Genetische Analyse und Kartierung einer <em>Turnip yellows virus</em> (TuYV)-Resistenz in Winterraps (<em>Brassica napus</em> L.)

Abstract

Turnip yellows virus (TuYV) belongs to the Poleroviruses within the Luteoviridae family. TuYV is transmitted in a persistent manner by aphids – mainly M. persicae (Schliephake et al., 2000; D’Arcy and Domier, 2005) - and is widespread worldwide (Graichen and Rabenstein, 1996; Thomas et al., 1993; Smith and Hinkes, 1985). The infection rate is depending on the vector activity, which increases with mild autumn and winter temperatures. Therefore, it is assumed that this virus will become even more important in the future due to global warming. Chemical measures against the vector i.e. insecticide sprayings or seed treatment are neither acceptable for ecological reasons nor very efficient (Read and Hewson, 1988; Walsh etal., 1989). Yield losses caused by TuYV have been estimated to range from 12 to 34 % (Graichen, 1997). Therefore, resistance to TuYV is an important goal of winter rapeseed breeding. This holds true in particular in the frame of an environmentally friendly winter rapeseed production, since resistant cultivars will considerably reduce
insecticide treatments. Besides this, red or violet discolorations of leaves and growth reductions resemble symptoms of nutrient deficiency, leading to the additional application of fertilizer which cannot be taken up by infected plants (Graichen, 1999). A prerequisite for succesfully integrating resistance to TuYV from wild or basic material into adapted cultivars is detailed knowledge on the inheritance of resistance and the availability of molecular markers, because rearing of virus infested aphids needed for artificial inoculation cannot be integrated efficiently into applied rapeseed breeding. Therefore, the goals of this study were (i) to achieve detailed information on the mode of inheritance of resistance to TuYV, and (ii) to develop molecular markers suited for marker based selection procedures. In order to obtain detailed information on the genetics of resistance derived from the resynthesized rapeseed line ‘R54’ and to identify segregating molecular markers a total of 111 DH-lines were analysed. In three years field trials a monogenic mode of inheritance (1 resistant : 1 susceptible segregation) was detected in December of the first two years after artificial inoculation of DH-lines using virus bearing aphids and assuming a threshold for resistance of E₄₀₅=0.1. With the progressing growing season a countinuous increase of the virus titre was observed also in the plants classified as resistant in December. However, the virus titre of these plants never reached the level of the susceptible ones. In contrast to these results much higher virus concentrations were already observed in December 2007 due to the extremly mild autumn. The phenotype data give hint that the TuYV-resistance is inherited by one dominant major-gene, which is responsible for the low virus titres in December until spring, complemented by further minorgenes for resistance. Additionally, pronounced effects of environment (temperature) are assumed. Based on these phenotypic data marker development using bulked segregant analysis (BSA) was started and two closely linked SSR marker, six closely linked and three cosegregating AFLP marker and two cosegregating diagnostic STS marker have been developed. Due to the SSRs identified the TuYV-resistance was first supposed to be located on chromosome N9 (A-Genome) based on the map by Piquemal et al. (2005). However, this localization could not be confirmed with further SSRs of chromosome N9. In more recent studies the linked SSRs were assigned to chromosome N4, and the analysis of additional SSRs from N4 led to the identification of one N4-derived SSR. Therefore, it is concluded that the TuYV-resistance is most likely located on chromosome N4.