Evidence for a high ancestral GC content in Drosophila

Study of the nucleotide composition in Drosophila, focusing on the saltans and willistoni groups, has revealed unanticipated differences in nucleotide composition among lineages. Compositional differences are associated with an accelerated rate of nucleotide substitution in functionally less constra ined regions. These observations have been set forth against the extended o pinion that the pattern of point mutation has remained constant during the evolution of the genus. A crucial assumption has been that the most recent common ancestor of the subgenus Sophophora had an elevated GC content. Unti l now, this assumption has been supported by indirect arguments, consisting of extrapolations from closely related outgroups and limited by the robust ness of mathematical descriptions concerning the extensive nucleotide compo sition differences among sequences. The present study seeks to test the ass umption of a high ancestral GC content using realistic representations of t he nucleotide substitution process to account for potential biases induced by the heterogeneous GC content of the taxa. The analysis of eight nuclear genes unambiguously corroborates that the common ancestor of Sophophora had an elevated GC content.