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, L ⁶, M, N and P ², was identified that possessed 10 copies of the Ac element, one of which was linked (29 map units) to L ⁶. 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 L ⁶ was present in the parent, a high frequency of L ⁶ 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 L ⁶ mutants was low (five in 124 088). A detailed analysis was made of the first 11 L ⁶ mutants recovered from parents carrying the L ⁶-linked Ac element. While none of the mutants possessed a tagged resistance gene, all lacked an RFLP marker closely linked to L ⁶, suggesting that deletions were responsible for loss of the L ⁶ 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.