Molecular evolution of trypsin genes in Drosophila.
|Title:||Molecular evolution of trypsin genes in Drosophila.|
|Abstract:||Nucleotide composition bias and codon usage bias are commonly observed in a wide range of organisms, but the causes of both are still under debate. One view is that codon usage bias and base composition bias are both the result of a directional mutation pressure in favor of AT or GC; others argue that codon usage bias is a result of natural selection acting to mold the codon choice to match the frequency of the corresponding tRNAs. A number of recent studies have indicated that nucleotide bias is especially marked in gene families that are undergoing concerted evolution. In this thesis, I use the Drosophila trypsin gene family as a model system to study the causes and consequences of concerted evolution in Drosophila, and to investigate the evolutionary forces that may be responsible for the observed nucleotide bias. From each of the two related Drosophila species, D. melanogaster and D. erecta, a 12kb genomic region was sequenced and eight trypsin genes were identified. Some members of this gene family have been evolving independently while the others have been evolving in a concerted fashion. In both species, the nonsynonymous codon positions have a moderate GC content, while the synonymous sites are very GC-rich. For the genes that have been undergoing concerted evolution, due to rapid gene conversion, their synonymous G + C content is much higher than that of the independently evolving genes. In addition, these genes are characterized by an elevated frequency of pyrimidines (C or T) at synonymous sites on the coding strand. A combination of selective constraints, directional DNA mutation pressure, and DNA repair bias could have resulted in the base composition pattern observed in these trypsin genes.|
|Collection||Thèses, 1910 - 2010 // Theses, 1910 - 2010|