Recent, extensive, and preferential insertion of members of the miniature inverted-repeat transposable element family Heartbreaker into genic regionsof maize

A 314-bp DNA element called Heartbreaker-hm1 (Hbr-hm1) was previously ident ified in the 3' untranslated region of a mutant allele of the maize disease resistance gene HM1. This element has structural features of miniature inv erted-repeat transposable elements (MITEs) and is a member of a large famil y of approximately 4,000 copies in the maize genome. Unlike previously desc ribed MITEs, most members of the Hbr family display over 90 % sequence iden tity. This, coupled with the insertion of an Hbr element into an allele of the HM? gene, suggested that this family might have spread recently through out the genome. Consistent with this view is the finding that Hbr insertion sites are remarkably polymorphic. Ten of ten loci containing Hbr elements were found to be polymorphic for the presence or absence of Hbr among a col lection of maize inbred lines and teosinte strains. Despite the fact that o ver 80% of the maize genome contain moderate to highly repetitive DNA, we f ind that randomly chosen Hbr elements are predominantly in single or low co py regions. Furthermore, when used to query both the public and private dat abases of plant genes, over 50% of the sequences flanking these Hbr element s resulted in significant "hits." Taken together, these data indicate that the presence or absence of Hbr elements is a significant contributory facto r to the high level of polymorphism associated with maize genic regions.