How does the underlying physiology enable or constrain the response to simultaneous selection on body size and development time? This work is collaborative with Drs. Fred Nijhout (Duke University) and Derek Roff (UC Riverside).
Two of the most important traits of any organism are body size and development time. Larger insects typically leave more offspring, whereas shorter development time minimizes the time an individual is exposed to predators and parasites. It has been known for over a century that body size is the product of growth rate and the duration of the growth period. However, most work on understanding the regulation of body size has focused only on the growth rate. Our work has identified, for the first time, the physiological mechanism (the timing of two hormonal events) that regulates the duration of the growth period (Davidowitz et al. 2003, 2004). These two hormonal events, together with growth rate, explain over 95% of the variation in body size in insects (D’Amico et al. 2001). These three factors interact non-linearly in response to environmental variation (Nijhout et al. 2006, 2010). Further work has shown that larvae of M. sexta modify each of the three physiological factors differently depending on the type of environmental variation they encounter (temperature versus diet quality) (Davidowitz et al. 2004). By exploiting different components of the physiological mechanism, larvae can respond to multiple sources of environmental variation somewhat autonomously, enabling them to cope with multiple sources of variation simultaneously. Our work has shown that these same three physiological factors also regulate development time in insects (Davidowitz et al. 2004), and that growth rate interacts differently with these two factors to determine final larval size and total development time of the last larval instar (Davidowitz et al. 2004, 2005, 2012). This physiological framework for the regulation of body size and development time in insects has broad applicability: we have shown it can be extended to protists, plants, amphibians and mammals (Davidowitz and Helm 2014). With a former postdoctoral fellow in my lab (Craig Stillwell), we have extended this work to examine the physiological regulation of sexual size dimorphism. Female insects are typically larger than males, yet we do not know how this size difference arises. Our research has identified how differential sensitivity between M. sexta males and females of the physiological factors that determine body size generates sexual size dimorphism (Stillwell & Davidowitz 2010a,b; Stillwell et al. 2010; Sillwell and Davidowitz 2014).