| Summary: | This contribution provides the first karyotype description of <i>Hemidactylus mercatorius</i> and discusses the interspecific chromosome diversification in the genus. Chromosomal analysis was performed on samples from different Malagasy populations using standard karyotyping, Ag-NOR staining, and banding methods (sequential C-banding + Giemsa, + Chromomycin A<sub>3</sub>, +4′,6-diamidino-2-phenylindole). Irrespective of sex or sampling locality, <i>H. mercatorius</i> shows a karyotype of 2n = 42 with metacentric (1, 18–21), submetacentric (4), subtelocentric (5, 11), and acrocentric pairs (all the remaining pairs). There was no heteromorphic chromosome pair and no clear distinction between macro- and microchromosomes. NORs were localised close to the centromeres of a medium acrocentric pair (14). Heterochromatic blocks were identified on the telomeric and centromeric regions of most chromosome pairs. A comparison with the karyotype of <i>H. mabouia</i> highlights that the different morphology of several chromosome pairs clearly distinguishes the two species, contrasting the previously proposed synonymy. The differences between the karyotypes of <i>H. mercatorius</i> and <i>H. mabouia</i> concern the number of biarmed and acrocentric elements, suggesting the occurrence of several chromosome inversions. Considering all the available karyotype data on <i>Hemidactylus</i> and its sister genus <i>Cyrtodactylus</i>, it is possible to advance an evolutionary hypothesis on their chromosomal evolution, starting from a common ancestor with 2n = 48 and all acrocentric elements. From this ancestral condition, the karyotype diversification in the two genera has been prevalently characterised by a progressive accumulation of fusions and inversions which have reduced the total chromosome count and increased the number of biarmed chromosomes.
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