Evolution of retroduplicated genes in drosophila

Abstract

Gene duplication is a major process that generates new genes. Duplication by retrotransposition (also called retroduplication) is thought to facilitate the evolution of novel functions in the new gene copy (retrogene), because retroduplication does not copy the original (parent) gene’s regulatory elements and therefore retrogenes are initially distinct from their parents. In this thesis, I explore the evolution of retroduplicated gene pairs (retrogenes and their parents) in the Drosophila lineage. In Chapter 2, I investigate three retroduplicated gene pairs, and find that these retrogenes, compared to their parents, tend to be less evolutionarily stable in terms of expression pattern and coding sequence. In Chapter 3, I use genetic analysis in Drosophila melanogaster to characterize the parent gene Sep2 and its retrogene Sep5, which encode cytoskeletal septin proteins. Sep2 and Sep5 have a redundant function in imaginal cell proliferation. Sep2 mutants are semisterile and have abnormal egg chambers; this Sep2 mutant phenotype is not rescued by overexpressing a Sep5 transgene, showing that Sep2’s function in oogenesis is unique. In contrast, Sep5 mutants appear wild-type, suggesting Sep5 lacks a unique function. In Chapter 4, I further explore the functions of Sep2 and Sep5. Sep2 is also required for male fertility; however, overexpressing Sep5 rescues the Sep2 mutant male sterility, showing this function is not unique to Sep2. I characterize the localization of Sep2 and Sep5 proteins in oogenesis using green fluorescent protein-tagged transgenes, finding they have similar localization patterns. Mitotic clonal analysis shows that Sep2 and Sep5 have a redundant function in follicle cells and are likely required for follicle cell proliferation. Mutations in either Sep2 or Sep5 enhance the embryonic lethal phenotype of bazooka, a key component in the establishment of cell polarity, suggesting a link between cell polarity and both Sep2 and Sep5 function. Altogether, my research provides insight into the evolution of duplicated genes by adding to the collection of functional studies and complementing the many genome-wide studies on duplicated genes, and also sheds light on our understanding of septin function, especially with respect to their functional redundancy which has not been well studied in complex multicellular animals.