Dissecting intraspecific variation in body size related traits in Drosophila melanogaster
by Amel Chtioui
Date of Examination:2024-02-26
Date of issue:2024-10-23
Advisor:Prof. Dr. Henner Simianer
Referee:Prof. Dr. Armin Schmitt
Referee:Dr. Nico Posnien
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Abstract
English
Understanding the evolution of complex morphological traits is one of the main challenges in biology. This requires the identification of the causative genomic regions and the molecular changes responsible for phenotypic differences. However, the genetic basis of many traits is complex as multiple genomic loci with small effects contribute to the phenotypic outcome. This complexity is further enhanced because genes rarely act individually; they are interconnected in complex gene regulatory networks (GRNs). To gain novel insights into the genetic architecture and the evolving nodes of GRNs, I studied natural variation in multiple body size related traits in a natural population of the fruit fly Drosophila melanogaster. Variation in compound eye size is pervasive in insects and it represents a complex quantitative trait. The development of the eye is controlled by a complex GRN composed of more than 5,000 genes. In Chapter I of this thesis, I employ the Drosophila melanogaster Genetic Reference Panel (DGRP) to genetically dissect intraspecific variation in compound eye size. I show tremendous variation in ommatidia number between 162 DGRP lines. To establish a link between phenotypic divergence and genetic changes, I performed genome wide association studies (GWAS). I identified 26 single nucleotide polymorphisms (SNPs) above the nominal threshold p-value of 10−05, annotated to 13 candidate genes. A functional validation screen of the three genes containing most associated SNPs (mbl, Trim9, CG15498) confirmed a potential involvement in eye size regulation. To better understand the cellular and developmental processes regulated by these genes, the gene Trim9 (Boyle et al., 2017; Frazer et al., 2009), which codes for a RING domain E3 ubiquitin ligase, was analyzed in detail. Knockdown of this gene during eye development resulted in an increase of apoptotic cells and a severe reduction or the entire loss of compound eyes. Our data suggests that Trim9 may be a negative regulator of apoptosis during eye development. The role of apoptosis in ommatidia number variation in Drosophila has not been observed yet, making this study an exciting start for upcoming research on morphological evolution. D. melanogaster wings and legs are two other highly variable structures that are associated with crucial tasks, such as motility to find food resources and mating partners and to avoid predators. Therefore, in chapter II, I quantified variation in wing size, wing shape and leg (i.e. tibia) length in females of 162 DGRP lines applying linear measurements and geometric morphometrics. To better understand the genetic bases and to identify the genes and pathways involved in variation in these traits, I performed a GWAS analysis. I found different genes, such as en, Gug, foxo and Hipk (for the wing size) and ed, MKp3, and salm (for the wing shape) to be associated with phenotypic variation. For tibia size variation, I detected dally and pico, two genes known to be involved in signaling pathways involved in leg development. As many of the identified candidate genes are involved in different signaling pathways, such as Egfr-, Notch-, Hippo-, Dpp- and Hh-signaling, my data is in line with current observations confirming the polygenic character of these traits. In summary, my thesis demonstrates that the integration of different genetic and functional methods and various datasets allows gaining major insights into the development and evolution of complex morphological traits.
Keywords: Body size; Drosophila melanogaster; Intraspecific variation; Eye size variation