The Close Relationship Between the A and B Genomes in Avena L. (Poaceae) Determined by Molecular Cytogenetic Analysis of Total Genomic, Tandemly and Dispersed Repetitive DNA Sequences

The genusAvena L. (Poaceae) consists of diploid, tetraploid,and hexaploid species, with the B genome known only in tetraploidspecies and the D genome in the hexaploid species. DNA:DNAinsitu hybridization, using total genomic DNA from diploidAvenastrigosa Schreb. (A_sgenome) as a probe, labelled all 28 chromosomesof the AB tetraploidAvena vaviloviana (Malz.) Mordv. stronglyand uniformly, revealing the close relationship between thesetwo genomes. Comparison of patterns of size-separated DNA restrictionfragments between the diploidA. strigosa and the tetraploidA.vaviloviana , using 32 different restriction enzymes, revealedno differences. Southern hybridization using total AB genomicDNA as a probe also gave no differences in banding patternsbetween the two genomes, even when a large excess of A genomicDNA was used as a block. From anA. vaviloviana genomic library,1800 colonies were blotted and probed sequentially with A andAB genomic DNA, but no colony was identified to be B genomespecific. DNA digests of AB genome tetraploids with restrictionenzymeHae III gave a strong band at 4.2 kb. Clone pAbKB3, derivedfrom the 4.2 kb band, was found to be part of aTy1-copia -likeretrotransposon present in A and B genome chromosomes. ClonedrRNA genes were used forin situ hybridization and showed thatdiploidA. strigosa has four major sites for 18S-25S rDNA andtwo pairs of sites for 5S rDNA (pairs on the same satellitedchromosome, on different chromosome arms), while 4xA. vavilovianahas eight major sites for 18S-25S rDNA and four pairs of sitesfor 5S rDNA (pairs on the same satellited chromosome, on differentchromosome arms). A repetitive sequence from rye pSc119.2, showeddispersed hybridization, while the telomeric sequence in clonepLT11 hybridized to telomeres. Again no discrimination was possiblebetween A and B genome chromosomes. The molecular similaritiesbetween the diploidA. strigosa and thebarbata group tetraploidsclearly indicate that thebarbata group of tetraploids arosefrom A_sdiploids through autotetraploidization. Avena ; evolution; repetitive sequences; in situ hybridization; retrotransposons; genome organization