Evolution and gene transfer in purple photosynthetic bacteria

A concern voiced in connection with recent sequencing of cytochrome c from the Rhodospirillaceae or purple non-sulphur photosynthetic bacteria is that molecular information might be of little use in deciphering bacterial phylogeny because of the possibility of lateral transfer of genes and the consequent scrambling of the genetic record. This could be true for many proteins, of course, but the immediate question is, is it true for cytochroms available for comparison? The evidence suggests that this is probably not so. The disagreement between cytochrome c sequences and the standard taxonomy of the Rhodospirillaceae in Bergey's Manual is not a problem. That reference, as its title indicates, is a manual of determinative rather than evolutionary bacteriology. Its goal is a reproducible system for identification of bacteria. If one claims that these determinative categories also have phylogenetic or evolutionary meaning, this is an assertion that must be proven. The argument outlined below suggests that molecular traits may eventually become more a dependable basis for classification of the Rhodospirillaceae than is gross morphology.