The nucleotype, the natural karyotype and the ancestral genome.

New knowledge of synteny and collinearity promises to unify genetics and to affect our perception of higher order genome structure. This exciting new synthetic approach emphasizes genomic similarities rather than diversity. Two other aspects of genomic form and organisation, offering potentially unifying concepts in genome studies are: the nucleotype, and the natural karyotype. Genome size varies greatly between eukaryotes, and shows many strikingly precise correlations with phenotypic characters, independent of information encoded in DNA. Such nucleotypic correlations, based on biophysical absolutes, apply to all species, irrespective of genome size or chromosome number, and set limits on the range of phenotypes which can be expressed by genic control. Thus, knowledge of nucleotypic effects has considerable predictive value which can help to unify our understanding of genomes. Other studies of reconstructed nuclei have shown that: (1) the basic haploid genome exists as a real structural unit in nuclear architecture; while (2) the mean spatial arrangement of its heterologues also exists as a natural karyotype which is predictable using a simple model. Recently reported conceptual alignments of the maize genomes, which reflect the circularized ancestral grass genome, show interesting similarities with the orders of centromeres in their natural karyotypes predicted by the Bennett model. The basis of this phenomenon (if repeated in other species), and of selection which retains the ancestral genome form despite changes in basic chromosome number, may need to be explained. Perhaps the overall 3-D structure of the genome has some critical functional significance, essential for development. If so, a knowledge of this common structure would further unify our understanding of genomes and their evolution.