Edited by mar F. Lyon, clinical Research Council, Didcot, united Kingdom, and approved September 28, 2006
All snake types exhibit hereditary sex determination with the ZZ/ZW form of sex chromosomes. To inspection the origin and evolution of line sex chromosomes, we constructed, by FISH, a cytogenetic map that the Japanese four-striped rat line (Elaphe quadrivirgata) v 109 cDNA clones. Eleven of the 109 clones were localized to the Z chromosome. Every human and chicken homologues the the line Z-linked genes were located on autosomes, suggesting that the sex chromosomes of snakes, mammals, and also birds were all derived from various autosomal bag of the usual ancestor. Us mapped the 11 Z-linked gene of E. Quadrivirgata to chromosomes that two various other species, the Burmese python (Python molurus bivittatus) and also the habu (Trimeresurus flavoviridis), to inspection the procedure of W chromosome differentiation. All and 3 the the 11 clones were localized come both the Z and also W chromosomes in P. Molurus and also E. Quadrivirgata, respectively, conversely, no cDNA clones were mapped come the W chromosome in T. Flavoviridis. Comparative mapping revealed that the sex chromosomes are only slightly distinguished in P. Molurus, whereas lock are totally differentiated in T. Flavoviridis, and also E. Quadrivirgata is at a transitional phase of sex-chromosome differentiation. The differentiation the sex chromosomes was probably initiated indigenous the distal region on the brief arm the the protosex chromosome that the typical ancestor, and then deletion and also heterochromatization developed on the sex-specific chromosome indigenous the phylogenetically primitive boids to the an ext advanced viperids.
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All snake varieties are subject to hereditary sex determination with sex chromosomes, as space mammals and birds, and they have actually female heterogamety (ZZ males and also ZW females). Comparative gene mapping in between human and also chicken revealed that person XX/XY and also chicken ZZ/ZW sex chromosomes have no homologies (1, 2), arguing that the sex chromosomes that mammals and also birds were obtained from various pairs that autosomes of the usual ancestor. Beçak et al. (3) discovered that there is close karyological similarity in between snakes and also birds, such as distinctive differentiation the macro- and also microchromosomes and continuous occurrence the ZW-type sex chromosomes. This recognize leads united state to predict the visibility of homology in between ophidian and also avian sex chromosomes. However, no attempts have actually yet to be made to inspection the conservation of the affiliation homologies of snake chromosomes to human and also chicken chromosomes by to compare gene mapping, back this strategy would provide basic information top top the genome evolution and the beginning of sex-chromosome differentiation in amniotes. In one more study (4), we constructed a preliminary cytogenetic map the the Japanese four-striped rat snake (Elaphe quadrivirgata) through 52 EST clones, which were isolated native the cDNA library that the mind tissue and were figured out as line homologues that human and also chicken orthologous gene by a search of the DNA database. That 52 EST clones, two genes, TAX1BP1 and also WAC, whose person homologues are situated on person chromosomes 7 and 10 respectively, were localized come the Z chromosome. In addition, snake homologues of 3 chicken Z-linked genes, DMRT1, ACO1/IREBP, and CHD1, were molecularly cloned by RT-PCR and were based on chromosome mapping. All three homologues were mapped come the quick arm the the line chromosome 2, suggesting that the sex chromosomes the snakes, mammals and also birds were differentiated independently from various autosomes of the typical ancestor. However, only a few genes were mapped top top the snake Z chromosome, and the homology the the line Z chromosome come human and chicken chromosomes has actually not to be investigated in detail.
It is speculated from the observations of in different way evolved sex chromosome pairs the heteromorphic sex chromosomes have occurred from a pair of homologous chromosomes (5). In this scenario, a gene mutation that conferred a sexual advantage an initial occurred on among the homologues, and also a partially heterozygous chromosomal an ar was in turn formed. Meiotic recombination in between the protosex chromosomes was suppressed around the heterologous region to preserve the linkage of sex-linked genes. The suppression of recombination favored the accumulation of gene mutations ~ above the sex-specific chromosome, top to countless deletions of the functionally inactivated gene and buildup of recurring DNA sequences (6, 7). The mammalian Y chromosome and avian W chromosome came to be highly degenerated and also extensively heteochromatized, through the exception of monotremes and also palaeognathous birds, which have less differentiated sex chromosomes (8–13). The person Y chromosome still contains 27 homologues that X-linked single-copy genes and also pseudogenes (14), and also chicken has likewise the Z and also W creates of six “gametologous” genes, which emerged by the cessation of recombination because of sex-chromosome differentiation, ATP5A1, CHD1, HINTZ, PKCI, SPIN, and also UBA2 (15–17). The degeneration status of the snake W chromosomes varies among varieties (3, 18, 19). The Z and also W chromosomes are homomorphic in the boid species. In contrast, the W chromosomes are very degenerated and heterochromatic in the poisonous snakes belonging to the Elapidae and also the Viperidae. The colubrid species, which have moderately identified sex chromosomes, room at an intermediate stage of sex-chromosome differentiation in between the Boidae and also the poison snakes. Thus, snakes space a great animal model for studying the evolutionary procedure of sex-chromosome differentiation in vertebrates.
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Here, us report a high-resolution cytogenetic map the the Japanese four-striped rat snake built with 105 EST clones. We show the conservation of the affiliation homologies of line chromosomes through human and chicken chromosomes and also discuss the genome evolution and also the origins of sex chromosomes in amniotes. Furthermore, us compare the frameworks of sex chromosomes among three line species, the Japanese four-striped rat snake (Colubridae), the Burmese python (Python molurus bivittatus, Pythonidae) and also the habu (Trimeresurus flavoviridis, Viperidae) to track the process of sex-chromosome differentiation throughout the advancement of snakes. First, the morphologies and G- and also C-banded trends of sex chromosomes were compared. Second, the cDNA clones localized to the sex chromosomes the the Japanese four-striped rat snake were comparatively mapped come the chromosomes the two other species. In addition, we cloned a sex chromosome-specific recurring DNA succession from the Japanese four-striped rat snake, i beg your pardon is additionally conserved in both the python and also the habu, and used it as a cytogenetic marker for comparative mapping of sex chromosomes. We additionally cloned two sexual-differentiation genes, DMRT1 and SOX9, from the habu and also determined their chromosomal areas in the three snake species to search for candidate genes of sex decision in snakes. Finally, we discuss the origin and also the procedure of differentiation of snake sex chromosomes.