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tional transfer whereas only a small fraction of nuclear genes could play a useful role if transferred to the mitochondrion. The pervasively unidirectional flow of mt genes to the nucleus may, therefore, be driven largely, perhaps even entirely, by a huge imbalance in the relative likelihood of gene movement and potential functionality in one direction versus the other.

EXPLOSIVE INVASION OF PLANT MITOCHONDRIA BY A GROUP I INTRON

Thus far, we have discussed intracellular horizontal evolution entirely as a means of relocating plant mt genes to the nucleus. As mentioned in the introduction, plant mt genomes are also well known to acquire foreign sequences by intracellular gene transfer, from both the chloroplast and nucleus. We have recently described (Cho et al., 1998; Cho and Palmer, 1999), and will briefly review here, a case of horizontal evolution that stands out in three respects: (i) It is the first case of cross-species acquisition of DNA by plant mt genomes; (ii) it is unparalleled with respect to how frequently the same piece of DNA has been acquired, over and over again, during angiosperm evolution; and (iii) all of these many invasions have occurred very recently, as an explosive wave within the last 10 million years or so.

The piece of DNA in question here is a homing group I intron. These introns encode site-specific endonucleases with relatively long target sites that catalyze their efficient spread from intron-containing to intron-lacking alleles of the same gene in genetic crosses. A few cases of the evolutionary spread of these introns by horizontal homing between species were known when, in 1995, we in collaboration with Jack Vaughn's group reported (Vaughn et al., 1995) that the angiosperm Peperomia had acquired, quite recently (Adams et al., 1998), a group I intron in its mt cox1 gene by long-distance horizontal transfer, most likely from a fungus. We subsequently discovered a closely related form of this intron, located at the same position in cox1, in a very distantly related angiosperm, Veronica. This stimulated us to use the Veronica intron as a probe against our survey blots of 281 angiosperm DNAs. As shown in Fig. 1 Lower, the intron probe hybridized strongly to relatively few DNAs, in an unusually patchy manner phylogenetically [and always to the same band as a cox1 exon probe (Fig. 1 Middle), indicating that the hybridizing region is always located in the same gene]. All told, 48 of the 281 angiosperm DNAs on the blots, scattered across most of the major groups represented, hybridized well to the intron (Cho et al., 1998).

The exceptionally patchy phylogenetic distribution of the intron (see Fig. 2 of Cho et al., 1998) caused us to sequence the intron from 30 diverse



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