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.
Highlights
Sex chromosomes have evolved independently across vertebrates multiple times [1,2,3].They mostly evolve from a pair of autosomes by the emergence of a sex-determining gene, and may further differentiate via the suppression of recombination leading to the loss of functional genes, accumulation of heterochromatin and/or repetitive elements in the sex-specific chromosome [4,5,6]
We studied 23 species from five families of Anguimorpha: Abronia campbelli, Abronia deppii, Abronia graminea, Abronia lythrochila, Abronia mixteca, Abronia smithi, Abronia taeniata, Barisia rudicollis, Celestus warreni, Gerrhonotus liocephalus (Anguidae), Heloderma exasperatum, Heloderma horridum (Helodermatidae), Shinisaurus crocodilurus (Shinisauridae), Varanus auffenbergi, Varanus cumingi, Varanus kordensis, Varanus olivaceus, Varanus primordius, Varanus salvadorii, Varanus salvator komaini (Varanidae), Xenosaurus grandis, Xenosaurus platyceps, Xenosaurus rectocollaris (Xenosauridae) (Table S1)
The Fluorescence in situ Hybridization (FISH) with rDNA probe revealed a signal on the 10th pair of macrochromosomes (Figure 8a)
Summary
Sex chromosomes have evolved independently across vertebrates multiple times [1,2,3]. They mostly evolve from a pair of autosomes by the emergence of a sex-determining gene, and may further differentiate via the suppression of recombination leading to the loss of functional genes, accumulation of heterochromatin and/or repetitive elements in the sex-specific chromosome [4,5,6]. Extensive accumulation of various repetitive elements such as rDNA loci, microsatellite motifs, and retrotransposons has been detected in sex chromosomes of many animal and plant species, often in heterochromatic regions [10,11,12,13,14,15,16,17,18,19]
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