Detailed Explanation of the Differences and Origins of Snake Sex Chromosomes

The sex of snakes is determined by sex chromosomes and belongs to the ZZ/ZW mechanism. To study the origin and evolution of snake sex chromosomes, Kazumi Matsubara and other researchers from the Creative Research Initiative Sousei (CRIS) at Hokkaido University in Japan used fluorescence in situ hybridization to obtain the cytogenetic map of the Japanese four-striped rat snake (Elaphe quadrivirgata). The article was published on November 28 in PNAS.

The map contains 109 cDNA clones, of which 11 are located on the Z chromosome. The human and bird genes corresponding to the snake Z chromosome genes are located on autosomes, indicating that the sex chromosomes of snakes, mammals, and birds originated from different pairs of autosomes in a common ancestor. The researchers also mapped the 11 Z-linked genes of E. quadrivirgata, the Burmese python (Python molurus bivittatus), and the pit viper (Trimeresurus flavoviridis) to study differences in the W chromosome. They found that all 11 clones were present on the Z chromosome of both P. molurus and E. quadrivirgata; 3 clones were present on the W chromosome of both P. molurus and E. quadrivirgata; and no cDNA was detected on the W chromosome of T. flavoviridis. Comparative mapping results showed that the sex chromosomes changed very little in P. molurus; however, large differences appeared in T. flavoviridis, and E. quadrivirgata is in a transitional stage of sex chromosome differentiation. This difference in sex chromosomes may have appeared in the distal region of the short arm of the proto-sex chromosome (protosex chromosome, editor's note) of the common autosomal ancestor, and then, during the evolutionary process from the most phylogenetically primitive boids to the more highly evolved viperids, deletion and heterochromatization progressed.