Nuclear DNA Variation Due to B Chromosomes

Abstract

Much attention is focused at present upon the causes and consequences of the variation in the amount of nuclear DNA which accompanies the divergence and evolution of a wide range of species. In some groups, such as the flowering plants, the magnitude of the variation is surprising. Even between diploids, the DNA values vary by a hundredfold (Sparrow et al., 1972; Rees, 1972; Rees and Jones, 1972). It is generally accepted that the structural basis of this variation is a lengthwise repetition, or sometimes deletion, of segments within chromosomes (Rees, 1972). A puzzling aspect of the phenomenon is that most organisms are normally conspicuously intolerant of the addition and, especially, the loss of chromosome material, even of minute segments. There is, however, one, by no means uncommon, chromosome system involving supernumerary or B chromosomes that is exceptional in this respect. It is characteristic of B chromosomes that their number varies...