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.

Highlights

  • Size and age at maturity are important life history traits

  • We found that the asymptotic mass, a, in juvenile hormone (JH)-treated last-instar larvae is a constant multiple of the initial mass of the instar (a = wiÃ11.7 ±0.2 SEM) across a large range of body sizes for several of our genetic strains

  • Our analysis of the growth kinetics of Manduca has revealed several interesting and some surprising features of the regulation of body size and development time that are likely to be widespread among insects, and that raise a number of questions for future research

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Summary

Introduction

Size and age at maturity are important life history traits. Body size can affect reproductive capacity [1,2,3] and development time can affect generation time [4], so both can affect the rate of increase of a population or the rate at which favorable genes can spread. Nutrient manipulation can induce an additional two-fold variation in pupal mass [9,10]. Such observations suggest that the small amount of phenotypic variation we observe under natural conditions must be due to the fact that body size is robust to normally-occurring genetic and environmental variation. This robustness must arise from the developmental and physiological mechanisms that regulate body size

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