Contrary to the chromosome constitution of most snakes (2n=36), the cobra karyotype shows diploid chromosome number at 38. To investigate karyotype differences and the process of chromosomal rearrangement between cobras and most other snakes, chromosomics (cytogenetics and genomics) was conducted in Siamese cobra (Naja kaouthia). Siamese cobra chromosome 2 (NKA2) is subtelocentric, differing with those of most other snakes which are submetacentric. Chicken BAC probes, comprising the chicken Z chromosome (GGAZ) and GGA12, GGA13, and GGA18 were almost mapped to NKA2 in the same order as on most other snake chromosome 2, suggesting that pericentric inversion or centromere repositioning occurred in the cobra lineage. GGA2p and GGA27 were localized to NKAZ, and were similar to those of most other snakes, but not for NKAW. In addition, large C-positive blocks were observed on the entire arm of NKAW. Hybridization signals of telomeric repeat (TTAGGG)n and two microsatellite motifs (AAGG and AGAT) were also observed interstitially at the same position as large C-positive blocks on NKAWq, suggesting that the amplification of telomeric repeat and microsatellite motifs are strongly associated with differentiation and heterochromatinization of sex chromosomes in Siamese cobra. One chicken GGAZ BAC clone (CH261-129A16), comprising various microsatellite motifs and transposons was located in NKAWq. This suggests that repetitive sequences are shared between the sex chromosomes of chicken and cobra, and supports the hypothesis of an ancestral super-sex chromosome in amniotes. Degeneration of sex chromosomes was probably convergent between birds and snakes.
Contrary to the chromosome constitution of most snakes (2n=36), the cobra karyotype shows diploid chromosome number at 38. To investigate karyotype differences and the process of chromosomal rearrangement between cobras and most other snakes, chromosomics (cytogenetics and genomics) was conducted in Siamese cobra (Naja kaouthia). Siamese cobra chromosome 2 (NKA2) is subtelocentric, differing with those of most other snakes which are submetacentric. Chicken BAC probes, comprising the chicken Z chromosome (GGAZ) and GGA12, GGA13, and GGA18 were almost mapped to NKA2 in the same order as on most other snake chromosome 2, suggesting that pericentric inversion or centromere repositioning occurred in the cobra lineage. GGA2p and GGA27 were localized to NKAZ, and were similar to those of most other snakes, but not for NKAW. In addition, large C-positive blocks were observed on the entire arm of NKAW. Hybridization signals of telomeric repeat (TTAGGG)n and two microsatellite motifs (AAGG and AGAT) were also observed interstitially at the same position as large C-positive blocks on NKAWq, suggesting that the amplification of telomeric repeat and microsatellite motifs are strongly associated with differentiation and heterochromatinization of sex chromosomes in Siamese cobra. One chicken GGAZ BAC clone (CH261-129A16), comprising various microsatellite motifs and transposons was located in NKAWq. This suggests that repetitive sequences are shared between the sex chromosomes of chicken and cobra, and supports the hypothesis of an ancestral super-sex chromosome in amniotes. Degeneration of sex chromosomes was probably convergent between birds and snakes.
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