show elevated (mt-specific) rates some 50–100 times higher than normal, putting them on a par with the very rapidly evolving mt genomes of mammals. Furthermore, analysis of several other species from the Pelargonium (Geraniaceae) and Plantago (Plantaginaceae) families shows a range of enhanced mt divergences in both families, as if sequential increases in the mt mutation rate had occurred during their evolution (Y.C., C.L.P., and J.D.P., unpublished work).
The magnitude and recency of these mutation rate shifts appear to be unprecedented for evolutionary lineages of species (as opposed to the well known but ephemeral mutator strains of laboratory mutant cultures, of wild strains of bacteria, or of human colon cancers). It remains to be seen whether the same sorts of underlying mechanisms are involved, such as changes in the fidelity and efficacy of DNA replication and mismatch repair (Modrich and Lahue, 1996).
Plant mt genomes continue to spring marvelous evolutionary surprises. The discovery that certain angiosperm groups are rapidly moving a large set of mt ribosomal proteins to the nucleus seems remarkable in two contexts: first, that they still have these so “easily transferred ” genes left to transfer after a roughly 2-billion-year period of mt existence; second, that animals lost all of these ribosomal protein genes at least 0.6 billion years ago (i.e., before they became animals) and that there has been absolutely no functional gene transfer within the long period of metazoan evolution (plants still do it, animals don't!). For reasons as yet unfathomable, rates of functional gene transfer appear to vary hugely across lineages and over time. The very recent and explosive burst of cox1 intron invasions into angiosperm mt genomes and the discovery of unprecedentedly large increases in the mt point mutation rate in two groups of angiosperms also speak to the surprising fluidity of the forces that control the rates of all manner of classes of mutations. These discoveries pave the way for more reductionist studies aimed at elucidating molecular mechanisms underlying these striking evolutionary patterns and rate changes. They also point to the opportunity afforded by microarray technology to mine new veins of molecular evolutionary gold by scaling up by orders of magnitude the Southern blot approach so successfully used thus far.
We thank Jeff Doyle, Jane Doyle, Peter Kuhlman, Jackie Nugent, Phil Roessler, and Andy Shirk for various contributions and Jeff Blanchard, Dan Daley, Will Fischer, Patrick Keeling, and Jim Whelan for helpful