ARE RICE CHROMOSOMES COMPONENTS OF A HOLOCENTRIC CHROMOSOME ANCESTOR

Comparative genomics reveals that cereal genomes are composed of simil ar genomic building blocks (linkage blocks). By stacking these blocks in a unique order, it is possible to construct a single ancestral 'chr omosome' which can be cleaved to give the basic structure of the 56 di fferent chromosomes found in wheat, rice, maize, sorghum, millet and s ugarcane. The borders of linkage blocks are defined by cereal centrome ric and telomeric sites. However, a number of studies have shown that telomeric heterochromatin has neocentromeric activity, implying that l inkage blocks are in fact defined by centromeric-like sites with conse rved sequences. The structure of the ancestral cereal genome thus rese mbles a holocentric chromosome, which is the chromosome structure shar ed by the closest relatives of the Gramineae, the Cypericeae and Junca ceae.