Abstract
Chameleons are well-known, highly distinctive lizards characterized by unique morphological and physiological traits, but their karyotypes and sex determination system have remained poorly studied. We studied karyotypes in six species of Madagascan chameleons of the genus Furcifer by classical (conventional stain, C-banding) and molecular (comparative genomic hybridization, in situ hybridization with rDNA, microsatellite, and telomeric sequences) cytogenetic approaches. In contrast to most sauropsid lineages, the chameleons of the genus Furcifer show chromosomal variability even among closely related species, with diploid chromosome numbers varying from 2n = 22 to 2n = 28. We identified female heterogamety with cytogenetically distinct Z and W sex chromosomes in all studied species. Notably, multiple neo-sex chromosomes in the form Z1Z1Z2Z2/Z1Z2W were uncovered in four species of the genus (F. bifidus, F. verrucosus, F. willsii, and previously studied F. pardalis). Phylogenetic distribution and morphology of sex chromosomes suggest that multiple sex chromosomes, which are generally very rare among vertebrates with female heterogamety, possibly evolved several times within the genus Furcifer. Although acrodontan lizards (chameleons and dragon lizards) demonstrate otherwise notable variability in sex determination, it seems that female heterogamety with differentiated sex chromosomes remained stable in the chameleons of the genus Furcifer for about 30 million years.
Highlights
Sex determination systems are variable in amniotes; this variability is unequally distributed across clades
Other scenarios assume that transitions between environmental sex determination (ESD) and genotypic sex determination (GSD) are possible in both directions [13,14]
The telomeric sequences were detected in terminal positions of all chromosomes and in the centromeric, pericentromeric, and interstitial regions of the sixth largest chromosome pairs (Figure 3a). rDNA loci were located in terminal position of the q arm of the third chromosome pair (Figure 4a)
Summary
Sex determination systems are variable in amniotes; this variability is unequally distributed across clades. Several clades such as viviparous mammals, caenophidian snakes, iguanas, softshell turtles, lacertids, and varanids and their relatives exhibited notable stability of sex chromosomes for several dozens of millions of years [1,2,3,4,5,6,7]. Other scenarios assume that transitions between ESD and GSD are possible in both directions [13,14] In any case, it seems that highly differentiated sex chromosomes tend to be evolutionary stable and recent molecular evidence disputed earlier reported transitions from GSD with highly differentiated sex chromosomes to ESD [15,16]. Poorly differentiated sex chromosomes in reptiles, e.g., in boas and pythons [17,18], tend to be prone to sex chromosome turnovers, but it is not yet clear whether they can be replaced only with other GSD systems or with
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