Persistent nuclear ribosomal DNA sequence polymorphism in the Amelanchier agamic complex (Rosaceae)

Individual plants of several Amelanchier taxa contain many polymorphicnucleotide sites in the internal transcribed spacers (ITS) of nuclearribosomal DNA (nrDNA). This polymorphism is unusual because it is notrecent in origin and thus has resisted homogenization by concertedevolution. Amelanchier ITS sequence polymorphism is hypothesized to be theresult of gene flow between two major North American clades resolved byphylogenetic analysis of ITS sequences. Western North American species plusA. humilis and A. sanguinea of eastern North America form one clade (A),and the remaining eastern North American Amelanchier make up clade B. Fiveeastern North American taxa are polymorphic at many of the nucleotide siteswhere clades A and B have diverged and are thought to be of hybrid origin,with A. humilis or A. sanguinea as one parent and various members of cladeB as the other parent. Morphological evidence suggests that A. humilis isone of the parents of one of the polymorphic taxa, a microspecies that werefer to informally as A. "erecta." Sequences of 21 cloned copies of theITS1- 5.8S gene-ITS2 region from one A. "erecta" individual are identicalto A. humilis sequence or to the clade B consensus sequence, or they areapparent recombinants of A. humilis and clade B ITS repeats. Amelanchier"erecta" and another polymorphic taxon are suspected to be relatively oldbecause both grow several hundred kilometers beyond the range of one oftheir parents. ITS sequence polymorphisms have apparently persisted inthese two taxa perhaps because of polyploidy and/or agamospermy (asexualseed production), which are prevalent in the genus.