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FIGURE 2. Summary of legume cox2 gene distribution and expression data in a phylogenetic context. The left two columns indicate the presence (+) or absence (-) of an intact cox2 gene in the mitochondrion (mt) or nucleus (nuc) of the indicated species. Bullets indicate genes containing small insertions or deletions that disrupt the reading frame or intron splicing. The right two columns indicate the presence (+) or absence (-) of detectable mitochondrial and nuclear cox2 transcripts in young leaves. Parentheses indicate transcripts present at low levels; the asterisk indicates transcripts that are not properly edited. Boxing highlights dual intact genes and/or dual transcription and proper processing (of dual cox2 genes, intact or not) in a given plant. Light rectangles and dark circles indicate loss or silencing of mt and nuclear cox2, respectively. The phylogenetic tree is one of three equally parsimonious trees obtained from parsimony analysis of a data set consisting of 2,154 bp of two chloroplast gene sequences (rbcL and ndhF) and 557 chloroplast restriction sites. Bootstrap values above 40% are shown. The figure is modified from Adams et al. (1999).

pear from the mitochondrion may be driven largely by mechanistic forces and chance mutations. These prior steps include reverse transcription (which could also occur after either of the next two steps), exit from the mitochondrion, entry into the nucleus, integration into the nuclear genome, gain of a nuclear promoter and other elements conferring properly

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