Structural Basis of Quantitative Variation in Nuclear DNA

Abstract

RECENT surveys show that variation in the amount of nuclear DNA, quite independently of change in chromosome number, is frequently associated with the divergence and evolution of both plant and animal species^1–10. The extent of such variation is often very great, particularly among the angiosperms. One of the many problems posed by these findings is the nature of the chromosome structural changes which give rise to the DNA variation. Two proposals have been put forward. The first is a differential polynemy, that is, a difference in the lateral multiplicity of DNA strands between chromosomes of different species^4,9. While this proposal has many theoretical attractions there is, to date, no direct evidence in its favour. Indeed, Callan¹¹ has adduced evidence which argues strongly against this view. The second possibility is that the nuclear DNA variation is caused by lengthwise incorporation or loss of chromosome segments such as result from duplication or deletion. Observations in Lolium¹ and in Chironomus³ hybrids at pachytene of meiosis and in polytene nuclei, respectively, show that the chromosomes of species with different nuclear DNA contents differ in respect of segmental duplications. There are therefore good grounds for attributing at least part of the DNA variation to such duplications. The following work in Allium provides further evidence to support this view. What is more, it shows that segmental duplications can account entirely for the DNA changes observed.