Evolution of polyphenic development in the beetle Onthophagus taurus (Coleoptera: Scarabaeidae)

Polyphenic development can be defined as a single genotype's ability to express two or more discretely different phenotypes in response to differences in environmental conditions. Polyphenic development is thought to play a pivotal role in the origin of morphological novelties. However, little is known about how polyphenisms evolve in natural populations, the developmental mechanisms that may mediate such evolution, and the consequences of such modification for patterns of morphological variation. This dissertation is designed to help fill this gap by examining the genetic, developmental, and ecological underpinnings of polyphenism-evolution in populations of the horn-dimorphic beetle Onthophagus taurus. Males of this species express two alternative morphologies in response to larval feeding conditions. Favorable conditions cause males to grow larger than a threshold body size and develop a pair of horns on' their heads. Males that encounter relatively poor conditions do not reach this threshold size, and remain hornless. Onthophagus taurus originally exhibited a circum-Mediterranean distribution, but became introduced to the Eastern US and Western Australia in the early 1970's. A morphometric examination of North Carolinian and Western Australian male O. taurus in combination with a breeding experiment showed that both populations have evolved highly divergent threshold body size, causing extant populations to express very different average scaling relationships between horn length and body size. Using archival museum collections to reconstruct scaling relationships in O. taurus' native range prior to introduction, a second morphometric study suggested that allometric differences between ancestral Mediterranean and descendant North Carolinian and Western Australian populations have evolved rapidly in less than 40 years since introduction to a new habitat, and that threshold evolution proceeded in opposite directions in North Carolinian and Western Australian populations. A hormone manipulation experiment was designed to elucidate the developmental mechanisms that underly allometric divergence between exotic populations. Results showed that populations that differ in their threshold location also differ in their sensitivity to the juvenile hormone (JH) analogue methoprene and the timing of the sensitive period for JH relative to other developmental events. Juvenile hormone had been shown in an earlier study to control horn expression in this species. These results suggests therefore that changes in the degree and timing of JH sensitivity may have mediated allometric divergences between Western Australian and North Carolinian populations. Lastly, comparative sampling of Eastern US and Western Australian populations showed that allometrically divergent populations also differ in the density of competing males and the competition exerted by other species with which resources are shared. These differences matched predictions from current models on how ecological factors are able to influence threshold locations in horn polyphenic beetle populations. Sampling beetle populations from a third exotic region, Eastern Australia, in which beetles express intermediate densities alongside intermediate allometric thresholds, corraborated these results.