Thermal plasticity and fitness measurements of wing spot and wing interference patterns in drosophila

Abstract

Wings play an important role in dipteran life. Wings are primarily used in flight and also play a role in mating behaviour in many insects. In Drosophila species, wings exhibit a variety of pigmentation patterns. Wing characteristics like size, colouration, and interference were found to be associated with male fitness. In some sexually dimorphic Drosophilids, a pigmented spot pattern is found in the top-right edge of the male wings. Our understanding of the thermal plasticity of wing spots in sexually dimorphic species is limited and is more biased towards spot size variation. In this study, I investigated the thermal effects on wing spot pigmentation in two spotted Drosophila species, Drosophila biarmipes and Drosophila suzukii. Both species were exposed to three different growth temperatures (18°C, 23°C, and 28°C). The results indicated that wing spot pigmentation exhibits thermal plasticity. Wing spot pigmentation, adjusted for wing size, was highest at lower temperatures in both species. Wing size and wing spot pigmentation were inversely correlated with temperature. Both the species demonstrated comparable responses to variations in developmental temperatures, irrespective of their different habitats and geographical backgrounds. Patterns across isofemale lines revealed strong genotype-by-environment (G×E) interactions indicating that genetic variations among the lines plays a critical role in shaping their response to environmental changes. Furthermore, the chemical properties of wing spots were examined using Raman spectral analysis. This analysis revealed a chemical distinction between pigmented and non-pigmented areas of the wing, with the pigmented regions enriched in carbon-carbon double bond compounds. The presence of C=C bonds was consistently abundant in the spotted regions of D. biarmipes and D. suzukii, two members of the suzukii subgroup. This study observed conserved thermal plasticity in the wing spots of two Drosophila species from contrasting ecological zones (tropical vs. temperate). To assess the fitness implications of wing spots, I exposed a set of recently collected outbred lines of D. biarmipes to various environmental stresses. The aim was to explore the association between wing spot pigmentation and stress tolerance to see for any presence of genes that increase the offspring fitness. Males with darker wing spot pigmentation were not linked to higher stress resilience as most of the traits showed negative or neutral trends, suggesting that darker wing spots may not confer better handling of stressful environments. However, body size was positively associated with darker wing spot pigmentation. This highlights the complexities of secondary sexual characteristics and their relationship with male fitness, emphasizing the role of body size in mate choice. In addition to pigmentation, transparent insect wings also produce structural colours against dark backgrounds, which arise due to light scattering. These structural colours, known as wing interference patterns (WIPs) were examined in this study. WIPs were analyzed across four levels: 1. Species level differences, 2. Geographical variations, 3. Altitudinal variations, and 4. Thermal response of WIPs. At the species level, the species belonging to tropical habitats exhibited a brighter pattern compared to the cold adapted species. Similarly, for geographical variations, D. melanogaster populations originating from the warmer environments ended up forming a brighter WIPs while the males coming from the colder habitats had darker WIPs. D. melanogaster males collected from the Western Himalayas were studied for their altitudinal differences in WIPs which showed an altitudinal cline. To further explore the evolutionary patterning of this trait, WIPs were also studied in two sibling species, D. melanogaster and D. simulans. Thermal plasticity experiments at three growth temperatures (18°C, 23°C, and 28°C) revealed the plastic nature of this trait. Sibling species D. melanogaster and D. simulans showed parallel patterns in WIPs, aligning with their genetic relatedness. Overall, this thesis explores the thermal characteristics of two wing pigmentation traitswing spots and WIPs. The findings reveal that wing spots exhibits thermal plasticity and the response is pretty conserved in two species from distinct geographical regions. Additionally, the WIPs are observed to be brighter in populations originating from a warmer climate or exposed to higher temperatures. A cline in WIP was also observed in D. melanogaster populations collected from different elevations.