Variable genome composition in TriticumxLeymus amphiploids

Three different amphiploid lines originated from crosses between wheat (Tri ticum L.) and lymegrass (Leymus Hochst.) were analysed by fluorescence in s itu hybridization (FISH) using total genomic DNA and 18S.26S ribosomal gene s (rDNA). Based on the genomic probes, these lines were the same in that th ey all were allohexaploids (2n=6x=42) containing 30 wheat and 12 lymegrass chromosomes. The ribosomal gene mapping further identified species origin o f the chromosomes, whereby the lymegrass parent was undoubtedly L. mollis a nd the wheat was likely to be a Triticum species having the AB genomes. Thi s rDNA mapping was also able to reveal differences in the genome compositio n among these lines, and such differences were mainly in the wheat nucleola r organizing regions (Nor). The first line (M) had two Nor-BI (1BS), the se cond line (G) had one Nor-BI (1BS), two Nor-B2 (6BS) and two Nor-Al (1AS), whereas the third line (U) had the same Nor loci as the second line but two sites each. The wheat ribosomal genes were variably expressed depending on the lines, but the lymegrass loci appeared inactive. All three lines had t he same Nor loci belonging to L. mollis! two Nor-m1 and two Nor-m2. Analysi s of restriction fragment length polymorphism (RFLP) of the rDNA confirmed the identity of L. mollis parent in all three lines and verified the differ ences in the wheat ribosomal genes among them. These amphiploids were, howe ver, similar in their restriction profiles, therefore indicating common ori gin. The molecular and cytogenetic evidence here suggested that these annua l, fully fertile amphiploids that originated from the same crosses 40 years ago became genetically differentiated and fixed in stable forms.