Different origins of bird and reptile sex chromosomes inferred from comparative mapping of chicken Z-linked genes

Abstract

Recent progress of chicken genome projects has revealed that bird ZW and mammalian XY sex chromosomes were derived from different autosomal pairs of the common ancestor; however, the evolutionary relationship between bird and reptilian sex chromosomes is still unclear. The Chinese soft-shelled turtle (Pelodiscus sinensis) exhibits genetic sex determination, but no distinguishable (heteromorphic) sex chromosomes have been identified. In order to investigate this further, we performed molecular cytogenetic analyses of this species, and thereby identified ZZ/ZW-type micro-sex chromosomes. In addition, we cloned reptile homologues of chicken Z-linked genes from three reptilian species, the Chinese soft-shelled turtle and the Japanese four-striped rat snake (Elaphe quadrivirgata), which have heteromorphic sex chromosomes, and the Siam crocodile (Crocodylus siamensis), which exhibits temperature-dependent sex determination and lacks sex chromosomes. We then mapped them to chromosomes of each species using FISH. The linkage of the genes has been highly conserved in all species: the chicken Z chromosome corresponded to the turtle chromosome 6q, snake chromosome 2p and crocodile chromosome 3. The order of the genes was identical among the three species. The absence of homology between the bird Z chromosome and the snake and turtle Z sex chromosomes suggests that the origin of the sex chromosomes and the causative genes of sex determination are different between birds and reptiles.

Highly conserved Z and molecularly diverged W chromosomes in the fish genus Triportheus (Characiformes, Triportheidae)

“…Noteworthy, it was evidenced that the end of the Wq has a high concentration of female-specific sequences (Figures 2-4), where an 18S rDNA cluster is also located in all Triportheus species so far analyzed (Artoni and Evolution of sex chromosomes in congeneric species CF Yano et al Bertollo, 2002;Nirchio et al, 2007;Diniz et al, 2009;Marquioni et al, 2013;Yano et al, 2014). Sex chromosomes carrying 18S rDNA sequences have already been reported in several other vertebrates, such as Characidium fishes (Scacchetti et al, 2015), cane toad Bufo marinus (Abramyan et al, 2009), Chinese softshell turtle Pelodiscus sinensis (Kawai et al, 2007) and tiger snake Notechis scutatus (O'Meally et al, 2010). However, the Triportheus case deserves further considerations because of the unusual and particular location of these sequences that do not occur in both homologs of the sex pair (only on the W chromosome) and in some autosomes.…”

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

“…Sex chromosomes bearing rDNAs/NORs have been reported in some vertebrates, such as mammals, reptiles, and amphibians (Kawai et al 2007;O'Meally et al 2010;Schmid et al 1983;Wiley 2003), suggesting that the accumulation of rDNAs is a common property of sex chromosome differentiation in vertebrates. Strong signals of rDNAs/NORs were observed on X or W chromosome in these cases; therefore, the copy number of the rRNA genes was considered to be higher in females than in males, implying a functional significance for female-specific amplification of NORs.…”

Extension, single-locus conversion and physical mapping of sex chromosome sequences identify the Z microchromosome and pseudo-autosomal region in a dragon lizard, Pogona vitticeps

“…There are very few sex-specific sequences reported for reptiles (Cooper et al, 1997;Halverson and Spelman, 2002), although repetitive satellite sequences are known to be interspersed throughout the chromosomes of snakes in high copy number (Singh et al, 1976(Singh et al, , 1980 and are concentrated in particularly high density on the W chromosome (Solari, 1994). A number of functional genes have recently been mapped to the Z and W chromosomes of three snakes, a gecko and a turtle (Matsubara et al, 2006;Kawaia et al, 2007;Kawagoshi et al, 2009).…”