Nuclear dna and the evolution of allotetraploid brassicae

Abstract

DNA amounts, by microdensitometry, are underestimated in nuclei with high DNA density. A method is described for correcting the DNA readings in DNA dense nuclei. DNA measurements in the allotetraploid Brassica napus (2n = 38), B. carimta (2n =34) and B. juncea (2n = 36), after applying the correction, were found to equal the sum of the diploid parents from which they are derived. This finding, contrary to previous reports, refutes the evidence for a nuclear DNA diminution subsequent to the formation of the allotetraploids. Interphase nuclei in newly synthesised allotetraploids are substantially larger than in long established allotetraploids. It is suggested that the reduction in nuclear size in tetraploids is associated with greater condensation of chromosome material which, in turn, may reflect an adaptive “switching off” of redundant gene copies in the tetraploid complement. © 1974 The Genetical Society of Great Britain.