Whole-chromosome fusions in the karyotype evolution ofSceloporus(Iguania, Reptilia) are more intense in sex chromosomes than autosomes

Abstract

17There is a growing body of evidence that the common ancestor of vertebrates had a bimodal 18 karyotype, i.e. consisting of large macrochromosomes and small microchromosomes. This 19 type of karyotype organization is preserved in most reptiles. However, certain species 20 independently experience microchromosome fusions. The evolutionary forces behind this are 21 unclear. We investigated the karyotype of the green spiny lizard, Sceloporus malachiticus, an 22 iguana species which has 2n=22, whereas the ancestral karyotype of iguanas had 2n=36. We 23 obtained and sequenced flow-sorted chromosome-specific DNA samples and found that most 24 of the microchromosome fusions in this species involved sex chromosomes. We found that 25 certain ancestral squamate chromosomes, such as the homologue of the Anolis carolinensis 26 chromosome 11, are repeatedly involved in sex chromosome formation in different species. 27To test the hypothesis that the karyotypic shift could be associated with changes in 28 recombination patterns, and to study sex chromosome synapsis and recombination in meiosis, 29 we performed synaptonemal complex analysis in this species and in S. variabilis, a related 30 species with 2n=34. We found that in the species studied the recombination patterns correlate 31 more with phylogeny than with the structure of the karyotype. The sex chromosomes had two 32 distal pseudoautosomal regions and a medial differentiated region. 33 34 42 different lineages share high homology [4,5]. The unimodal organization originated 43 independently in different lineages by fusion of microchromosomes with each other and with 44 the macrochromosomes [6,7]. 45This parallel process is interesting in the context of the search for general patterns of 46 karyotype and genome evolution in animals. Fixation of chromosomal rearrangements is 47 recombine in a meiotic act. It has two components: the interchromosomal, which reflects 87 recombination via independent chromosome segregation, and the intrachromosomal, which 88 reflects crossover recombination. We also studied sex chromosome synapsis and 89 recombination to validate the FISH and sequencing data on their structure. 90 Materials and methods 91The male specimens of S. malachiticus and S. variabilis were obtained from the pet trade. To 92 confirm the species identification, the 5'-fragment of the mitochondrial COI gene was 93 sequenced using protocols and primers described previously [17]. 94The primary fibroblast cell cultures of S. malachiticus were obtained in the Laboratory of 95 Animal Cytogenetics, the Institute of Molecular and Cellular Biology, Russia, using the 96 protocols described previously [18,19]. All cell lines were deposited in the IMCB SB RAS 97 cell bank ("The general collection of cell cultures", 0310-2016-0002). Metaphase 98 chromosome spreads were prepared from chromosome suspensions obtained from early 99 passages of primary fibroblast cultures as described previously [20-22].100C-like DAPI staining was performed in the following way. The slides were incub...

Amplified Fragments of an Autosome-Borne Gene Constitute a Significant Component of the W Sex Chromosome of Eremias velox (Reptilia, Lacertidae)

“…In humans and other placental mammals, the sex chromosomes are constituted of the ancestral X/Y chromosome region and the added autosome-derived region [57]. In reptiles, such cases are known, for example, in geckos, anoles, and fence lizards [58][59][60][61]. In lacertids, the most well-known example is the European common lizard (Z. vivipara), which acquired a multiple sex chromosome system (Z 1 Z 2 Z 1 Z 2 /Z 1 Z 2 W) via a sex chromosomeautosome fusion [62].…”

Bridging the Gap between Vertebrate Cytogenetics and Genomics with Single-Chromosome Sequencing (ChromSeq)

“…The study of sex chromosome evolution—and neo-sex chromosomes in particular—has also taken great advantage of the ChromSeq approach. This technique was implemented to reveal the genetic contents of neo-sex chromosomes in two iguanian lizard groups, which independently experienced multiple whole-chromosome fusions: the anoles (Dactyloidae) and the fence lizards ( Sceloporus , Phrynosomatidae) [ 55 , 82 ]. Despite the evolutionary independence of the fusions, repeated fusions of the same ancestral chromosomes were identified, which suggests that such fusions may probably occur non-randomly or may have non-neutral consequences and be fixed by selection.…”

Characterization of a pericentric inversion in plateau fence lizards (Sceloporus tristichus): evidence from chromosome-scale genomes

“…Differentiation in the fundamental number of chromosomes is hypothesized to be a factor responsible for driving the rapid diversification of the genus ( Hall 2009 ; Leaché and Sites Jr. 2009 ). While many species have the ancestral karyotype formula of 2 n = 36 chromosomes, Sceloporus exhibits substantial karyotype variation (ranging from 2 n = 22 to 46), sex chromosome evolution ( Lisachov et al 2020 ), and large genome rearrangements ( Leaché and Sites Jr. 2009 ). Existing genomic resources for Sceloporus include an annotated genome for S. undulatus ( Westfall et al 2020 ), a de novo assembled shotgun genome for S. occidentalis and partial genomes for 34 other species ( Arthofer et al .…”