Microchromosome polymorphism in the sand lizard, Lacerta agilis Linnaeus, 1758 (Reptilia, Squamata)

Germline-Restricted Chromosomes and Autosomal Variants Revealed by Pachytene Karyotyping of 17 Avian Species

Karyotypes of less than 10% of bird species are known. Using immunolocalization of the synaptonemal complex, the core structure of meiotic chromosomes at the pachytene stage, and centromere proteins, we describe male pachytene karyotypes of 17 species of birds. This method enables higher resolution than the conventional analyses of metaphase chromosomes. We provide the first descriptions of the karyotypes of 3 species (rook, Blyth's reed warbler, and European pied flycatcher), correct the published data on the karyotypes of 10 species, and confirm them for 4 species. All passerine species examined have highly conservative karyotypes, 2n = 80-82 with 7 pairs of macrochromosomes (including the ZZ sex chromosome pair which was not unambiguously distinguished from other macrochromosomes in most species) and 33-34 pairs of microchromosomes. In all of them, but not in the common cuckoo, we revealed single copies of the germline-restricted chromosomes varying in size and morphology even between closely related species. This indicates a fast evolution of this additional chromosome. The interspecies differences concern the sizes of the macrochromosomes, morphology of the microchromosomes, and sizes of the centromeres. The pachytene cells of the gouldian finch, brambling, and common linnet contain heteromorphic synaptonemal complexes indicating heterozygosity for inversions or centromere shifts. The European pied flycatcher, gouldian finch, and domestic canary have extended centromeres in several macro- and microchromosomes.

Snake W Sex Chromosome: The Shadow of Ancestral Amniote Super-Sex Chromosome

: 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.

Evolutionary Variability of W-Linked Repetitive Content in Lacertid Lizards

Lacertid lizards are a widely radiated group of squamate reptiles with long-term stable ZZ/ZW sex chromosomes. Despite their family-wide homology of Z-specific gene content, previous cytogenetic studies revealed significant variability in the size, morphology, and heterochromatin distribution of their W chromosome. However, there is little evidence about the accumulation and distribution of repetitive content on lacertid chromosomes, especially on their W chromosome. In order to expand our knowledge of the evolution of sex chromosome repetitive content, we examined the topology of telomeric and microsatellite motifs that tend to often accumulate on the sex chromosomes of reptiles in the karyotypes of 15 species of lacertids by fluorescence in situ hybridization (FISH). The topology of the above-mentioned motifs was compared to the pattern of heterochromatin distribution, as revealed by C-banding. Our results show that the topologies of the examined motifs on the W chromosome do not seem to follow a strong phylogenetic signal, indicating independent and species-specific accumulations. In addition, the degeneration of the W chromosome can also affect the Z chromosome and potentially also other parts of the genome. Our study provides solid evidence that the repetitive content of the degenerated sex chromosomes is one of the most evolutionary dynamic parts of the genome.