Cytogenetic studies were done on three Pomacentridae species of the genus Chromis. The karyotype of C. multilineata consisted of 48 acrocentric chromosomes (FN = 48), C. insolata had 2n = 46-47 (3-4M+6SM+36-38A; FN = 56) and C. flavicauda had 2n = 39 (9M+6SM+24A; FN = 54). Robertsonian polymorphisms were detected in C. insolata and C. flavicauda. All three species had small heterochromatic blocks restricted to centromeric regions. Nucleolar organizer regions (NORs) were detected in the telomeric position of a medium acrocentric chromosome pair in C. multilineata and in non-homologous chromosomes in both C. flavicauda and C. insolata. FISH with a telomeric probe detected no internal telomeric sequences in C. flavicauda and C. insolata. 5S rRNA genes were observed in a pericentromeric region of two large metacentric chromosome pairs in C. flavicauda and two large acrocentric pairs in C. insolata. The karyotype structure and the number and location of the 5S rDNA loci in these two species indicated that the 5S rRNA-bearing acrocentric chromosomes were directly involved in the origin of the polymorphisms observed. These data reinforce the idea that Robertsonian rearrangements have been involved in molding the karyotype in the subfamily Chrominae

Despite substantial progress, there are still several gaps in our knowledge about the process of sex chromosome differentiation. The degeneration of sex-specific chromosome in some species is well documented, but it is not clear if all species follow the same evolutionary pathway. The accumulation of repetitive DNA sequences, however, is a common feature. To better understand this involvement, fish species emerge as excellent models because they exhibit a wide variety of sex chromosome and sex determining systems. Besides, they have much younger sex chromosomes compared to higher vertebrates, making it possible to follow early steps of differentiation. Here, we analyzed the arrangement of 9 repetitive DNA sequences in the W chromosomes of 2 fish species, namely Leporinus reinhardti and Triportheus auritus, which present well-differentiated ZZ/ZW sex system, but differ in respect to the size of the sex-specific chromosome. Both W chromosomes are almost fully heterochromatic, with accumulation of repeated DNAs in their heterochromatic regions. We found that microsatellites have strongly accumulated on the large W chromosome of L. reinhardti but not on the reduced-size W chromosome of T. auritus and are therefore important players of the W chromosome expansion. The present data highlight that the evolution of the sex chromosomes can diverge even in the same type of sex system, with and without the degeneration of the specific-sex chromosome, being more dynamic than traditionally appreciated.

The chromosomes of two neotropical freshwater fish species, namely Liposarcus anisitsi (Siluriformes, Loricariidae) and Leporinus elongatus (Characiformes, Anostomidae), were investigated by means of C-banding, Ag-NORs, fluorochrome staining and banding by hot saline solution (HSS) treatment, to reveal patterns of heterochromatin differentiation. The karyotype of L. anisitsi is described for the first time. Staining with the GC-specific fluorescent antibiotic mithramycin (MM) revealed bright signals in some C-banded blocks in both species, suggesting that these MM+ heterochromatin contains GC-rich DNA. Banding by denaturation employing HSS, followed by Giemsa staining, yielded corresponding results documenting the thermal stability of GC-rich DNA part of heterochromatin positive after C-banding. In L. elongatus the Ag-NOR also followed the above banding patterns. However, in L. anisitsi the Ag-NOR was MM+ but negatively stained after C-banding and HSS treatment. L. elongatus also showed C-banded segments that were negative for mithramycin staining and HSS treatment. The results obtained evidence the heterochromatin heterogeneity in these fish species.

The family Serranidae (Perciformes) represents a group of species widely distributed in tropical and subtropical waters, most of them presenting economical value. Some species are associated to coral reefs and lack genetic information. Cytogenetical analyses were carried out on three Serranidae species from Northeastern Brazilian coast, Epinephelus adscencionis, Alphestes afer and Servatius flaviventris. All of them presented a karyotype typical of Perciformes, composed by 48 acrocentric chromosomes (2n=48, FN=48). Active NORs were located at interstitial position on 24^th pair in E. adscencionis and A. afer and on 16^th pair in S. flaviventris. Besides the major NOR-bearing pair, extra ribosomal sites were also detected at telomeric position on long arms of the 2^nd chromosomal pair of E. adscencionis. Heterochromatic blocks were restricted to centromeric or pericentromeric regions in all chromosomes, without any differences among the three species. The cytogenetical evidences support the monophyletism proposed for the subfamily Epinephilinae. It is possible that the karyotype structure associated to peculiar behavioral and biological features could be restraining chromosomal changes. These factors, not acting over the morphological diversification, would contribute for the observed asynchrony between karyotype (endophenotype) and morphology (exophenotype) of Serranidae species.

The Lutjanidae or snappers are a family of perciform fishes, mainly marine but with some members living in estuaries and entering fresh water to feed. Some are important food fish. Cytogenetic data for Lutjanidae are scarce. In the present work, we cytogenetically characterized through conventional Giemsa staining techniques, Ag-NOR and C-banding the species Ocyurus chrysurus, Lutjanus analis, L. alexandrei, L. cyanopterus, L. jocu and L. synagris, all found along the Brazilian coast. Karyotype analysis of all six species showed a modal value of 2n = 48 acrocentric chromosomes. Single NORs were found at pericentromeric position on the long arms of the 2nd pair in O. chrysurus, L. alexandrei and L. cyanopterus, on the 5th pair in L. analis and on the 23rd pair in L. synagris. The species L. jocu presented multiple NORs located on the 2nd pair at a pericentromeric region and on the 5th pair at a telomeric region. Heterochromatic blocks were identified at the centromeric region of all chromosomes of the studied species. These results indicate that, despite of the chromosomal stability of this family, a relative structural diversification seems to have occurred in the chromosome evolution of the group. Such diversification was evidenced by divergent number and location of ribosomal sites among species. The NOR-bearing pairs represented an efficient cytotaxonomic marker for most of the analyzed species. The data suggest that the presence of interstitially located single NORs on a large acrocentric pair should represent a basal condition for lutjanids