Differential transmission of homoeologous chromosomes in heterozygous amphiploids: rRNA loci of Thinopyrum distichum in an amphiploid with Triticum durum

Abstract

The segregation of two sets of homoeologous Thinopyrum chromosomes from Th. distichum (5J and 6J) was followed in segregating BC1F2 progeny from the backcross (2n = 42) of the amphiploid of Triticum durum × Th. distichum) (2n = 56) to T. durum (2n = 28). The segregation pattern was followed by the presence or absence of the 18S + 26S rRNA locus using in situ hybridization and Southern analysis of the TaqI digest. The BC1F2 progeny segregated for four to seven NOR chromosomes. This confirmed that one homoeologue carried a nucleolar organizing region (NOR), while in the other it was absent or greatly reduced in copy number. That is, for example, 5J1 and 6J1 carry a NOR site, while 5J2 and 6J2 both lack NORs. Both in situ hybridization and Southern analysis confirmed that presence or absence of NORs did not segregate at random. The NOR on 5J had a 2.9-kb TaqI fragment and this NOR was present in the progeny less frequently than expected. The 6J NOR that carried a 1.1- and 0.8-kb fragment was transmitted to the progeny with a normal frequency. The results are discussed with respect to the implications for creating reconstructed genomes and recovering the genetic variation of a polyploid in addition lines.Key words: segregation, homoeologue, rRNA, amphiploid, polyploid, reconstructed genome, in situ hybridization, TaqI fragment.