Clayberg, C. D.
The gene for high level resistance to tobacco mosaic virus (Tm\2) has been reported by Soost to be linked to nv (see TGC8:35). The general interest in breeding for resistance to this virus has prompted work at this station to ascertain the tightness of the linkage. The work is being done in conjunction with a program of backcrossing the resistance into several commercial varieties. Plants of the genotype Tm\2 nv/ + + were crossed by susceptible varieties in the winter of 1957. From the resulting progeny segregants heterozygous for Tm\2, but possibly not carrying nv, were grown in the field in summer 1958. Open-pollinated, selfed seed was collected from these and small progenies were grown therefrom in winter 1958. Families from 75 plants so tested all contained nv/nv segregants. Thus none of the 75 plants represented a breaking of the Tm\2-nv linkage. This indicates that if Tm\2 and nv are separate genes they are probably less than 2 units apart, large families will be required to break the linkage. It is also quite possible that they are pleiotropic effects of the same gene, in which case the resistance would be of use only in F1 hybrids.