M. bocourti had 2n = 56 as reported in previous study , which is in accordance with the previous study conducted by . Such 2n is also same as for the other species of the Bagridae family namely Coreobagrus ichikawai, Hemibagrus menoda, H. nemurus (from India), M. albolineatus, M. singaringan, Pelteobagrus nudiceps and Sperata acicularis[4, 7]. However, the karyotype of the species under study was composed by 22 m + 22 sm + 12 st/a chromosomes, which differs from the previous study by  that reported the karyotype of M. bocourti consisting of 24 m + 18 sm + 14 st/a chromosomes. This fact suggests that some pericentic inversions have occurred in the karyotype differentiation of this species. In fact, the occurrence of chromosomal rearrangements has been considered a relatively common evolutionary mechanism inside the Bagridae family reviewed .
Karyotype diversification processes in species are subject to multiple factors, whether intrinsic (genomic or chromosomal particularities) or extrinsic (historic contingencies). Among these, restricted gene flow between populations is an important factor for fixation of karyotype changes. For example, after the occurrence of an inversion, it can be lost in the polymorphic state or, under the proper conditions, spread in the population until it is fixed. Inversions maintain areas of imbalance between alleles in loci within or influenced by these rearrangements, leading to an adaptive condition, primarily along environmental gradients. This could occur, particularly in relation to possible historical expansion and adaptation to new environments for a review see .
Physical chromosome mapping of repetitive sequences
The in situ investigation of 9 classes of repetitive DNA sequences resulted in useful characteristics for comparative genomics at the chromosomal level, providing new insights into heterochromatin composition of the species M. bocourti. In fact, all of the used probes generated evident signals on both euchromatic and heterochromatic chromosomal regions, although they were preferentially located in the latter.
Ribosomal RNA genes are among the most mapped sequences in fish chromosomes. Accordingly, they can be excellent genetic markers for the comparative genomic studies, evolutionary studies as well as the genetic identification of fish species . In higher eukaryotes, the ribosomal RNA genes (rDNA) comprise two repetitive DNA families, the 45S and the 5S rDNA. The 45S rDNA is formed by tandemly repeated units composed by three transcribed regions, the 18S, 5.8S, and 28S rRNA generating regions separated by internal transcribed spacers (ITS 1 and ITS 2) and by non transcribed spacer (NTS) sequences. On the other hand, the 5S rDNA is formed by tandemly repeated units of a coding region for the 5S rRNA and a non transcribed spacer (NTS) [10, 11]. In M. bocourti, both the 5S and 18S rDNA genes mapped at the telomeric position of distinct sm chromosomal pairs, not occupying a syntenic position. This feature seems to be the most common situation revealed in fishes, and such pattern is also more common in vertebrates . Since the major and 5S rDNA families are transcribed by different RNA polymerases, these functional differences may require different physical locations for these genes .
Another multigene family is represented by the U2 snRNA, which is one of the components of the small nuclear ribonucleoprotein particles (snRNP) and responsible for mRNA splicing . The U2 snRNA was also located in the short arm of a sm chromosomal pair and not syntenic with the 5S or 18S rDNAs. This result is quite similar to the one found for the fishes Halobatrachus didactylus, Batrachoides manglae and Thalassophryne maculosa, but it differs from Amphichthys cryptocentrus and Porichthys plectrodon, in which the U2 snRNA signals are very widely scattered through the genome . In fact, it has been proposed a trend for the U2 snRNA genes to accumulate in a specific chromosome pair over the course of the evolutionary history inside the Batrachoididae family . However, in order to propose any a trend for the U2 snRNA genes accumulation in the Bagridae family, their location should be studied in more members of this family.
Microsatellites, also known as simple sequence repeats, consist of very short motifs (1–6 nucleotides in length) repeated in tandem arrays. Generally, they are located in the heterochromatic regions (telomeres, centromeres and in the sex chromosomes) of fish genomes, where a significant fraction of repetitive DNA is expected to be localized. In M. bocourti the microsatellites (CA)15 and (GA)15 are abundantly distributed in telomeric regions of all chromosomes and such pattern is similar to another catfishes such as, in Imparfinis schubarti, Steindachneridion scripta, and Rineloricaria latirostris; in the zebrafish Danio rerio and in the wolf fish Hoplias malabaricus. However, an intriguing feature exclusive for M. bocourti was the strong accumulation of both microsatellites at the telomeric regions of one specific chromosomal pair, indicating that these microsatellites may be used as chromosomal markers in this fish species.
sequences are present in the telomeres of vertebrate chromosomes, and the study of these sequences provides insight into the chromosomal rearrangements that have occurred during karyotype evolution of distinct organisms [6, 18]. FISH with the telomeric probe (TTAGGG)n revealed hybridization signals on each telomere of all chromosomes and ITS were not observed, which indicates that Robertsonian fusions or chromosomal translocations might be not involved in the karyotypic evolution of M. bocourti.
Transposable elements (TEs) represent another important class of repetitive DNA that is widely studied in the genome of many organisms with the Rex retrotransposon class being the most studied one within fish species reviewed in . These retroelements were characterized for the first time in the genome of the swordtail fish Xiphophorus. The in situ investigation of some retroelements in many species indicated that they are compartmentalized in heterochromatic regions and it can be correlated with their role in the structure and organization of centromeres or with the reduced selective pressure acting on heterochromatic regions, which are poor in gene content . However, in some other fish species, despite a preferential localization to the centromeric region, TEs have a widely scattered distribution over all chromosomes, with intense hybridization signals in some specific regions [21, 22]. Here, the physical mapping of different Rex elements showed that they are generally dispersed throughout the genome both heterochromatic and euchromatic regions in M. bocourti. Rex1 and 3 are quite accumulated in the telomeric region of several chromosomes while Rex6 showed a more dispersed pattern throughout the genome, including heterochromatin and euchromatin regions. Overall, the results indicate that TEs are important structural components of the heterochromatic regions and have played an important role in the evolutionary history of M. bocourti genome. Generally this distribution pattern is non-random and seems to have some relation to specific characteristics of subregions of the host genomes . Importantly, retroelements increase their copy number by retrotransposition and can be substrate for homologous recombination to form various categories of DNA rearrangements including deletions, inversions, translocations, duplications and amplifications . Therefore, it would be interesting to study the chromosomal distribution of Rex1, 3 and 6 in more members of the Bagridae family to explain the mechanism of the evolutionary dynamics of these retrotransposable elements.