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

Abstract

The genus Avena L. (Poaceae) consists of diploid, tetraploid, and hexaploid species, with the B genome known only in tetraploid species and the D genome in the hexaploid species. DNA:DNA in situ hybridization, using total genomic DNA from diploid Avena strigosa Schreb. (A8 genome) as a probe, labelled all 28 chromosomes of the AB tetraploid Avena vaviloviana (Malz.) Mordv. strongly and uniformly, revealing the close relationship between these two genomes. Comparison of patterns of size-separated DNA restriction fragments between the diploid A. strigosa and the tetraploid A. valviloviana, using 32 different restriction enzymes, revealed no differences. Southern hybridization using total AB genomic DNA as a probe also gave no differences in banding patterns between the two genomes, even when a large excess of A genomic DNA was used as a block. From an A. vaviloviana genomic library, 1800 colonies were blotted and probed sequentially with A and AB genomic DNA, but no colony was identified to be B genome specific DNA digests of AB genome tetraploids with restriction enzyme HaeIII gave a strong band at 4.2 kb. Clone pAbKB3, derived from the 4.2 kb band, was found to be part of a Tyl-copia-like retrotransposon present in A and B genome chromosomes. Cloned rRNA genes were used for in situ hybridization and showed that diploid A. strigosa has four major sites for 18S-25S rDNA and two pairs of sites for 5SrDNA (pairs on the same satellited chromosome, on different chromosome arms), while 4x A. vaviloviana has eight major sites for 18S-25S rDNA and four pairs of sites for 5S rDNA (pairs on the same satellited chromosome, on different chromosome arms). A repetitive sequence from rye pSc119.2, showed dispersed hybridization, while the telomeric sequence in clone pLT11 hybridized to telomeres. Again no discrimination was possible between A and B genome chromosomes. The molecular similarities between the diploid A. strigosa and the barbara group tetraploids clearly indicate that the barbara group of tetraploids arose from A8 diploids through autotetraploidization.