1Horticulture Research International, Wellesbourne, Warwick, CV35 9EF, UK; 2Saskatoon Research Centre, Saskatchewan, Canada; 3IPG, Poznan, Poland; 4IPMB, Ceske Budejovice, Czech Republic

Background and objectives
Turnip mosaic virus (TUMV) is one of the most important viruses affecting field vegetables worldwide. The interaction between TUMV isolates and Brassica napus has been shown to follow the classic gene-for-gene pattern [1, 2). Resistance genes in the differential B. napus lines are being characterised and mapped. Studies are also under way to identify the viral virulence/avirulence determinant for each resistance gene in order to dissect the interactions and devise robust strategies for the sustainable deployment of these genes for effective control of TUMV.

Materials and methods
Isolates of TUMV used in this study have been well characterised, their pathotypes on the B. napus differentials determined [1, 2] and the nucleotide sequences of their coat protein coding regions determined [3]. The B. napus oilseed rape line N-o-1 possessing resistance to certain TUMV isolates was crossed to a susceptible line of oilseed rape and one of the resulting F1 plants was subjected to microscope culture to produce a segregating population of doubled haploid (DH) lines for genetic mapping [4]. The interaction of the UK 1 isolate of TUMV (avirulent on N-o-1), a mutant of this isolate (UK 1 M), which is virulent on N-o-1 and the other sequenced isolates with the N-o-1 line were determined as described for other interactions [2]. The interactions between a range of TUMV isolates and the DH lines were also determined.

Results and conclusions
The segregation of resistance (15 lines) and susceptibility (13 lines) in the 28 DH lines tested, allowed the resistance gene to be positioned on linkage group N6 of the B. napus A genome, indicating that the gene probably originated from B. rapa. The resistance gene TuMV 01 (TuRB01) is the first gene for resistance to a virus to be mapped in a Brassica species.

A comparison between the nucleotide sequence of the coat protein-coding region of the wild-type UK 1 isolate of TUMV (avirulent on plants possessing TuRB01) and that of the mutant UK IM isolate (virulent on plants possessing TuRB01) revealed a single nucleotide difference which did not give rise to any change in the predicted amino acid sequence. The coat protein nucleotide sequences of these two isolates were aligned with those of a further six TUMV isolates. A comparison of these with the phenotypes of the isolates on plants possessing TuRB01, revealed that all isolates with G at position 9 were avirulent and all isolates with A at position 9 were virulent. This is consistent with the hypothesis that the single nucleotide substitution at position 9 of the coat protein coding region of TUMV conditions the interaction with TuRB01. This hypothesis is being investigated further. Further resistance genes in B. napus and B. rapa are being mapped and their virulence/avirulence determinants in TUMV are being sought. The selective combining of such genes, using marker-assisted breeding, will make durable resistance to TUMV a realisable breeding objective.

1 . Walsh JA, 1989. Annals of Applied Biology 115, 89-99.
2.. Jenner CE, Walsh JA, 1996. Plant Pathology 45, 848-56.
3.. Lehmann P, Petrzik K, Jenner C et al., 1997. Physiological and Molecular Plant Pathology 51, in press.
4. Sharpe AG, Parkin IAP, Keith DJ, Lydiate DJ, 1995. Genome 38, 1112-21.