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Fishes represent a useful model for evolutionary studies, given their diversity of species and habitats. In this study, we investigate the chromosomes of three unexplored Harttia fish species from the Amazonian region and compare the obtained data with previous analyses. Our data reveal that both

Harttia dissidens

and Harttia sp. 3 exhibit the same number of chromosomes in their cells (54), but that they differ in the karyotype organization.

Harttia guianensis

possesses 58 chromosomes, being thus the first representative from north Brazil to present this feature for both sexes. Although otherwise rather common in Harttia species, we observed no chromosomal differences between sexes in all but one species. Namely in Harttia sp. 3, we revealed signs of initial differentiation between homologues of one chromosome pair in males but not in females. Altogether, our data bring new evidence strengthening the view that Harttia spp. represent an informative model for studying patterns of karyotype and sex chromosome dynamics in teleost fishes.

Abstract
A remarkable morphological diversity and karyotype variability can be observed in the Neotropical armored catfish genus

Harttia

. 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 H. dissidens (20m + 26sm + 8a) and Harttia sp. 3 (16m + 18sm + 14st + 6a), and different 2n in H. guianensis (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 H. dissidens and H. guianensis at the scale of our analysis. However, one single chromosome pair in Harttia 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 Harttia is a vital model for the investigation of teleost karyotype and sex chromosome dynamics.