Many modern crops, including wheat, cotton, canola, Arabica coffee, leek, oat and peanut, are allopolyploids—species that contain two or more chromosome sets derived from different species—which were created through sexual hybridization. However, allopolyploidization is a relatively rare phenomenon. “Strong barriers prevent sexual hybridization between most species,” explains Luca Comai of the University of California, Davis. “This demonstration that allopolyploids and chromosome addition lines (the approach should also enable the transfer of single chromosomes) can arise by an asexual mechanism might open significant new opportunities for crop improvement.”
The most striking outcome of the study arose from grafting stems of transgenic, hygromycin-resistant cigarette tobacco (Nicotiana tabacum, an herbaceous species with 48 chromosomes) to stems of transgenic, kanamycin-resistant tree tobacco (Nicotiana glauca, a woody species with 24 chromosomes). After fusion of the tissues from the different species had occurred, the graft site was excised and cultured on regenerative media that contained both kanamycin and hygromycin. Analysis of 45 doubly resistant plants derived from 12 grafted plants indicated not only that they had 72 chromosomes, which is the sum of the chromosome numbers of their parents, but also that the new species—tentatively named Nicotiana tabauca—outgrows its progenitor species while having many other traits that are intermediate between those of its parents.
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