Instability of the L6 gene for rust resistance in flax is correlated with the presence of a linked Ac element.

In a programme aimed at tagging rust-resistance genes in flax with the maize transposable element Ac, a primary transformant of a line called 'Forge' that is homozygous for four rust-resistance genes, L6, M, N and P2, was identified that possessed 10 copies of the Ac element, one of which was linked (29 map units) to L6. Descendants of this plant, which had from 8 to 15 copies of Ac, were crossed to a rust-susceptible line and the progeny screened for rust-susceptible mutants. When the Ac linked to L6 was present in the parent, a high frequency of L6 mutants was observed (29 mutants in 30,575). By contrast, when this Ac was absent, no such mutants were observed in 9258 progeny. The background frequency of L6 mutants was low (five in 124,088). A detailed analysis was made of the first 11 L6 mutants recovered from parents carrying the L6-linked Ac element. While none of the mutants possessed a tagged resistance gene, all lacked an RFLP marker closely linked to L6, suggesting that deletions were responsible for loss of the L6 specificity. In many of the mutants, one or more RFLP markers in the vicinity of the linked Ac were also absent. These findings suggest that the linked Ac may be inducing chromosome breakage.