Jumping Genes Cross Plant Species Boundaries

In recent years horizontal, or lateral, gene transfer has been seen frequently in prokaryotes and also occasionally in eukaryotes. This lateral transfer involves the movement of genetic material between species as opposed to the vertical transfer of genes from parent to progeny. Horizontal transfer has been observed between the mitochondrial DNA of different plant species. Until now, however, no one had found evidence for horizontal transfer in the nuclear DNA of plants.

In a new study published online in the open access journal PLoS Biology, Xianmin Diao, Michael Freeling, and Damon Lisch analyzed the genomes of millet and rice, two distantly related grasses that diverged 30–60 million years ago. They found evidence for a case of horizontal gene transfer; despite significant genetic divergence from millions of years of mutations, they carry some nuclear DNA segments that are surprisingly similar. These segments contain transposon-related sequences (MULEs); transposons are genetic elements capable of independently replicating and inserting the copy into new positions in DNA.

To dispel alternative explanations for these similar segments, the authors investigated whether they could have been the result of positive selection acting to preserve these sequences. However, similarity of non-coding regions of the sequences in millet and rice was as high as the coding regions, and even synonymous mutations, which do not alter the protein sequence, were very few. The authors discount another explanation—that these sequences might occur within a genomic region subject to a lower mutation rate in general—with the help of maize (as complete genomic sequence data for the surrounding region in millet is not available). The sequences did not show the similar degree of reduced variation predicted for below-average mutation rates.

Since neither selection nor low mutation frequency can explain the similar DNA between the grasses, the authors conclude, a transposon must have carried it between millet and rice long after these species diverged. Interestingly, the authors also found similar sequences in bamboo, raising the question of how common horizontal transfer may be between plant species. Given that plant mitochondrial genes appear “particularly prone to horizontal transfer,” the authors note, “it is remarkable that these results represent the first well-documented case of horizontal transfer of nuclear genes between plants.” But as researchers begin to explore the growing databases of plant genomic sequences, they can determine whether this finding constitutes an anomaly—or points to a significant force in plant genome evolution.

Citation: Diao X, Freeling M, Lisch D (2006) Horizontal transfer of a plant transposon. PLoS Biol 4(1): e5.

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