Conservation of chromosomes syntenic with avian autosomes in squamate reptiles revealed by comparative chromosome painting.

In contrast to mammals, birds exhibit a slow rate of chromosomal evolution. It is not clear whether high chromosome conservation is an evolutionary novelty of birds or was inherited from an earlier avian ancestor. The evolutionary conservatism of macrochromosomes between birds and turtles supports the latter possibility; however, the rate of chromosomal evolution is largely unknown in other sauropsids. In squamates, we previously reported strong conservatism of the chromosomes syntenic with the avian Z, which could reflect a peculiarity of this part of the genome. The chromosome 1 of iguanians and snakes is largely syntenic with chromosomes 3, 5 and 7 of the avian ancestral karyotype. In this project, we used comparative chromosome painting to determine how widely this synteny is conserved across nine families covering most of the main lineages of Squamata. The results suggest that the association of the avian ancestral chromosomes 3, 5 and 7 can be dated back to at least the early Jurassic and could be an ancestral characteristic for Unidentata (Serpentes, Iguania, Anguimorpha, Laterata and Scinciformata). In Squamata chromosome conservatism therefore also holds for the parts of the genome which are homologous to bird autosomes, and following on from this, a slow rate of chromosomal evolution could be a common characteristic of all sauropsids. The large evolutionary stasis in chromosome organization in birds therefore seems to be inherited from their ancestors, and it is particularly striking in comparison with mammals, probably the only major tetrapod lineage with an increased rate of chromosomal rearrangements as a whole.