PERSISTENT NUCLEAR RIBOSOMAL DNA-SEQUENCE POLYMORPHISM IN THE AMELANCHIER AGAMIC COMPLEX (ROSACEAE)

Individual plants of several Amelanchier taxa contain many polymorphic nucleotide sites in the internal transcribed spacers (ITS) of nuclear ribosomal DNA (nrDNA). This polymorphism is unusual because it is not recent in origin and thus has resisted homogenization by concerted ev olution. Amelanchier ITS sequence polymorphism is hypothesized to be t he result of gene flow between two major North American clades resolve d by phylogenetic analysis of ITS sequences. Western North American sp ecies plus A, humilis and A. sanguinea of eastern North America form o ne clade (A), and the remaining eastern North American Amelanchier mak e up clade B. Five eastern North American taxa are polymorphic at many of the nucleotide sites where clades A and B have diverged and are th ought to be of hybrid origin, with A. humilis or A. sanguinea as one p arent and various members of clade B as the other parent. Morphologica l evidence suggests that A. humilis is one of the parents of one of th e polymorphic taxa, a microspecies that we refer to informally as A. ' 'erecta.'' Sequences of 21 cloned copies of the ITS1-5.8S gene-ITS2 re gion from one A. ''erecta'' individual are identical to A. humilis seq uence or to the clade B consensus sequence, or they are apparent recom binants of A. humilis and clade B ITS repeats. Amelanchier ''erecta'' and another polymorphic taxon are suspected to be relatively old becau se both grow several hundred kilometers beyond the range of one of the ir parents. ITS sequence polymorphisms have apparently persisted in th ese two taxa perhaps because of polyploidy and/or agamospermy (asexual seed production), which are prevalent in the genus.