Evidence for different origin of sex chromosomes in snakes, birds, and mammals and step-wise differentiation of snake sex chromosomes

Abstract

All snake species exhibit genetic sex determination with the ZZ͞ZW type of sex chromosomes. To investigate the origin and evolution of snake sex chromosomes, we constructed, by FISH, a cytogenetic map of the Japanese four-striped rat snake (Elaphe quadrivirgata) with 109 cDNA clones. Eleven of the 109 clones were localized to the Z chromosome. All human and chicken homologues of the snake Z-linked genes were located on autosomes, suggesting that the sex chromosomes of snakes, mammals, and birds were all derived from different autosomal pairs of the common ancestor. We mapped the 11 Z-linked genes of E. quadrivirgata to chromosomes of two other species, the Burmese python (Python molurus bivittatus) and the habu (Trimeresurus flavoviridis), to investigate the process of W chromosome differentiation. All and 3 of the 11 clones were localized to both the Z and W chromosomes in P. molurus and E. quadrivirgata, respectively, whereas no cDNA clones were mapped to the W chromosome in T. flavoviridis. Comparative mapping revealed that the sex chromosomes are only slightly differentiated in P. molurus, whereas they are fully differentiated in T. flavoviridis, and E. quadrivirgata is at a transitional stage of sex-chromosome differentiation. The differentiation of sex chromosomes was probably initiated from the distal region on the short arm of the protosex chromosome of the common ancestor, and then deletion and heterochromatization progressed on the sex-specific chromosome from the phylogenetically primitive boids to the more advanced viperids.comparative map ͉ chromosome homology ͉ FISH ͉ sex-determining gene ͉ reptile A ll snake species are subject to genetic sex determination with sex chromosomes, as are mammals and birds, and they have female heterogamety (ZZ males and ZW females). Comparative gene mapping between human and chicken revealed that human XX͞XY and chicken ZZ͞ZW sex chromosomes have no homologies (1, 2), suggesting that the sex chromosomes of mammals and birds were derived from different pairs of autosomes of the common ancestor. Beçak et al. (3) found that there is close karyological similarity between snakes and birds, such as distinct differentiation of macro-and microchromosomes and constant occurrence of ZW-type sex chromosomes. This finding leads us to predict the presence of homology between ophidian and avian sex chromosomes. However, no attempts have yet been made to investigate the conservation of the linkage homologies of snake chromosomes to human and chicken chromosomes by comparative gene mapping, although this approach would provide fundamental information on the genome evolution and the origin of sex-chromosome differentiation in amniotes. In another study (4), we constructed a preliminary cytogenetic map of the Japanese four-striped rat snake (Elaphe quadrivirgata) with 52 EST clones, which were isolated from the cDNA library of the brain tissue and were identified as snake homologues of human and chicken orthologous genes by a search of the DNA database. Of 52 EST clones, two genes, T...

Evolutionary stability of sex chromosomes in snakes

“…Z-specificity versus autosomal or pseudoautosomal position was tested by qPCR in a control autosomal gene and six genes linked to ACA6 (homologous to Z in viperids and colubrids [11,12]) in every species. Due to the large phylogenetic divergences within snakes, and also between snakes and their tested outgroups, not all primers worked efficiently in qPCR.…”

“…Partial genetic content of the Z chromosome is known in colubrids and viperids, and is highly syntenic to chromosome 6 (ACA6) of ACA [11,12]. The W chromosome in these lineages is greatly degenerated and lacks the known Z-linked genes.…”

“…Molecular evidence for the homology of sex chromosomes based on knowledge of gene linkage to sex chromosomes is available only for four species from two families: Colubridae (Elaphe quadrivirgata, Thamnophis elegans) and Viperidae (Protobothrops flavoviridis, Sistrurus miliarius) [11,12]. The genetic content of the Z chromosome in these colubroid snakes reveals the strong homology of this chromosome with chromosome 6 of the green anole, Anolis carolinensis (ACA) [11], the model squamate species for reptile genomics with a fully sequenced and adequately annotated genome [13].…”

Disruption of FEM1C-W gene in zebra finch: evolutionary insights on avian ZW genes

“…The ancestral chromosomes of avian sex chromosomes are different from those of eutherian mammals and snakes (Fridolfsson et al 1998;Nanda et al 1999;Matsubara et al 2006), although recent evidence indicates surprising shared gene content of the avian Z and monotreme X chromosomes (Rens et al 2007). The gene content of the Z chromosome appears to have changed little during avian evolution, as shown by comparative painting analysis (Itoh and Arnold 2005;Nishida-Umehara et al 2007;Tsuda et al 2007), even in Ratites (Struthioniformes) which are considered to be an ancestral avian species whose sex chromosomes are morphologically relatively undifferentiated (Ogawa et al 1998;Shetty et al 1999).…”

Molecular cloning and characterization of the repetitive DNA sequences that comprise the constitutive heterochromatin of the W chromosomes of medaka fishes

“…These sex chromosomes share similar properties; the X and Z chromosomes are large and gene-rich, whereas the Y and W chromosomes are smaller, highly heterochromatic, and contain only a small number of genes. However, recent cytogenetic mapping studies showed no homology among human, chicken, and snake sex chromosomes, suggesting different evolutionary origins of these sex chromosomes (Nanda et al 1999;Matsubara et al 2006). Therefore, the accumulation of heterochromatin and the gene loss on Y and W chromosomes are considered to be a convergent property of sex chromosome differentiation.…”