The evolution of sex chromosomes from autosomes has been documented many times in different organisms ; recently reviewed e.g. in ; but see e.g. ]. During their evolution, sex chromosomes go progressively through several steps. Briefly, the first step is the acquisition of sex determining locus or loci. Subsequently, the genetic content of both members of the pair diverge. The specialization of sex chromosomes for their sex-specific roles [e.g. ] selects for the reduction of the interchange of genetic material between sex chromosomes and thus for lower levels of recombination. However, lack of recombination leaves the unpaired sex chromosomes (Y and W) without the possibility to correct mutations in coding sequences, which leads to an unusually low content of functional genes. Moreover, cessation of recombination opens doors for the accumulation of various repeats on sex chromosomes (e.g. microsatellites, transposons, rDNA sequences; ). Alternatively, the accumulation of repetitive sequences may not be a consequence of reduced recombination, but its cause . By generating asynchrony in the DNA replication pattern of X and Y, respectively Z and W chromosomes, it can reduce the crossing-over frequency between them [e.g. ]. The accumulation of repeats on a heterogametic sex chromosome (Y or W) may be so massive that the chromosome is finally much larger than its homologous counterpart in the pair. The heterogametic sex chromosome may even become the largest chromosome in the genome such as the Y chromosome in the plant Silene latifolia , . On the other hand, in some lineages, heterogametic sex chromosomes may progressively decrease in size [e.g.  and such degeneration can result in their elimination from the genome [e.g. . In yet other cases, sex chromosomes may stay homomorphic for a long evolutionary time [e.g. [10, 12, 13]]. In many organisms, the heterogametic sex chromosome has been found to be highly heterochromatinized [e.g. [12, 14]. The heterochromatinization may be a mechanism for the defence against the activity of transposable elements or other repetitive sequences to safeguard genome integrity [e.g. [15, 16].
Squamate reptiles, the lineage encompassing lizards, snakes and amphisbaenians, represent an interesting group for the exploration of the evolution of sex chromosomes, as they possess substantial variability in sex determining mechanisms [17–19]. Squamate reptiles include species with environmental sex determination, i.e. without sex chromosomes; species with homomorphic sex chromosomes, and those with heteromorphic sex chromosomes. All three situations can be found even in a single family, for example in dragon lizards or eye-lid geckos [20–22]. Sex chromosomes are at various stages of the general process of sex chromosome evolution in different squamate species and they evolved within squamates independently several times as supported by differences in their size, shape and type (male or female heterogamety) but also by molecular-cytogenetic tests of synteny of sex chromosomes and phylogenetic distribution of sex determining systems [10, 19, 20, 23].
At the time of submitting, to our knowledge, the accumulation of repeats during the degeneration of sex chromosomes has been studied only in a single lineage of squamate reptiles, in that of snakes [7, 24, 25]. Pythons, the group in the rather basal position of snake phylogeny , with homomorphic sex chromosomes, do not show any accumulation of repeats, while the degenerated W sex chromosomes in many advanced snakes from the crown clade Colubroidea such as colubrids or elapids, exhibit a massive accumulation of repeats [24, 25]. For example, the W chromosome in an elapid snake Notechis scutatus is composed almost entirely of repetitive sequences, including 18S rDNA and the banded krait minor-satellite (Bkm) repeats . The Bkm repeats consist of tandem arrays of 26 and 12 copies, respectively, of two tetranucleotides, GATA and GACA . Bkm-related repeats are also accumulated on the heterogametic sex chromosomes in many vertebrates, including humans, and also in plants [28–33]. It was speculated that the Bkm-related repeats are functional, playing a role in the transcriptional activation of sex chromosome heterochromatin . A common origin of the Bkm-related repeats across different eukaryotic lineages was assumed [e.g. , however, a convergent evolution is also likely . Recently, based on the results of chicken W chromosome painting in snakes, O'Meally and colleagues  concluded that heterogametic sex chromosomes in birds and derived snakes may share repetitive sequences. They suggested that this observation could be explained by yet undetected synteny of parts of the sex chromosomes between these lineages. The homology of repetitive sequences accumulated on sex chromosomes could be tested by the evaluation of the identity of repeats on sex chromosomes in other lineages of squamates phylogenetically nested between snakes, birds and vertebrates with the Bkm-related repeats.
The aim of the present study is to compare the distribution of microsatellite sequences on differently differentiated sex chromosomes in two lizard species with independently evolved sex chromosomes and to determine whether the distribution of microsatellite repeats on sex chromosomes corresponds to the stage of their heteromorphism or heterochromatinization. Multiple sex chromosomes (X1X1X2X2/X1X2Y) are heteromorphic and fully euchromatic in the first studied species, the gecko Coleonyx elegans from the family Eublepharidae . On the other hand, the ZZ/ZW sex chromosomes in the second species, Eremias velox from the family Lacertidae, are homomorphic and the W chromosome is highly heterochromatic . Moreover, the gekkotan lizards represent one of the basal groups of squamate reptiles, while lacertids are much more closely related to snakes . The knowledge of repetitive sequences on the sex chromosomes in the two selected species should therefore be informative for the determination of the homology of the Bkm-related and other repeats across vertebrate lineages.