B-CHROMOSOMES - A PHYSIOLOGICAL ENIGMA

The concept of the 'nucleotype' embraces aspects of the nuclear DNA th at affect the phenotype other than through the coding sequences. In th is context, we consider one particular situation, namely DNA variation due to the presence of supernumerary B chromosomes (Bs); these Bs, wh ich are additional to the basic complement of A chromosomes, generate a spectrum of DNA amounts, and thus nucleotypes, within many species. We consider the physiological consequences of carrying extra chromosom es that appear by and large to lack coding sequences, and focus on two species with contrasting case histories: rye (Secale cereale L.) and chives (Allium schoenoprasum L.), which illustrate 'selfish' and 'adap tive' explanations of the persistence of B chromosomes.