Multiplicity of Ribosomal RNA Genes in Vicia Species with Different Nuclear DNA Contents

Abstract

THERE is widespread variation between species of higher plants in amounts of nuclear DNA. In ten species a fifty-seven-fold variation in amounts of DNA per nucleus for equivalent cells was recorded¹, and later reports showed wide ranges in the DNA contents of diploid species within single families (forty-fold in the Ranunculaceae² and sixty to eighty-fold in the Droseraceae³). Most strikingly, large differences exist within a single genus between species with the same diploid chromosome number, for example, a three-fold range in the genus Lathyrus⁴ and between five and seven times as much nuclear DNA in Vicia faba as in V. sativa^5–7. The structural basis of these variations is unclear although models of either a single stranded chromosome, in which DNA content increases by longitudinal multiplication of DNA sequences, or a multi-stranded chromosome, where increase is by lateral increase of strands, have been suggested (see reviews, refs 8, 9).