期刊名称：Proceedings of the National Academy of Sciences
印刷版ISSN：0027-8424
电子版ISSN：1091-6490
出版年度：1999
卷号：96
期号：4
页码：1475-1479
DOI：10.1073/pnas.96.4.1475
语种：English
出版社：The National Academy of Sciences of the United States of America
摘要：To estimate patterns of molecular evolution of unconstrained DNA sequences, we used maximum parsimony to separate phylogenetic trees of a non-long terminal repeat retrotransposable element into either internal branches, representing mainly the constrained evolution of active lineages, or into terminal branches, representing mainly nonfunctional "dead-on-arrival" copies that are unconstrained by selection and evolve as pseudogenes. The pattern of nucleotide substitutions in unconstrained sequences is expected to be congruent with the pattern of point mutation. We examined the retrotransposon Helena in the Drosophila virilis species group (subgenus Drosophila) and the Drosophila melanogaster species subgroup (subgenus Sophophora). The patterns of point mutation are indistinguishable, suggesting considerable stability over evolutionary time (40-60 million years). The relative frequencies of different point mutations are unequal, but the "transition bias" results largely from an {approx}2-fold excess of G*C to A*T substitutions. Spontaneous mutation is biased toward A*T base pairs, with an expected mutational equilibrium of {approx}65% A + T (quite similar to that of long introns). These data also enable the first detailed comparison of patterns of point mutations in Drosophila and mammals. Although the patterns are different, all of the statistical significance comes from a much greater rate of G*C to A*T substitution in mammals, probably because of methylated cytosine "hotspots." When the G*C to A*T substitutions are discounted, the remaining differences are considerably reduced and not statistically significant.
