The living world is filled with limitless three-dimensional variation in size, shape, and the presence of novel structures that only appear in one or a few species. Although there are several examples of genetic alterations that cause the loss of morphological structures, the questions remain as to how novel structures form and how shape changes. Using the highly divergent Drosophila genitalia as a model of recent shape evolution, we can begin to uncover how relevant pathways that pattern and control growth are modified to create these diverse morphological forms. Specifically examining the posterior lobe, a recently evolved novelty of the melanogaster clade that is rapidly diverging in shape and required for male fertility, we are taking a candidate gene approach to investigate the origination and modification of this organ. After characterization of the development of this tissue, we have begun investigating the Pox neuro (Poxn) gene, a transcription factor that has been implicated in male genital development. Poxn is required for proper posterior lobe development and growth, which raises the question of how it obtained this role in a novel setting, and stimulates the hypothesis that Poxn contributes to posterior lobe shape variation. Investigations of Poxn, in combination with several other candidate genes (morphogens, signaling molecules, cell cycle control genes), will allow us to understand the flexible points in growth control pathways as well as how nascent genetic programs are established. This will ultimately lead to insights into how novel structures form and how shape evolves
