Chromosome rearrangements play an important role in polyploid evolution. Our preliminary phylogenetic studies have confirmed that the highly variable species Poa annua is an allotetraploid between Poa infirma and Poa supina. However, according to a detailed karyotype analysis by Koshy (1968), the karyotype of Poa annua did not match the combined pattern of the two parents, and therefore has experienced major chromosome rearrangements. Our data also suggests that Poa annua has evolved recently and that the parental genomes are very distinct. Thus, with all reasons above and the ready availability of materials from both parental species, Poa annua serves as an ideal model organism for studying polyploid chromosome rearrangements. Fluorescent in situ hybridization (FISH) with 45S rDNA as the probe showed three telomeric loci in Poa annua metaphase cells, meaning that at least one of them does not have a homologous locus in the Poa annua genome. Furthermore, in some cells, only two 45S rDNA loci could be observed. To confirm and further examine these potential rearrangements, 45S rDNA FISH will be performed in the parental species, as well as genomic in situ hybridization (GISH) on Poa annua. Many crop species are ancient polyploids, but due to various reasons, they may not be the best models for understanding the mechanistic underpinning of polyploid evolution. Thus, with Poa annua as a model, mechanisms of chromosome rearrangements in polyploid evolution may be applied to crop improvement efforts on these and other crops species.