Genome size evolution in vertebrates: trends and constraints.

1. Studies on the genomic evolution in vertebrates have highlighted the differences existing between anamniotes and amniotes, both in quantitative and compositional terms. 2. These differences do not seem to depend on a different tendency to genic amplification, but rather on the existence of more strict and efficient constraints in amniotes. 3. Some constraints, that may be defined as "intrinsic", would act directly on the genome; among these particularly important is the chiasma frequency during meiosis. 4. Other, "extrinsic", constraints, would act indirectly through genic products or through cell morphometric parameters. 5. The genome size increase seems to depend on various mechanisms. The most wide-spread one seems to be the amplification of interspersed repetitive and non-repetitive sequences.