Recent work revealed that, in the genomes of polyploid wheat, there exists a class of low_copy and chromosome_specific sequences that are labile upon polyploid formation. This class of sequences was proposed to play a critical role in the stabilization and establishment of nascent plant polyploids as new species. To further study this issue, five wheat chromosome 7B_specific sequences, isolated from common wheat (Triticum aestivum L.) by chromosome microdissection, were characterized. The sequences were studied by genomic Southern hybridizations on a collection of polyploid wheats and their diploid progenitors. Four sequences hybridized to all polyploid species, but at the diploid level to only species closely related to the B_genome of polyploid wheat. This indicates that these sequences originated with the divergence of the diploid species, and was then vertically transmitted to polyploids. One sequence hybridized to all species at both the diploid and polyploid levels, suggesting its elimination after the polyploid wheat formation. The hybridization of this sequence to two synthetic polyploid wheats indicated that sequence elimination is a rapid event and probably related to methylation status of the sequence. Based on the above results, we suggest that selective changes of low_copy sequences occur rapidly after polyploid formation, which may contribute to the differentiation of chromosomes in newly formed allopolyploid wheats.
