The distribution of quantitative DNA changes associated with the evolution of diploid Festuceae

Abstract

Increase in nuclear DNA amount associated with the evolution of diploid species in the genus Festuca and the genus Lolium was achieved by an allocation of approximately equal amounts of DNA to each chromosome of the complement. Thus, each chromosome of the haploid complement of Festuca drymeja has approximately 0·17 pg more DNA than its homoeologous counterpart in F. scariosa. Each chromosome of Lolium temulentum has approximately 0·15 pg more DNA than its L. perenne homoeologue. Because small chromosomes within the complement acquire as much extra DNA as the larger chromosomes, it follows that the relative difference in size and DNA content is greater between the smaller than the larger homoeologous chromosomes of species with different DNA amounts. For this reason we should expect, and indeed have confirmed, that pairing at meiosis in interspecific hybrids is less effective for small than large bivalents. At the same time, however, observations at pachytene in species hybrids show that, despite large DNA differences, homoeologous chromosomes are capable of complete and effective pairing in a high proportion of pollen mother cells. In other words the effect of large differences in DNA amount upon the pairing of homoeologous chromosomes is surprisingly small. A further consequence to be expected of the kind of DNA change which we have described is that the chromosomes of species with high DNA amounts will, in general, be more alike in size and DNA content than chromosomes within complements of species with low nuclear DNA amounts. © 1982, The Genetical Society of Great Britain.