AN EPISODIC CHANGE OF RDNA NUCLEOTIDE SUBSTITUTION RATE HAS OCCURRED DURING THE EMERGENCE OF THE INSECT ORDER DIPTERA

We have studied the potential reasons for a conspicuous deviation of s ubstitution rates in Dipteran ribosomal genes. Systematic pairwise rel ative-rate tests reveal that a significant increase in substitution ra te is characteristic for Diptera, but not for the other insects analyz ed. Estimation of sequence change in specific lineages reveals that mo st of these substitutions took place during the evolution of the Dipte ran stem lineage. When related to the paleontologically documented per iods of absolute time, the substitution rate in the stem lineage of th e Diptera underwent an at least 20-fold increase compared to other ins ect groups and subsequently dropped by a factor of 10 before the diver sification of the major Dipteran subgroups. Systematic comparisons of nucleotide composition show that this episodic change in substitution rate was accompanied by a significant increase in A+T content of Dipte ran rDNA. Our data suggest that the episodic evolution of the Dipteran rDNA has most probably been caused by a change of directional mutatio n pressure which must have occurred during the evolution of the stem l ineage of the Diptera.