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FIGURE 3. Phylogenetic evidence for horizontal transfer of cox1 introns. (A) Maximum-likelihood tree of 30 angiosperm cox1 introns. Numbers on the tree are bootstrap values. Plus signs on the tree mark 25 inferred gains of the intron among these taxa. Symbols to the immediate right of names are as in Fig. 3B. Numbers at far right indicate number of 3′-flanking nucleotides changed by co-conversion (see text). Bold branches mark four small clades of introns thought to have originated from the same intron gain event. (B) Organismal tree from a maximum-likelihood analysis of a combined data set of chloroplast rbcL and mt cox1 coding sequences. Numbers are bootstrap values. Symbols mark the nine major groups of angiosperms represented in this analysis. The figure is modified from Cho et al. (1998).

species of angiosperms, we are confident that the intron has been horizontally acquired at least hundreds of times during angiosperm evolution and probably over 1,000 times. Equally remarkably, all of these transfers seem to have occurred very recently, in the last 10 million years or so of angiosperm evolution.

Many fascinating questions can be asked about the evolution of this wildly invasive group I intron. What does the inevitably complex historical network of horizontal transfers look like; i.e., who is the donor and who is the recipient in each specific instance of intron transfer? Phylogenetic evidence suggests at least one, perhaps initiating long-distance transfer of the intron from a fungus to a flowering plant (Cho et al., 1998). Have many or most of the transfers occurred via this long-distance route, in

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