopulational variability in its thermal dependence. Nevertheless, its heritability remains to be determined.

Quantitative genetic variation of metabolism in the nymphs of the sand cricket, Gryllus firmus, inferred from an analysis of inbred-lines

“…But, so far as we are aware, no study has addressed the heritability of metabolic rate. Repeatability studies suggest that metabolic rate is consistent over time in insects, and is consistent with a high heritability (Nespolo et al, 2003a;Chappell and Rogowitz, 2000;Terblanche et al 2004). The low broad sense heritabilities we found in morphological traits of adults (0-0.18) are lower than obtained in previous studies in adults of the seven inbred stocks, (Roff and Sokolovska 2004).…”

“…metabolic rate, a general proxy of the "intensity" of functioning of every metacellular animal, represents nearly the same state variable in very different animals). The repeatability of standard metabolic rate is high in several species of insects Nespolo et al, 2003a;Terblanche et al, 2004), which is consistent with a high heritability. On the other hand, both broadand narrow-sense heritabilities of metabolic rate appear to be relatively low in vertebrates.…”

“…One of the least known aspects of insect physiology is the genetic variance of physiological traits, especially metabolic rate (Hoffmann, 2000;Nespolo et al, 2003a). This is rather surprising as flowthrough respirometry has been intensely used to study a very wide range of factors correlated with metabolic rate, such as age (Hack, 1997), diet composition (Zanotto et al, 1997), reproductive cost (Prestwich and Walker, 1981;Hack, 1997Hack, , 1998, flight and locomotion , and adaptive features such as seasonal acclimation (Forlow and MacMahon, 1988), dry habitat adaptations (Cooper, 1993), altitude (Hadley and Massion, 1985), latitude across populations (Rourke, 2000) and species differences (Davis et al, 2003).…”

“…However, as we advanced in the introduction, physiology is a very heterogeneous category and generalizations within this category may be misleading. The difficulties of measuring metabolic rate in animals have impeded accurate estimates of heritabilities, as reflected in generally low sample sizes: for example, the cited studies above were all performed using sample sizes of less than 26 families (Dohm et al 2001;Nespolo et al 2003b;Bacigalupe et al 2004). Estimates of heritabilities of metabolism in mammals obtained with larger sample sizes appear to give large and significant narrow-sense heritabilities (Nespolo et al, unpublished results; Koteja, personal communication).…”

“…On the other hand, both broadand narrow-sense heritabilities of metabolic rate appear to be relatively low in vertebrates. For instance, Dohm et al, (2001) presented only negative estimates for Mus, Nespolo et al (2003b) report h 2 = 0.14 (non-significant) for basal metabolic rate in a field rodent. The purpose of the experiments reported herein was to determine if the broad-sense heritability of metabolic rate in the sand cricket, Gryllus firmus is consistent with the high repeatabilities estimated in invertebrates or with the low heritabilities reported in vertebrates.…”