The role of toolkit genes in the evolution of complex abdominal color patterns in Drosophila guttifera

Author

Tillu Bollepogu Raja, Michigan Technological UniversityFollow

Date of Award

2017

Document Type

Campus Access Dissertation

Degree Name

Doctor of Philosophy in Biochemistry and Molecular Biology (PhD)

Administrative Home Department

Department of Biological Sciences

Advisor 1

Thomas Werner

Committee Member 1

Victor Busov

Committee Member 2

Ramakrishna Wusirika

Committee Member 3

Oliver Gailing

Abstract

How gene-regulatory networks drive complex morphological structures and patterns is an intriguing question in evolutionary and developmental biology. All animals share a set of genes known as the “genetic toolkit”, which orchestrate the building of the basic body plan. Some of these toolkit genes have been co-opted into new developmental pathways, thereby leading to many evolutionary novelties. One example is the formation of color patterns in animals. We developed the fruit fly Drosophila guttifera (D. guttifera) as a model to study complex wing and body color patterns. The coloration of D. guttifera is striking, as this species displays stark black spot, stripe, and less obvious shade patterns on its wings, thorax, and abdomen. Previously, we have shown that the Wingless (Wg) morphogen is sufficient to induce the pigmentation gene yellow (y), which is necessary for the production of black melanin spots on the wings of D. guttifera. Our current research focuses on the abdominal pigmentation pattern of the same species, which consists of four distinct sub-patterns: one pair of dorsal, median, and lateral rows of spots, plus a dorsal midline shade. Our RNA in situ hybridization data in developing pupal abdomens show that the pigmentation gene y is expressed in the same pattern as the adult melanin pattern. In our search for regulators of y gene expression on the abdomen, we found that the toolkit gene wg foreshadows the entire spotted pattern just before y transcription starts, while abdominal-A (abd-A), decapentaplegic (dpp), hedgehog (hh), and zenknullt (zen) are also expressed at that time, but only in distinct subsets of the abdominal pattern. Using a transgenic reporter assay in D. guttifera pupae, we identified a ~1 kb fragment within the y intron, containing a cis-regulatory element (CRE) that drove DsRed reporter expression in a pattern closely resembling the adult abdominal spot pattern. Further sub-divisions of this CRE and subsequent transgenic reporter analyses revealed a core CRE, which induces stripe expression of the reporter gene DsRed across the abdomen, when a cluster of putative repressor sites was excluded from the CRE. Our data suggest that abdominal spot patterns of fruit flies may have evolved from an ancestral stripe pattern through partial repression of portions of the stripes. We next elucidated the molecular mechanisms causing another pigment-forming gene, tan (t), to be co-expressed with y in identical spot expression patterns on the wings and abdomen. We identified two t CREs, which drive t in spotted patterns on wings and the abdomen. Detailed analyses of the t CRE that drives abdominal spots revealed that this CRE also has core-stripe activity and contains putative repressor elements. Thus, even the t abdominal spots may develop through stripe repression. Taken together, our data indicate that similar regulatory architectures of the y and t genes cause them to be co-expressed in complex patterns on the abdominal tergites, thus providing one of the first examples of how complex morphological patterns develop through the concerted functions of two pigment-forming genes.

Recommended Citation

Bollepogu Raja, Tillu, "The role of toolkit genes in the evolution of complex abdominal color patterns in Drosophila guttifera", Campus Access Dissertation, Michigan Technological University, 2017.

https://doi.org/10.37099/mtu.dc.etdr/319