Hybridization in larch-alternating Melampsora epitea (M-larici-epitea)

Crossing and selfing experiments were carried out with six field collection s and five isolates belonging to three formae speciales, larici-epilea typi ca (LET), larici-daphnoides (LD), and larici-retusae (LR), of willow rust, Melampsora epitea. European larch (Larix decidua) was inoculated with basid iospores produced on overwintered telial leaves and the resulting spermagon ial lesions on larch needles subsequently paired in vitro. In two crosses b etween LR and LD involving a total of 439 lesion pairs, only two cultures o btained were identified as hybrids. These two cultures were non-pathogenic to the maternal host Salix burjatica cv. Korso and weakly pathogenic to the paternal host S. daphnoides cv. Meikle. No identifiable hybrids were obtai ned from 56 lesion pairs between LET and LD. In a cross between LET as rece ptor and LD as donor, one-third of the lesions formed aecia. In the recipro cal combination, however, less than 1% developed aecia. All F-1 cultures be tween LET and LR were weakly pathogenic to the parental hosts S. viminalis cv. Mullatin and S. burjatica cv. Korso. When three of the F-1 cultures wer e used to produce telia, only one developed mature teliospores. Subsequent selfing experiments showed that this culture was predominantly self-sterile , with only 1% of lesions producing aecia. In selfing and crossing two path otypes within the same f. spy., 20-40% of needles produced aecia and the ra te of aecial formation was similar in both directions. The results obtained suggest that M. epitea is heterothallic and the sexual compatibility is co ntrolled by a pair of alleles at a locus. If is concluded that the three f. spp are genetically different populations and ecologically fit new pathoty pes cannot arise easily as a result of hybridization between them. Within a f. sp., however, many pathotypes exist or will occur due to exchange of ge nes for virulence during the sexual life-cycle.