TRANSFER OF RESISTANCE TO POTATO LEAFROLL VIRUS, POTATO VIRUS-Y AND POTATO VIRUS-X FROM SOLANUM-BREVIDENS TO SOLANUM-TUBEROSUM THROUGH SYMMETRICAL AND DESIGNED ASYMMETRIC SOMATIC HYBRIDIZATION

Resistance to potato leafroll virus (PLRV), potato virus Y (PVY(o)) an d potato virus X (PVX) was studied in symmetric and asymmetric somatic hybrids produced by electrofusion between Solanum brevidens (2n=2x=24 ) and dihaploid S. tuberosum (2n=2x=24), and also in regenerants (B-hy brids) derived through protoplast culture from a single somatic hybrid (chromosome number 48). All of the somatic hybrids between S. brevide ns and the two dihaploid lines of potato cv. Pito were extremely resis tant to PLRV and PVY(o) and moderately resistant to PVX, irrespective of their chromosome number and ploidy level (tetraploid or hexaploid). Most (56%) of the asymmetric hybrids of irradiated S. brevidens and t he dihaploid line of potato cv. Pentland Crown (PDH40) had high titres of PVY(o) similar to those of PDH40, whereas the rest of the hybrids had PVY(o)) titres less than a tenth of those in PDH40. Three B-hybrid s had a highly reduced chromosome number (27, 30 and 34). but were how ever as resistant to PLRV, PVY(o) and PVX as S. brevidens. Two asymmet ric hybrids and one B-hybrid were extremely resistant to PLRV but susc eptible to both PVY and PVX. The results suggested that resistance to PLRV in S. brevidens is controlled by a gene or genes different from t hose controlling resistance to PVY and PVX, and the gene(s) for resist ance to PVY and PVX are linked in S. brevidens.