Abstract
Heteromorphic sex chromosomes, particularly the ZZ/ZW sex chromosome system of birds and some reptiles, undergo evolutionary dynamics distinct from those of autosomes. The W sex chromosome is a unique karyological member of this heteromorphic pair, which has been extensively studied in snakes to explore the origin, evolution, and genetic diversity of amniote sex chromosomes. The snake W sex chromosome offers a fascinating model system to elucidate ancestral trajectories that have resulted in genetic divergence of amniote sex chromosomes. Although the principal mechanism driving evolution of the amniote sex chromosome remains obscure, an emerging hypothesis, supported by studies of W sex chromosomes of squamate reptiles and snakes, suggests that sex chromosomes share varied genomic blocks across several amniote lineages. This implies the possible split of an ancestral super-sex chromosome via chromosomal rearrangements. We review the major findings pertaining to sex chromosomal profiles in amniotes and discuss the evolution of an ancestral super-sex chromosome by collating recent evidence sourced mainly from the snake W sex chromosome analysis. We highlight the role of repeat-mediated sex chromosome conformation and present a genomic landscape of snake Z and W chromosomes, which reveals the relative abundance of major repeats, and identifies the expansion of certain transposable elements. The latest revolution in chromosomics, i.e., complete telomere-to-telomere assembly, offers mechanistic insights into the evolutionary origin of sex chromosomes.
Highlights
A fundamental aspect of the life history of sexually reproducing organisms is the fusion of haploid gametes produced by meiosis through the mechanisms of independent assortment and genetic recombination of chromosomes from two parental genomes to form a zygote
Sex chromosomes were discovered by Nettie Stevens in 1905
Recent progress on the evolution of sex chromosomes in several amniotes has supported long-standing hypotheses and, for many other amniotes, has revealed that there is no single narrative for how these regions form and evolve
Summary
A fundamental aspect of the life history of sexually reproducing organisms is the fusion of haploid gametes produced by meiosis through the mechanisms of independent assortment and genetic recombination of chromosomes from two parental genomes to form a zygote. This scenario is supported by evidence of the same linkage homology blocks that perform the role of sex chromosomes in several amniote lineages, or by unrelated sex chromosomes sharing partial linkage homologies across distantly related groups [18,25,27,28,30,31,32,58] This was probably caused by multiple random selections from a limited number of linkage homologies, or a stronger tendency for a linkage homology to be co-opted owing to its gene content, as a result of enrichment of the genes involved in gonad differentiation, and the possibility of homologous sex-determining systems [7]. Evolutionary dynamics of repeat-mediated sex chromosome formation are discussed
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