Identification of the linkage group of the Z sex chromosomes of the sand lizard (Lacerta agilis, Lacertidae) and elucidation of karyotype evolution in lacertid lizards

Abstract

The sand lizard (Lacerta agilis, Lacertidae) has a chromosome number of 2n = 38, with 17 pairs of acrocentric chromosomes, one pair of microchromosomes, a large acrocentric Z chromosome, and a micro-W chromosome. To investigate the process of karyotype evolution in L. agilis, we performed chromosome banding and fluorescent in situ hybridization for gene mapping and constructed a cytogenetic map with 86 functional genes. Chromosome banding revealed that the Z chromosome is the fifth largest chromosome. The cytogenetic map revealed homology of the L. agilis Z chromosome with chicken chromosomes 6 and 9. Comparison of the L. agilis cytogenetic map with those of four Toxicofera species with many microchromosomes (Elaphe quadrivirgata, Varanus salvator macromaculatus, Leiolepis reevesii rubritaeniata, and Anolis carolinensis) showed highly conserved linkage homology of L. agilis chromosomes (LAG) 1, 2, 3, 4, 5(Z), 7, 8, 9, and 10 with macrochromosomes and/or macrochromosome segments of the four Toxicofera species. Most of the genes located on the microchromosomes of Toxicofera were localized to LAG6, small acrocentric chromosomes (LAG11-18), and a microchromosome (LAG19) in L. agilis. These results suggest that the L. agilis karyotype resulted from frequent fusions of microchromosomes, which occurred in the ancestral karyotype of Toxicofera and led to the disappearance of microchromosomes and the appearance of many small macrochromosomes.

Mammalian X homolog acts as sex chromosome in lacertid lizards

“…We would have preferred to use the ACA instead of the GGA genome in this step, but physical mapping in ACA is still not sufficient. However, using GGA genome is substantiated by the high level of conservation in gene synteny between GGA and squamates (see, for example, Alföldi et al, 2011;Pokorná et al, 2011aPokorná et al, , 2012Srikulnath et al, 2014). In the next step, we identified the unusually numerous groups of the TSE transcripts without SNPs grouped according to the linkage of their orthologs to particular GGA chromosomes.…”

“…Primer pairs were designed for the amplification of the 120-200 bp exon fragment of autosomal 'control' genes, candidate Z-specific genes from the transcriptome of TSE and of genes identified as Z-linked in LAG in the previous study by Srikulnath et al (2014) using the Primer-BLAST software (Ye et al, 2012). As this was not technically possible in several cases, we designed primers from genes with the closest topology in the chromosomes/scaffolds of the green anole.…”

“…A relative female-to-male gene dosage ratio (r) of 0.5 is expected for Z-specific genes, that is, the genes lacking in the degenerated part of the W chromosome, and 1.0 for (pseudo)autosomal genes. For adequate biological replicates, the Z-specificity of the tested genes was determined in three pairs of TSE, and also in three pairs of LAG for comparison with the previous study (Srikulnath et al, 2014).…”

“…In any case, more robust testing of the hypotheses of the ancestral sex determination and on the evolution of sex-determining mechanisms in amniotes is largely precluded by our very limited knowledge about the homology of sex chromosomes and sex-determining genes among particular groups. The homology of sex chromosomes can be inferred from the knowledge of the gene content of sex chromosomes; however, it has been identified in only a few amniote lineages such as viviparous mammals (see, for example, Kohn et al, 2004) and monotremes (see, for example, Rens et al, 2007), birds (see, for example, Zhou et al, 2014), several turtle species (see, for example, Kawagoshi et al, 2012Kawagoshi et al, , 2014, advanced snakes (Matsubara et al, 2006;Vicoso et al, 2013;Rovatsos et al, 2015), iguanas (see, for example, Alföldi et al, 2011;Rovatsos et al, 2014a-c), a gecko (Kawai et al, 2009) and recently also a lacertid lizard (Srikulnath et al, 2014).…”

“…The recent study by Srikulnath et al (2014) used physical gene mapping in a single lacertid species, the sand lizard (Lacerta agilis (LAG)), in order to reconstruct the homology of individual lacertid chromosomes with chromosomes of other sauropsids. Based on the cytogenetic mapping of nine tested genes, the authors concluded that the Z chromosome of LAG is the fifth pair of the complement, and its gene content is partially homologous to the chicken (Gallus gallus (GGA)) chromosomes 6 (GGA6) and 9 (GGA9) and to the green anole (Anolis carolinensis (ACA)) chromosome 3 (ACA3).…”