Evol Ecol Res 6: 49-62 (2004)     Full PDF if your library subscribes.

The effects of environmental variation on a mechanism that controls insect body size

Goggy Davidowitz,1* Louis J. D’Amico2 and H. Frederik Nijhout2

1Department of Ecology and Evolutionary Biology, University of Arizona, Tucson, AZ 85721 and  2Department of Biology, Duke University, Durham, NC 27708, USA

Author to whom all correspondence should be addressed.
e-mail: goggy@email.arizona.edu


Adult body size in animals is determined by the duration of the growth period and the amount of mass gained during that period. Few models of body size regulation distinguish between these two components or explicitly address the mechanisms that control the duration of the growth period. Body size in the tobacco hornworm Manduca sexta is controlled by three underlying physiological factors: growth rate; timing of the onset of juvenile hormone decay (which initiates the processes leading to pupation), as measured by the critical weight; and the timing of prothoracicotropic hormone (PTTH, which stimulates ecdysteroid secretion) and ecdysteroid secretion, as measured by the interval to cessation of growth (ICG, the time interval between the attainment of the critical weight and entry into the pre-pupal wandering stage). The critical weight and the ICG determine the duration of the growth period, while growth rate determines how much mass accumulates during that period. We studied how phenotypic plasticity of body size in M. sexta, in response to variation in temperature and diet quality, is affected by phenotypic plasticity of these three physiological determinants of body size. We show that plasticity of size in response to diet quality is regulated by variation in growth rate and critical weight, while plasticity of size in response to temperature is regulated by variation in growth rate and the ICG. These results demonstrate the importance of the timing of hormonal events in the regulation of phenotypic plasticity. We suggest that the differential sensitivity of the physiological processes that regulate body size may enable insects to adjust adult body size in response to simultaneous variation in multiple types of environmental stimuli.

Key words: body size, critical weight, endocrine regulation, growth rate, interval to cessation of growth, Manduca sexta.

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