Episodic change of rDNA during the emergence of the Diptera.
M. Friedrich and D. Tautz. 1997.
Mol Biol Evol 14: 644-653
An episodic change of rDNA nucleotide substitution rate
has occurred during the emergence of the insect order
Diptera.
"The claim of a sister group relationship between the Diptera and the peculiar insect order Strepsiptera (Whiting and Wheeler 1994) based on 18S rDNA sequences must be seen in the light of these problems, since a recent study shows that the rDNA genes of the latter group have also evolved under a strong compositional bias towards A+T (Chalwatzis et al. 1994)."
Abstract. We have studied the potential reasons for a conspicuous deviation of substitution rates in Dipteran
ribosomal genes. Systematic pairwise relative-rate tests reveal that a significant increase in
substitution rate is characteristic for Diptera, but not for the other insects analyzed. Estimation of
sequence change in specific lineages reveals that most of these substitutions took place during the
evolution of the Dipteran stem lineage. When related to the paleontologically documented periods
of absolute time, the substitution rate in the stem lineage of the Diptera underwent an at least
20-fold increase compared to other insect groups and subsequently dropped by a factor of 10
before the diversification 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 Dipteran rDNA. Our data suggest that the episodic evolution of the
Dipteran rDNA has most probably been caused by a change of directional mutation pressure which
must have occurred during the evolution of the stem lineage of the Diptera.
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