Characterizing mutagenesis in Fusarium circinatum

Abstract

Spontaneous mutagenesis can be divided into three main steps: the introduction of DNA damage and lesions, damage recognition and DNA repair. All sources of spontaneous mutagenesis originate from within the cell itself, e.g., polymerase errors cause DNA mismatches and reactive oxygen species alter the chemical composition of DNA bases. The combined effects of all these processes influence spontaneous genomic mutation rates, which are thought to be a characteristic of individual species and/or groups of species. Although much is known about different mutagens and how they cause mutations the sequence context of these mutations are less well understood. The results of this MSc study on mutation in the filamentous fungus Fusarium circinatum showed that the 5ʹ and 3ʹ neighbouring bases of a single nucleotide polymorphism can significantly influence the type of substitution that occurred leading to the formation of mutational motifs. This was the case for both sets of genes examined (core housekeeping and non-ribosomal protein synthetase genes), whose evolution is known to differ. The fact that none of the identified motifs are shared between the two sets of genes could indicate that the cellular mutagens and/or repair machinery function differently for the two gene groups. Furthermore, none of the mutable motifs that have been identified for the well-known mutagens in model organisms could be detected in the fungus, which suggests that mutagens and/or DNA repair mechanisms of this fungus are unique. Although limited information is available for non-model eukaryotes, an estimate for the rate at which mutations arise across the genome of F. circinatum could be a good starting point for comparisons of its evolutionary rate to those of its close relatives. This was accomplished using a fluctuation analysis involving nitrate non-utilizing mutation reversion. Although mutation rate determined in this study is probably not precisely accurate, it represents a good starting point for future comparative studies on the evolutionary rate of Fusarium species. As a whole this study laid the foundation for a better understanding of spontaneous mutagenesis at specific sites in certain groups of genes as well as across the genome of the economically important plant pathogen F. circinatum.

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