most of them. These many losses appear to reflect remarkably high(and variable) rates of functional transfer of mt ribosomal protein genes to the nucleus in angiosperms. The recent transfer of cox2 to the nucleus in legumes provides both an example of interorganellar gene transfer in action and a starting point for discussion of the roles of mechanistic and selective forces in determining the distribution of genetic labor between organellar and nuclear genomes. Plant mt genomes also acquire sequences by horizontal transfer. A striking example of this is a homing group I intron in the mt cox1 gene. This extraordinarily invasive mobile element has probably been acquired over 1,000 times separately during angiosperm evolution via a recent wave of cross-species horizontal transfers. Finally, whereas all previously examined angiosperm mtDNAs have low rates of synonymous substitutions, mtDNAs of two distantly related angiosperms have highly accelerated substitution rates.
The evolutionary dynamics of plant mitochondrial (mt) genomes have long been known to be unusual compared with those of animals and most other eukaryotes at both the sequence level (exceptionally low rate of point mutations) and structural level (high rates of rearrangement, duplication, genome growth and shrinkage, and incorporation of foreign DNA). The rate of synonymous substitutions (a useful approximation of the neutral point mutation rate) was shown in the 1980s to be lower in angiosperm mitochondria than in any other characterized genome, and fully 50–100 times lower than in vertebrate mitochondria (Wolfe et al., 1987; Palmer and Herbon, 1988). This gulf largely persists despite the more recent discovery of modest substitutional rate heterogeneity within angiosperms (Eyre-Walker and Gaut, 1997; Laroche et al., 1997) and vertebrates (Martin et al., 1992; Waddell et al., 1999).
Angiosperms have by far the largest mtDNAs, at least 200 kb to over 2,000 kb in size (larger than some bacterial genomes) (Palmer 1990, 1992). These genomes grow and shrink relatively rapidly; for example, within the cucumber family, mt genome size varies by more than six-fold (Ward et al., 1981). Plant mitochondria rival the eukaryotic nucleus (especially the plant nucleus) in terms of the C-value paradox they present: i.e., larger plant mt genomes do not appear to contain more genes than smaller ones, but simply have more spacer DNA (intron content and size also do not vary significantly across angiosperms). This paradox extends to plant/animal comparisons. For example, the one sequenced angiosperm mt genome (from Arabidopsis; Unseld et al., 1997; Marienfeld et al., 1999) is 367 kb in size yet contains only one more RNA gene and twice the number of protein genes (27 vs. 13) than our own mt genome, which is over 20 times smaller (16.6 kb). Angiosperm mtDNAs are large in part because of fre-