Cytogenetic Evidence for Sex Chromosomes and Karyotype Evolution in Anguimorphan Lizards

Abstract

Anguimorphan lizards are a morphologically variable group of squamate reptiles with a wide geographical distribution. In spite of their importance, they have been cytogenetically understudied. Here, we present the results of the cytogenetic examination of 23 species from five anguimorphan families (Anguidae, Helodermatidae, Shinisauridae, Varanidae and Xenosauridae). We applied both conventional (Giemsa staining and C-banding) and molecular cytogenetic methods (fluorescence in situ hybridization with probes for the telomeric motifs and rDNA loci, comparative genome hybridization), intending to describe the karyotypes of previously unstudied species, to uncover the sex determination mode, and to reveal the distribution of variability in cytogenetic characteristics among anguimorphan lizards. We documented that karyotypes are generally quite variable across anguimorphan lineages, with anguids being the most varying. However, the derived chromosome number of 2n = 40 exhibits a notable long-term evolutionary stasis in monitors. Differentiated ZZ/ZW sex chromosomes were documented in monitors and helodermatids, as well as in the anguids Abronia lythrochila, and preliminary also in Celestus warreni and Gerrhonotus liocephalus. Several other anguimorphan species have likely poorly differentiated sex chromosomes, which cannot be detected by the applied cytogenetic methods, although the presence of environmental sex determination cannot be excluded. In addition, we uncovered a rare case of spontaneous triploidy in a fully grown Varanus primordius.

Cytogenetic Analysis of the Members of the Snake Genera Cylindrophis, Eryx, Python, and Tropidophis

“…These microchromosomes are tiny and dot-like; therefore, we cannot safely conclude whether the intense signal derives from ITRs or the extended arrays of terminal telomeres. A similar pattern has been identified in other reptilian species, such as the dragonsnake Xenodermus javanicus [ 66 ], monitors, and helodermatids [ 28 , 63 ]. One potential explanation is that microchromosomes often have higher recombination rates than autosomes in vertebrates, including birds and snakes [ 67 , 68 , 69 ].…”

“…Considering that the chromosome morphology of this species is shared with other sand boas (except for E. colubrinus, as mentioned above), we suppose that ITRs in this species are likely an outcome of cryptic intrachromosomal rearrangements, such as inversions. Furthermore, the centromeric satellite content is very dynamic, and even closely related species might have a different composition of repeats [ 23 , 62 , 63 , 64 ]. FISH with telomeric probes did not detect ITRs in T. melanurus; however, the presence of the interstitial heterochromatin on the sixth chromosome pair might suggest a possible fusion point (e.g., tandem fusion), which might also explain the lower chromosome number (2n = 34) in this species compared with the typical snake karyotype (2n = 36).…”

Sex Chromosomes and Master Sex-Determining Genes in Turtles and Other Reptiles

“…In a later study, the occurrence of a ZW system of sex chromosomes was confirmed or suggested by cytogenetics in more species. More precisely, in A. lythrochila a putative W chromosome was identified on a minichromosome by C-banding, and both C-banding and CGH revealed a female specific signal in two other Anguidae species: Celestus warreni and Gerrhonotus liocephalus [246].…”

Extreme genetic divergence indicated by chromosome rearrangements

“…Varanidae, a widespread family of reptiles found in Africa, the Middle East, Asia and Australia, provides an ideal model system for testing the role of chromosome rearrangements in speciation. Varanids are distinct from many other species of lizards in that they have a remarkably conserved karyotype where all species so far karyotyped have 2n = 40 chromosomes (King and King 1975;Dutt 1968;King et al 1982;De Smet 1981;Matsubara et al 2014;Patawang et al 2017;Johnson Pokorná et al 2016;Iannucci et al 2019;Augstenová et al 2021). All variability in their karyotypes is due to differences in chromosome morphology on chromosomes 3-8.…”