Schematic representation of the process of chromosome rearrangements and the change of GC-content in sauropsids. Phylogenetic relationships of major groups of sauropsids are based on molecular phylogenetic data [3, 6, 7]. This model postulates that chromosome size-dependent GC heterogeneity occurred in the ancestral karyotype. This feature is retained almost intact in the lineages of birds and turtles where interchromosomal rearrangements are infrequent. Some chromosome fusions have occurred in the lepidosaurian lineage and GC3 of genes has been altered according to the size of the chromosome on which they are borne.