“Synergism and antagonism of proximate mechanisms enables and constrains the response to simultaneous selection on body size and development time: an empirical test using experimental evolution”

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Goggy Davidowitz, Derek Roff, and H. Frederik Nijhout

Antagonism and synergism at developmental level determine evolutionary change to simultaneous selection at phenotypic level

Larval growth of Manduca sexta, the tobacco hornworm. From left to right, eggs, 1st, 2nd, 3rd, and 4th instar larva. The two larvae on the right are both 5th instar: one at the beginning, and one at the end, of the 5th instar. Over 90% of growth occurs during the 5th (last) larval instar, where growth increases tenfold in just five days and when peak body size is attained. Development time is the time it takes from hatching from the egg on the left, to peak size on the right.
(Credit: Goggy Davidowitz)

Why are insects the size that they are, and why does it take them as long to develop as it does? We know that natural selection can favor the evolution of traits such as body size and development time, but we are less clear on how this happens. How do the underlying developmental, physiological, endocrine and molecular mechanisms that regulate such traits control their response to selection? In this study, Davidowitz et al. imposed ten generations of simultaneous selection on four combinations of body size and development time (Big/Short, Big/Long, Small/Short, Small/Long, and a Control) in the tobacco hornworm (Manduca sexta, Sphingidae) to address that question. They show how some of the underlying physiological and developmental mechanisms act against each other, whereas others act together. Together, these explain 93% of the response of development time and 100% of the response of body size to selection. The researchers developed a novel method to reduce the highly complex processes underlying body size and development time to only three measurable traits. Each of these three traits can either constrain or enable the response to selection of body size and development time, depending on the specific direction of simultaneous selection. By greatly reducing the complexity inherent in the regulation of phenotypic traits, this method – which can be applied broadly across taxa – may facilitate genotype-phenotype mapping, a longstanding goal of biology. Read the Article