Transposable Elements and the Evolution of Virus Resistance in Drosophila melanogaster

Abstract

Viral infection results in a significant fitness cost on organisms, which driving evolutionary change in antiviral defences. Previous research in Drosophila has shown that there is large genetic variation in pathogen resistance, and that a significant amount of genetic variation to virus resistance can be explained by major-effect polymorphisms in a small number of genes.

In this thesis I will explore the role of transposable elements in the evolution of virus resistance. We discover a Doc retrotransposon insertion in the Drosophila gene Veneno (Ven; CG9684), a gain of function mutation which confers resistance to Drosophila A Virus (DAV). We show that the insertion acts by creating a truncated transcript Ven_tra, which is sufficient to induce DAV resistance in Drosophila cell culture. Although this transcript includes sequences of Doc element origin, we show that it doesn’t require these sequences for its resistant phenotype. We also show that resistance doesn’t rely on the functioning of the key protein domains Ven_tra encodes: an MYND Zinc finger and a Tudor domain. We work on further characterizing this allele by narrowing down the pathways it interacts and find it does not require a functional siRNA pathway to cause resistance, and fail to show its reliance on other immune pathways, leaving us with a limited understanding of its mechanism. This adds it to a growing list of poorly understood restriction factors, suggesting that virus resistance in Drosophila relies largely on these idiosyncratic restriction factors targeting specific viruses.

Furthermore, we examine another instance of the Doc element insertion into the sequence of the gene CHKov1 which confers resistance to the Drosophila melanogaster sigma virus (DMelSV). We show that the Doc insertion in CHKov1 leads to the expression of a transcript that includes sequences from both CHKov1 and the Doc element’s ORF1, encoding a gene-TE chimera which is sufficient to cause resistance.

Finally, we explore ways in which virus resistance can be artificially created in arthropods through genetic engineering. We attempt to use CRISPR-associated gene 13 (Cas13) to generate DAV resistance in Drosophila melanogaster in the hope that the system could be extended to confer resistance to other viruses (such as Zika) in other arthropods (such as Aedes aegypti).