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Figure 5. Schematic representation of the protokaryotypes of amniotes and tetrapods and their evolutionary processes.The schematic diagrams of the vertebrate chromosomes are modified from the genome and/or chromosome maps of Ambystoma mexicanum/A. tigrinum [27], the green anole (Anolis carolinensis) (the Ensembl Anole Lizard Genome Browser, http://www.ensembl.org/Anolis_carolinensis) [19], the Japanese four-striped rat snake (Elaphe quadrivirgata) [31], [32], and chicken (the Ensembl Chicken Genome Browser, http://www.ensembl.org/Gallus_gallus), and the protokaryotype of teleost fishes [17], [24]. Genetic linkages that are homologous to chicken macrochromosomal arms and/or macrochromosomes (GGA1p, 1q, 2p, 2q, 3, 4q, and GGA5–8) and microchromosomes (GGA4p and GGA9–28) are represented by 10 and 21 differently colored bars, respectively, and segments drawn with diagonal lines indicate the chicken Z chromosome. The chromosome numbers of the chicken microchromosomes are shown to the left of the chromosomes for the reptilian and amphibian species and the ancestral amniote, tetrapod, and teleost fish. The ancestral amniotes and tetrapods had at least 10 large genetic linkage groups, which corresponded to chicken macrochromosomes. At least 14 and eight pairs of microchromosomes, which were homologous to chicken microchromosomes, were also contained in the protokaryotypes of amniotes and tetrapods, respectively. The macrochromosomal genetic linkages of tetrapods have been highly conserved in amphibians, non-avian reptiles, and birds for over 360 million years. Fusions between macro- and microchromosomes and/or between microchromosomes occurred independently in the amphibian, squamate, crocodilian, and mammalian lineages, although the fusions occurred very rarely or less frequently in the testudian and avian lineages. Homologies with chicken macro- and microchromosomal linkage groups are much lower in human [22]–[24]. In the salamander, linkage 4 and 13, linkage 8 and 12, and linkage 15 and 17 are each contained in the same linkage [26], [27], [49] (Figure S5). The divergence times are cited from Hedges et al. [2] and Benton & Donoghue [3]. MYA, million years ago. Image published in: Uno Y et al. (2012) Image reproduced on Xenbase with permission of the publisher and the copyright holder. This image is reproduced with permission of the journal and the copyright holder. This is an open-access article distributed under the terms of the Creative Commons Attribution license Larger Image Printer Friendly View

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