Proximate mechanisms of condition-dependent exaggerated weapon growth in the Asian rhinoceros beetle Trypoxylus dichotomus (Kono)

Abstract

Sexual selection as an evolutionary force is responsible for generating some of the most iconic morphologies in the animal kingdom. My dissertation utilizes the Asian rhinoceros beetle, Trypoxylus dichotomus (Kono), to investigate the proximate mechanisms behind both the evolution of shape in a sexually selected weapon, as well as how sexually selected weaponry evolves exaggerated growth responses based on an organism’s condition. To determine how horns are shaped developmentally, I have performed RNAi mediated knockdown of candidate developmental patterning genes predicted to play a role during horn growth. In order to investigate the proximate signals beetle horns utilize to become sensitive to nutrition, I utilized comparative endocrinology to investigate the role conserved insect endocrine signals play in the evolution of trait exaggeration. Finally I utilized next-generation sequencing to analyze differences in gene expression in a diversity of morphological traits across both nutritional condition and sex. This dissertation provides the first evidence of role for members of the Sp-family of zinc-finger transcription factors in the development of rhinoceros beetle horns (Chapter 2). I have also demonstrated that despite being correlated with body size during development, juvenile hormone does not play a role during horn exaggeration in T. dichotomus (Chapter 3), contrary to the role this hormone plays during condition-dependent exaggeration in other insects. Finally, this dissertation demonstrates that patterns of gene expression across traits in organisms exposed to different conditions vary with each tissue’s individual conditional sensitivity and that this effect is much greater not only within sexually dimorphic tissue, but as a consequence of sexual dimorphism itself. (Chapter 4). Furthermore, many genes predicted to be involved in nutritionally sensitive growth are, in fact, involved in the maintenance of sexual dimorphism. Together my dissertation provides insight into the molecular and physiological mechanisms behind exaggerated weapon growth in beetles.