Molecular cytogenetic studies on the process of genomic and chromosomal reorganization in amphibians and reptiles

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To delineate the process of genomic and karyotypic evolution of tetrapods, we constructed comparative cytogenetic maps of Pelodiscus sinensis, Crocodylus siamensis, Elaphe quadrivirgata, Varanus salvator, Xenopus tropicalis, and Xenopus laevis. Our results provide the possibility that the ancestral karyotype of tetrapods had at least 10 large linkage groups of macrochromosomes and many microchromosomal pairs, which have been conserved in the chicken. We identified all nine homoeologous chromosome groups of X laevis, which were homologous within the same groups except for two inversions, and the genetic linkages were highly conserved between X. tropicalis and X. laevis except for one fusion and one inversion. In E. quadrivirgata, microchromosomal genes showed significantly higher GC content than macrochromosomal genes as in the chicken, suggesting that the chromosome size-dependent GC heterogeneity had already occurred before the lepidosaur-archosaur split, 275 million years ago.