Molecular diversity and physical mapping of 5S rDNA in wild, and cultivated oat grasses (Poaceae : Aveneae)

5S rDNA repeats studied in five genera of Aveneae have lengths between 285 and 329 bp (Avena sativa, Avena macrostachya, 26 species of Helictotrichon, Pseudarrhenatherum longifolium, Lagurus ovatus, and Trisetum spicatum). In only a single species (Helictotrichon aetolicum) an additional repeat of 4 56 bp occurs infrequently. Variation is largely due to insertions or deleti ons in the nontranscribed spacer as determined from sequences of 163 indepe ndent clones. The 5S gene of the Aveneae studied is conserved in length and sequence except for Helictotrichon bromoides and Helictotrichon marginatum in which duplications occur at two different sites. This new type of dupli cation and all duplications reported to date in 5S genes of angiosperms are shown to center on defined palindromic sequences. The "uncommon" 5S gene s equences detected in some Aveneae are not necessarily nonfunctional as pseu dogenes because the essential features of the internal control region are m aintained even after such duplication events. In each instance such gene se quences have spacers with unmodified structure, indicating that change in g ene sequence is not necessarily coupled with change in adjacent spacers. Th e value of 5S spacer sequences for genomic identifications in Aveneae is ex emplified in A. macrostachya (perennial), A, sativa (annual), and several d iploid taxa of the genus Helictotrichon. (C) 2001 Academic Press.