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The existence of evolutionary rate variation has previously been demonstrated between different orders, different species and even between different regions of the same gene. To examine rate variation between closely related species of rodents we have sequenced the adenine phosphorybosyltransferase (APRT) gene from Mus spicilegus, Mus pahari, Mastomys hildebrandtii, Stochomys longicaudatus and Gerbillus campestris and compared these sequences with the previously published Mus musculus, Rattus norvegicus and Mesocricetus auratus APRT sequences. The alignment of these eight rodent APRT sequences reveals two large insertions within the introns: an insertion with sequence similar to a B1 repetitive element is found within Mastomys and an insertion with sequence similar to a B2 repetitive element is found within M. pahari. A phylogeny for the rodent APRTs agrees with the previously published rodent phylogeny based on other molecular and morphological data. The relative rate test which is often used to test for variation in rates of evolution in different lineages is shown here to be sensitive to the choice of outgroup and therefore should be used with great caution. This sensitivity is detectable only with closely related species and results from the prevalence of homoplastic substitutions. Rate variation is demonstrated within the APRT exons and introns and between the rodent species (with the most significant difference being a rate difference in M. spicilegus). In addition, some third codon positions are shown to be more prone to substitution than others. This clearly demonstrates that even between very closely related species there is ample evidence of major differences in rates of evolution among species, among regions of the gene and among different positions within the gene. We also demonstrate that standard methods of analysis might not detect this variation.
Rodents are thought to have relatively high rates of evolution, twice as fast as the rates for mammals in other orders. However, the uniformly high rates of evolution inferred for the order Rodentia from Mus musculus and Rattus norvegicus are not consistently found for other rodent species. Using a maximum likelihood phylogenetic algorithm (DNAML), we show here that Mus spicilegus has a fivefold different rate of evolution in 1100 bp around the adenine phosphoribosyltransferase gene (APRT) since its divergence from a common ancestor with Mus musculus. A greater than threefold difference in rates is also found in a comparison of the number of evolutionary events directly detected from the APRT sequences of these two closely related Mus species. The evolutionary events can be directly detected, since M. spicilegus, M. musculus, and the four rodent outgroup species used to determine the ancestral sequence are so closely related. One of the major differences between M. spicilegus and M. musculus that might affect evolutionary rate is the degree of commensalism with man. The Mus species therefore provide a useful model for testing various hypotheses for the causes of rate variations between genes, and possibly, between lineages.
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The complete genomic sequence of the rat APRT gene is described and compared with published mammalian sequences. The rat APRT gene organization is typical of other rodent APRTs with five exons, one large intron of 993 bp, and three smaller introns averaging 145 bp. Because complete sequences for mouse and Chinese hamster APRT are also known, it is possible to compare the evolutionary rates of change in the exons with those of the introns. The latter provide one possible estimate of underlying rates of change. It is shown that the APRT exons have differential rates of evolution in rodents and have had a recent and rapid burst of substitutions within the mouse lineage. Rates of change in the exons do not appear to be strongly correlated with the rates of change in the introns.
The processes of spontaneous mutation are known to be influenced by neighboring DNA. Imperfect nearby repeats in the neighboring DNA have been observed to mutate to form perfect repeats. The repeats may be either direct or inverted. Such a mutational process should create perfect direct and inverted repeats in intergenic DNA. A larger than expected number of direct repeats has generally been observed in a wide range of species in both coding and noncoding DNA. Simulations are carried out to determine how this process might influence the repetitive structure of genomic DNA. These simulations show that small repeats created by this kind of a mutational process can explain the excess number of repeats in intergenic DNA. The simulations suggest that this mechanism may be a common cause of mutations, including single-base changes. The influences of the distance between imperfect repeats and of their degree of similarity are investigated.