Isolation and characterization of Tnd-1, a retrotransposon marker linked to black root rot resistance in tobacco
In Nicotiana debneyi, resistance to a wide range of black root rot (Chalara elegans) isolates is conferred by a single dominant gene. This gene has been transferred to cultivated tobacco (Nicotiana tabacum) and was recently discovered to be linked in coupling to a 1050-bp random amplified polymorphic DNA (RAPD) marker generated with the UBC 418 primer. We have cloned and sequenced the UBC4181050 marker and found it to be part of a retrotransposon. This retrotransposon is a remnant of the N. debneyi genome and is the first to be isolated from this species. Transposon N. debneyi (Tnd)-1 is present in the tobacco genome as two directly repeated copies, and in multiple copies in the donor species N. debneyi and in a number of related Nicotiana species. The retrotransposon appears to have been introduced into the Nicotiana genome after the development of the Suavolentes progenitors. The gene associated with black root rot resistance co-segregates with the retrotransposon in tobacco and is thought to be contained within the introgressed fragment marked by Tnd-1. The retrotransposon will therefore be a useful species-specific landmark that can be used for future cloning of the resistance gene.