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Insects display the extraordinary amount of morphological variation, particularly in regard to their colorations. This diversity in coloration exists both in terms of the array of colors as well as the patterning of pigments. Previous studies have revealed that melanin, ommochrome, and pteridine are the most common pigments constituting insect color patterns. Among these three, the melanin pathway has been best studied so far and presents the foundation of the current understanding of insect coloration. At the same time, though, most of this insight relied on work performed in model systems such as Drosophila and Tribolium, which have significant limitations in their color palettes. Our knowledge on the roles of the melanin pathway in aposematic coloration, which is possessed by a wide variety of non-model insects, remains extremely limited. Other than the melanin pathway, functional studies on ommochrome and pteridine pathways have primarily focused on the eyes, whereas their roles in overall body coloration are much less understood. Furthermore, because these three pigmentation mechanisms have traditionally been studied separately from each other, it is still unknown how they are coordinated in a single species. Presently, there are significant gaps within single pigmentation pathways, between different pathways, as well as between model and non-model insects, all of which limits our general understanding of evolution and diversity of coloration in insects.
To address such lack of knowledge, in this dissertation we performed a comprehensive analysis involving all three major pigment pathways in Oncopeltus fasciatus (milkweed bug), a hemimetabolous insect featuring distinct orange/black aposematic color patterns. We first analyzed the pigmentation roles of the core melanin genes (TH, DDC, lac2, ebony, tan, yellow, black, and aaNAT) using RNAi. We found that although the melanin pathway is generally conserved in Oncopeltus, the pathway is employed differently between body regions. Based upon these findings, we proposed that melanin coloration can be processed in two distinct modes, “painting” and “erasing”, depending on the overall coverage of dark coloration in a specific body region. These principles of melanin patterning can be generally applied to most of the previously studied species, which in turn provides a practical framework for future studies on a wider range of insects. In addition to the melanin pathway, we also determined the roles of core ommochrome (vermilion, cinnabar, and scarlet), and pteridine genes (Punch, purple, and white) in the coloration of Oncopeltus. Our results show that the pigmentation roles of ommochrome pathway is restricted to the nymphal and adult eyes, whereas the contributions of pteridine pathway extend into the forewings and the body. Our latter results are especially interesting, indicating that the pteridine pathway can be utilized differently between embryonic and postembryonic stages. In addition, we found that the overall orange/black color patterns of Oncopeltus are formed by a two-component overlay: pteridine pigments in the body tissue and melanin coloration in the cuticle. Such multi-layer concept provides a novel insight into the coloration of insects with complex color patterns that are commonly observed in nature. Overall, our work has now established Oncopeltus as a new system for pigmentation studies, and laid out the foundation for future studies focusing on the regulatory mechanisms.
Liu, Jin, "Unraveling The Molecular Mechanisms Of Aposematic Pigmentation In Oncopeltus Fasciatus" (2016). Wayne State University Dissertations. 1554.