| Summary: | A remarkable morphological diversity and karyotype variability can be observed in the Neotropical armored catfish genus <i>Harttia</i>. These fishes offer a useful model to explore both the evolution of karyotypes and sex chromosomes, since many species possess male-heterogametic sex chromosome systems and a high rate of karyotype repatterning. Based on the karyotype organization, the chromosomal distribution of several repetitive DNA classes, and the rough estimates of genomic divergences at the intraspecific and interspecific levels via Comparative Genomic Hybridization, we identified shared diploid chromosome numbers (2n = 54) but different karyotype compositions in <i>H. dissidens</i> (20m + 26sm + 8a) and <i>Harttia</i> sp. 3 (16m + 18sm + 14st + 6a), and different 2n in <i>H. guianensis</i> (2n = 58; 20m + 26sm + 2st + 10a). All species further displayed similar patterns of chromosomal distribution concerning constitutive heterochromatin, 18S ribosomal DNA (rDNA) sites, and most of the surveyed microsatellite motifs. Furthermore, differences in the distribution of 5S rDNA sites and a subset of microsatellite sequences were identified. Heteromorphic sex chromosomes were lacking in <i>H. dissidens</i> and <i>H. guianensis</i> at the scale of our analysis. However, one single chromosome pair in <i>Harttia</i> sp. 3 males presented a remarkable accumulation of male genome-derived probe after CGH, pointing to a tentative region of early sex chromosome differentiation. Thus, our data support already previously outlined evidence that <i>Harttia</i> is a vital model for the investigation of teleost karyotype and sex chromosome dynamics.
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