A Quantitative Analysis of Growth and Size Regulation in Manduca sexta: The Physiological Basis of Variation in Size and Age at Metamorphosis

Abstract

Body size and development time are important life history traits because they are often highly correlated with fitness. Although the developmental mechanisms that control growth have been well studied, the mechanisms that control how a species-characteristic body size is achieved remain poorly understood. In insects adult body size is determined by the number of larval molts, the size increment at each molt, and the mechanism that determines during which instar larval growth will stop. Adult insects do not grow, so the size at which a larva stops growing determines adult body size. Here we develop a quantitative understanding of the kinetics of growth throughout larval life of Manduca sexta, under different conditions of nutrition and temperature, and for genetic strains with different adult body sizes. We show that the generally accepted view that the size increment at each molt is constant (Dyar’s Rule) is systematically violated: there is actually a progressive increase in the size increment from instar to instar that is independent of temperature. In addition, the mass-specific growth rate declines throughout the growth phase in a temperature-dependent manner. We show that growth within an instar follows a truncated Gompertz trajectory. The critical weight, which determines when in an instar a molt will occur, and the threshold size, which determines which instar is the last, are different in genetic strains with different adult body sizes. Under nutrient and temperature stress Manduca has a variable number of larval instars and we show that this is due to the fact that more molts at smaller increments are taken before threshold size is reached. We test whether the new insight into the kinetics of growth and size determination are sufficient to explain body size and development time through a mathematical model that incorporates our quantitative findings.

Do respiratory limitations affect metabolism of insect larvae before moulting: an empirical test at the individual level

“…Even if there is some within-instar tracheal growth in final-instar M. sexta (Helm and Davidowitz, 2013), it is unlikely to compensate for the rapid increase in oxygen demand during exponential tissue growth before the instar-specific critical mass (cf. Nijhout et al, 2006;Grunert et al, 2015). In fact, tracheal volume, which is typically assumed to correlate positively with oxygen supply capacity, decreases within earlier instars in M. sexta (Callier and Nijhout, 2011) and in fifth-instar Schistocerca americana apparently because growing tissues compress tracheae (Lease et al, 2006).…”

“…In fact, tracheal volume, which is typically assumed to correlate positively with oxygen supply capacity, decreases within earlier instars in M. sexta (Callier and Nijhout, 2011) and in fifth-instar Schistocerca americana apparently because growing tissues compress tracheae (Lease et al, 2006). According to the ODIM hypothesis, moulting is triggered at the critical mass at which oxygen demand meets the supply capacity, but growth still continues after it, although with a decelerating rate until the cessation of growth for moulting (or entering the wandering stage in the final instar) (Nijhout et al, 2006;Grunert et al, 2015). At the cessation of growth, the gut is purged and moulting begins (except in the final instar where the wandering stage begins after gut purging), which results in changes in MR independently of the ODIM mechanism (cf.…”

“…The critical mass (sensu Nijhout et al, 2006Nijhout et al, , 2010Callier and Nijhout, 2011;Shingleton, 2011) can be seen as a developmental checkpoint where physiological changes are induced (Callier and Nijhout, 2013), potentially associated with constraints for growth. Suggested candidate mechanisms include the limited capacity of the exoskeleton to expand (Bennet-Clark, 1963), potentially hypoallometric growth of the gut, compromising nutrient absorption efficiency (see Esperk and Tammaru, 2004;Grunert et al, 2015), and availability of metabolic intermediates acting as precursors for growth . In D. melanogaster, the regulation of the critical mass is affected by insulin signalling, the production of prothoracicotrophic hormone and growth of the imaginal discs (Shingleton, 2011, and references therein), so these mechanisms potentially contribute to size-sensing and moult induction.…”

“…The lack of expected response under hyperoxia does not necessarily refute the ODIM hypothesis because it is possible that other mechanisms start to constrain growth and set the critical mass before the constraint that is due to oxygen supply is met under hyperoxia. It is evident that other mechanisms are involved in moult induction (Callier and Nijhout, 2011;Grunert et al, 2015), but the ODIM mechanism appears to be the dominant one under benign growth conditions. Empirical tests of the ODIM hypothesis have concentrated on the critical mass.…”

“…Respiration rate (with respiration rate we refer to the rate of gas exchange -O 2 consumption or CO 2 production rate -throughout) is expected to first increase with mass but then level off at the critical mass, resulting in a change in the regression slope of respiration rate to mass at the critical mass. Moreover, growth decelerates after attainment of the critical mass because of increasing oxygen limitation to growing tissues, which means that the critical mass is at the inflection point of the instar-specific growth trajectory (Grunert et al, 2015). Limited oxygen supply should also suppress metabolism (Wigglesworth, 1972;Downer, 1981), so that the increase in metabolic rate decelerates towards the end of a larval instar.…”