Karyotype Variation in South American Elymus (Triticeae)

Karyotypes of eight tetraploid (2n = 28) South American species of Elymus, were symmetrical (arm ratios: 0 70-0.78) and uniform (intrachromosomal asymmetry index: 0.105-0 133). Significant differences in arm ratio and asymmetry index were found among tetraploid species and also among geographically isolated populations within one species. Variation was nearly continuous, and significant differences were found only between the extreme values. The opposite situation was found among three hexaploid species (2n = 42) that can be readily differentiated by significant differences in karyotype parameters Significantly higher intrachromosomal asymmetry indexes in E. erianthus (6x, 2n = 42) and E mendocinus (8x, 2n = 56) indicated relatively larger differences in chromosome size between the genomes of the progenitors of these polyploid species Low variation in the tetraploid species and high variation in the hexaploid and octoploid species were also found in the number and morphology of chromosomes with secondary constrictions (CSCs). Tetraploid species with the genome formula SSHH (S from Pseudoroegneria and H from Hordeum) showed only four CSCs from the S genome of Pseudoroegneria. The presence of CSCs from both S and H genomes in synthetic SSHH tetraploids and in natural SSHHHH hexaploids indicated absence of suppression of nucleolar organizing region (NOR) activity (amphiplasty) between the NORs from the S and H genomes. On the basis of karyotype and molecular data, it was hypothesized that the nucleolar organizing regions from the H genome were eliminated or highly reduced during the evolution of the tetraploid species.