Genome structure and evolution in the allohexaploid weed Avena fatua L-(Poaceae)

Allohexaploid wild oat, Avena fatua L. (Poaceae; 2n = 6x = 42), is one of t he world's worst weeds, yet unlike some of the other Avena hexaploids, its genomic structure has been relatively little researched. Consequently, in s itu hybridisation was carried out on one accession of A. fatua using an 18S -25S ribosomal DNA (rDNA) sequence and genomic DNA from A. strigosa (AA-gen ome diploid) and A. clauda (CC-genome diploid) as probes. Comparing these r esults with those for other hexaploids studied previously: (i) confirmed th at the genomic composition of A. fatua was similar to the other hexaploid A vena taxa (i.e., AACCDD), (ii) identified major sites of rDNA on three pair s of A/D-genome chromosomes, in common with other Avena hexaploids, and (ii i) revealed eight chromosome pairs carrying intergenomic translocations bet ween the A/D- and C-genomes in the accession studied. Based on karyotype st ructure, the identity of some of these recombinant chromosomes was proposed , and this showed that some of these could be divided into two types, (i) t hose common to all hexaploid Avena species analysed (3 translocations) and (ii) one translocation in this A. fatua accession not previously observed i n reports on other hexaploid Avena species. If this translocation is found to be unique to A. fatua, then this information, combined with more traditi onal morphological data, will add support to the view that A. fatua is gene tically distinct from other hexaploid Avena species and thus should retain its full specific status.