Most species of the genus Harttia inhabits the headwaters of small tributaries, but some species are restricted to the main channel of some rivers. This feature, combined with limited dispersal ability, leads to the formation of small isolated populations with reduced gene flow. Currently, there are 23 taxonomically defined and recognized species, and 17 of these are found in Brazil, distributed in several hydrographic basins. Despite this diversity, few chromosomal data for the species belonging to this genus are found in the literature. Thus, this study analyzed, by classical and molecular cytogenetics methodologies, the chromosomal diversity of this genus, to discuss the processes that are involved in the evolution and karyotype differentiation of the species of the group. Seven species of Harttia were analyzed: H. kronei, H. longipinna, H. gracilis, H. punctata, H. loricariformis, H. torrenticola, and H. carvalhoi. The chromosomal diversity found in these species includes different diploid and fundamental numbers, distinct distribution of several repetitive sequences, the presence of supernumerary chromosomes in H. longipinna and multiple sex chromosome systems of the type XX/XYY in H. carvalhoi and XXXX/XXY in H. punctata. Lastly, our data highlight the genus Harttia as an excellent model for evolutionary studies.

ABSTRACT. We examined chromosomes of three species of the genus Hypostomus, in order to contribute to the understanding of the karyotype evolution of this group. Specimens of H. ancistroides and H. nigromaculatus displayed differences in karyotype formulas, distribution and location of heterochromatin and nucleolus organizer regions when compared to other populations of the same species. We made the first cytogenetic characterization of H. tapijara, an endemic species in the Ribeira de Iguape River. These specimens had 2n = 66 chromosomes, while H. ancistroides showed 2n = 68 and H. nigromaculatus 2n = 76 chromosomes. Physical mapping of 18S and 5S rDNA sites of the three species showed simple, multiple and syntenic clusters. Synteny of ribosomal sites was found in H. ancistroides and H. tapijara, and an interspersed pattern between these sites in all chromosomes bearing the synteny was observed. We conclude that the genus Hypostomus has a high chromosome complexity that is accompanied by great morphological variation. It is evident that this group comprises an interesting model for understanding the chromosome evolution of Neotropical ichthyofauna.

In this study, which is the first karyotype analysis of Hypostomus iheringii, nine specimens collected in Córrego da Lapa (tributary of the Passa-Cinco River) showed a diploid number of 80 chromosomes. Silver nitrate staining and fluorescence in situ hybridization (FISH) with an 18S rDNA probe revealed the presence of multiple nucleolus organizer regions (NORs) (chromosome pairs 13, 20, and 34). FISH with a 5S rDNA probe showed that this cistron was only present in chromosome pair 2. When the karyotypes of individual animals were compared, unique heterochromatic polymorphisms were detected on chromosome pairs 1 and 5. Specifically, specimens had heterochromatic blocks (h+h+) on both chromosomes, one chromosome with heterochromatic blocks (h+h-) or chromosomes that lacked heterochromatic blocks (h-h-). Considering that heteromorphic pattern is not correlated with variation in size, the process of heterochromatinization might act on the long arms of these chromosomes. In summary, all chromosomal markers indicate that the karyotype of Hypostomus iheringii is highly differentiated and that the heterochromatinization of chromosomal segments may have contributed to its karyotypic differentiation.

Ageneiosus is the most widely distributed genus of the family Auchenipteridae among South American river basins. Although chromosome studies in the family are scarce, this genus has the largest number of analyzed species, with 2n = 54 to 56 chromosomes, differing from the rest of the family (2n = 58). This study aimed to analyze Ageneiosus inermis from the Araguaia River basin. The diploid number found was of 56 chromosomes. Heterochromatin was allocated in terminal region of most chromosomes, plus a pericentromeric heterochromatic block in pair 1, a pair distinguished by size in relation to other chromosomes pairs. AgNORs were detected in only one submetacentric chromosome pair, which was confirmed by FISH. 5S rDNA was present in only one metacentric chromosome pair. Hybridization with [TTAGGG] n sequence marked the telomeres of all chromosomes, in addition to an ITS in the proximal region of the short arm of pair 1. The repetitive [GATA] n sequence was dispersed, with preferential location in terminal region of the chromosomes. Ageneiosus has a genomic organization somewhat different when compared to other Auchenipteridae species. Evidences indicate that a chromosomal fusion originated the first metacentric chromosome pair in A. inermis, rearrangement which may be a basal event for the genus.Ageneiosus é o gênero da família Auchenipteridae mais amplamente distribuído em bacias da América do Sul. Apesar dos estudos cromossômicos nesta família serem escassos, este gênero tem o maior número de espécies analisadas, com número diploide variando de 54 a 56 cromossomos, o que difere do restante da família (2n = 58). Este estudo objetivou analisar Ageneiosus inermis da bacia do rio Araguaia. O número diploide encontrado foi de 56 cromossomos. A heterocromatina se mostrou localizada na região terminal da maioria dos cromossomos, além de um bloco heterocromático pericentromérico no par 1, um par facilmente distinguível no cariótipo pelo seu maior tamanho quando comparado aos outros pares do complemento. AgRONs foram detectadas em somente um par de cromossomos submetacêntricos, que foi confirmado pela FISH. 5S rDNA se mostrou presente em somente um par de cromossomos metacêntricos. A hibridização com a sequência [TTAGGG] n marcou os telômeros de todos os cromossomos, além de um ITS (sequência telomérica intersticial) na região proximal do braço curto do par 1. A sequência repetitiva [GATA] n se mostrou dispersa, com localização preferencial na região terminal dos cromossomos. Ageneiosus apresenta uma organização genômica um pouco diferente quando comparada a outras espécies de Auchenipteridae. As evidências indicam que uma fusão cromossômica originou o primeiro par de cromossomos metacêntricos de A. inermis, rearranjo que parece ser um evento basal para o gênero.

Among the more than 30 families of the order Siluriformes, Heptapteridae is composed of 189 species distributed into 24 genera. Rhamdia, which has wide distribution throughout the Neotropical region, presents only 11 valid species, with 8 being in the Brazilian territory. Rhamdia quelen is the only species considered as widely distributed in almost all Brazilian hydrographic basins. It is a Neotropical fi sh species with a confusing taxonomical history. Classic and molecular cytogenetics data for three populations of this species from two large Brazilian hydrographic basins (Paraná and Araguaia) are presented here. The diploid number found for the three analyzed populations was 58 chromosomes, but with distinct karyotypic formulae. The presence of B chromosomes was detected in the two Araguaia River populations with intra-and interindividual variation. C-banding evidenced little heterochromatin in the three analyzed populations. FISH with 18S rDNA probes evidenced a single chromosome pair bearing this site, confi rming the presence of simple NORs, as visualized through silver nitrate staining. The site of 5S rDNA was observed in only one pair of chromosomes, but differing in the marked pair and their location. Based in the differences of the karyotypic formulae and rDNA 5S found between populations on this study and many others available in the literature, it is suggested that this group represents a species complex, and that a new and detailed taxonomical review is necessary.