The role of Genomic Constraints upon Evolutionary Changes in Genome Size and Chromosome Organization

While the large excess of genomic DNA sequences involved in C-value variation is non-coding, the genome size distribution within several plant genera follows well defined discontinuous patterns. Such discontinuous distributions, it is proposed, are governed by constraints fundamental to the complex organization of genomes. It is also shown how such genomic constraints might decide the pattern of distribution of excess DNA within chromosome complements and predetermine future evolutionary patterns in karyotype organization. The molecular processes underlying the discontinuous pattern of genome evolution in Lathyrus are examined and the pattern of genome size variation accompanying phylogenetic differentiation in Nicotiana and Brassica investigated. The possible roles of genomic constraints in the evolution of stable patterns in genome organization are discussed.