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 be en transferred to cultivated tobacco (Nicotiana tabacum) and was recently d iscovered to be linked in coupling to a 1050-bp random amplified polymorphi c DNA (RAPD) marker generated with the UBC 418 primer. We have cloned and s equenced the UBC418(1050) marker and found it to be part of a retrotranspos on. This retrotransposon is a remnant of the N. debneyi genome and is the f irst to be isolated from this species. Transposon N. debneyi (Tnd)-1 is pre sent 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 Nicotian a species. The retrotransposon appears to have been introduced into the Nic otiana genome after the development of the Suavolentes progenitors. The gen e associated with black root rot resistance co-segregates with the retrotra nsposon in tobacco and is thought to be contained within the introgressed f ragment marked by Tnd-1. The retrotransposon will therefore be a useful spe cies-specific landmark that can be used for future cloning of the resistanc e gene.