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which case all of the fungi must themselves be closely related [because all of the plant introns are (Cho et al., 1998)]? Or have most transfers, perhaps all but the first, occurred via “short”-distance transfer, i.e., from angiosperm to angiosperm? These two alternative models make contrasting phylogenetic predictions as elaborated elsewhere (Cho et al., 1998). Has transfer, especially if largely plant-to-plant, been mediated by vectoring agents, and if so which ones (e.g., viruses, bacteria, aphids, mycorrhizal fungi, etc.)? Or has it occurred by transformation-like uptake of DNA from the environment or by the occasional direct fusion (perhaps pollen-mediated) of two unrelated plants? To some extent, the answers are probably yes, yes, and yes; considering the large number of independent transfers, each a unique and rare (except on the evolutionary timescale) historical event, almost any imaginable kind of vector and method of intron transfer could have been used at least once. Why has the intron burst on the angiosperm scene in such a rampant manner only so recently? Has this recent wave of lateral transfers been triggered by some key shift in the intron's invasiveness within angiosperms, and if so, what has changed? We hope to provide at least partial answers to some of these fascinating but challenging questions over the coming years.

In passing, we note that the overwhelmingly horizontal evolution of this remarkable group I intron is in striking contrast to the vertical pattern of evolution of the 23 other introns in angiosperm mt genomes, all of which are group II introns (Unseld et al., 1997). We have used probes for 11 of these introns in our Southern blots surveys (Qiu et al., 1998; Y.-L.Q. and J.D.P., unpublished data). All 11 are present in most or all major groups of angiosperms, and in many other groups of vascular plants, and thus were clearly present in the common ancestor of all angiosperms. These group II introns appear to have been transmitted in a strictly vertical manner, including occasional to frequent losses.

HIGHLY ACCELERATED SUBSTITUTION RATES IN TWO LINEAGES OF PLANTS

As already emphasized, our wide-scale Southern blot survey for presence or absence of mt genes and introns is predicated entirely on the uncommonly low rates of nucleotide substitutions observed to date in plant mitochondria. If a lineage of plants were to sustain for very long a radically higher substitution rate (say at the 50- to 100-fold higher level characteristic of mammalian mitochondria), then all of its mt genes might hybridize poorly or not at all, as if the genome no longer existed. Poor hybridization with all mt probes tested was observed for two of the 281 angiosperms on our blots, Pelargonium hortorum (the common garden geranium) (Fig. 1, lane 4) and Plantago rugelii (plantain, a common lawn



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