Title

Genomic instability of the histone repeats in the Drosophila virilis group

Date of Completion

January 2008

Keywords

Biology, Molecular|Biology, Genetics

Degree

Ph.D.

Abstract

Tandemly repeated DNA, such as the fly histone genes, exists in most organisms, encodes critical functions, and is typically driven to extreme levels of homogeneity among family members; what little variation exists are often single nucleotide polymorphisms. In contrast to other members of its genus, the Drosophila virilis histone repeats display signatures of extensive variation in length and composition as well as several H1 sequence variants. To place these unexpected differences in an evolutionary context, the histone repeats in five additional species from the virilis group were characterized. First, we find that the extensive polymorphic lengths observed in the histone repeats of virilis are also present in the five species and are due to precisely the same region, the H4:H2A intergenic spacer. Second, histone quartet repeats, thus far only seen in D. virilis and D. americana. are also extant in the other four species. Third, we show the histone repeat copy number varies between the species and an analysis of their core histone genes indicates polymorphisms are prevalent when compared to their counterpart species, Drosophila melanogaster. Lastly, a detailed study of the H1 gene shows three base pair indels as well as single nucleotide polymorphisms are common to this gene within all five species. These results reveal the atypical histone repeats of D. virilis are widespread in the virilis clade which led to speculating what might be responsible for such unusual signatures. In D. virilis, the H4:H2A spacer contains a complex insertion comprised of transposable element remnants, satellite repeats, and an H2A specific tandem repeat. Investigation of the H4:H2A spacer in the five related species shows similar insertion type sequences suggesting a correlation exists between the presence of transposable elements and the seemingly increased instability of the histone repeats within this group. The results of this study prove an ancestral transposon insertion occurred within the histone gene family before diversification of the clade. Additionally, the features of the histone genes of this particular species group suggest mobile genetic elements have played a key role in the molecular evolution of this tandemly repeated gene family. ^