No abstract

B chromosomes are additional genetic elements to the standard complement. They display distinctive features and have been found in 15% of eukaryote species. In this study, we analyzed 4 populations of Crenicichla lepidota from hydrographic system of Laguna dos Patos/RS (Brazil). All specimens showed 2n = 48 with 6m + 42st - a, FN = 54, with a secondary constriction on the first pair of the complement. Among the 18 samples analyzed, 6 individuals belonging to the Gasômetro and Saco da Alemoa populations presented 1-3 small-sized heterochromatic B chromosomes, with intra- and interindividual variation. Simple AgNORs coincident with 18S rDNA and CMA3 positive/DAPI negative sites were present in all populations. The extra chromosomes did not exhibit any 18S rDNA sites. The meiotic analyses showed heteropycnotic regions in leptotene and zygotene stages, which may be related to the presence of B chromosomes. During pachytene were found 24 bivalents and 1 spatially separated, as well as during metaphases I and diplotene, indicating that there is no association between B chromosomes and those of the A complement. During diakinesis, an unusual meiotic configuration was observed, revealing a proximity between the bivalent and chromosome B (univalent), that might be the result of a heterochromatin affinity between these chromosomes. In anaphase I, late migration of B chromosomes was detected. The low frequency of B chromosomes in the Cichlidae family and in Crenicichla suggests its recent origin in this group and may be ascribable to animal exposure to deleterious effects under certain environmental conditions. Moreover, this is the first report in C. lepidota.

ABSTRACT. Analyses conducted using repetitive DNAs have contributed to better understanding the chromosome structure and evolution of several species of insects. There are few data on the organization, localization, and evolutionary behavior of repetitive DNA in the family Lygaeidae, especially in Brazilian species. To elucidate the physical mapping and evolutionary events that involve these sequences, we cytogenetically analyzed three species of Lygaeidae and found 2n (♂) = 18 (16 + XY) for Oncopeltus femoralis; 2n (♂) = 14 (12 + XY) for Ochrimnus sagax; and 2n (♂) = 12 (10 + XY) for Lygaeus peruvianus. Each species showed different quantities of heterochromatin, which also showed variation in their molecular composition by fluorochrome Physical mapping in Lygaeidae staining. Amplification of the 18S rDNA generated a fragment of approximately 787 bp. The alignment of the consensus sequence with sequences from other species of Heteroptera deposited in the GenBank revealed a similarity of 98% with small differences. Fluorescent in situ hybridization with the 18S rDNA fragment revealed that this ribosomal gene was located in 1 autosomal pair at different positions in the three species. No cytogenetic data are available for these Brazilian species. The basal number and the possible chromosomal changes that occurred among the different species, as well as the evolution of these DNA sequences, are discussed.

B chromosomes are additional elements to standard karyotypes observed in different species of fishes, especially in Curimatidae. However, despite studies demonstrating the occurrence of Bs, little is known about their origin and evolution. To better understand the genomic composition and evolutionary processes involving B chromosomes, microdissection of B microchromosomes in Steindachnerina insculpta was conducted. Chromosome painting revealed the totally hybridized B and markings on A chromosomes both in S. in sculpta and in Cyphocharax spilotus, demonstrating a strong homology between these different species. In specimens of C. modestus, which do not have Bs, the signals were observed on A chromosomes. Cloning and sequencing of some B fragments revealed that the B microchromosome in S. insculpta is composed of repetitive elements, homologous to the DIRS-4 LTR retrotransposon of Xenopus (Silurana) tropicalis. FISH with clone pSi48 with the DIRS-4 retroelement revealed signals on all A chromosomes in the 2 species and also on the B, suggesting the insertion of repetitive elements in these species.

In the present work, six curimatid species were analyzed: Cyphocharax voga (Hensel, 1870), Cyphocharax spilotus (Vari, 1987), Cyphocharax saladensis (Meinken, 1933), Cyphocharax modestus (Fernández-Yépez, 1948), Steindachnerina biornata (Braga & Azpelicueta, 1987) and Steindachnerina insculpta (Fernández-Yépez, 1948) collected from two hydrographic basins. All samples presented 2n=54 meta-submetacentric (m-sm) chromosomes and FN equal to 108, and 1 or 2 B microchromosomes in the mitotic and meiotic cells of the six sampled populations showing inter-and intraindividual variation. The analysis of the meiotic cells in Cyphocharax saladensis, Cyphocharax spilotus, and Cyphocharax voga showed a modal number of 54 chromosomes in the spermatogonial metaphases and 27 bivalents in the pachytene, diplotene, diakinesis and in metaphase I stages, and 27 chromosomes in metaphase II; in Cyphocharax modestus, Steindachnerina biornata, and Steindachnerina insculpta, spermatogonial metaphases with 54 chromosomes and pachytene and metaphase I with 27 bivalents were observed. The B microchromosome was observed as univalent in the spermatogonial metaphase of Cyphocharax spilotus, in the pachytene stage in the other species, with the exception of Cyphocharax saladensis, and Steindachnerina biornata in metaphase I. New occurrences of the B microchromosome in Cyphocharax voga, Cyphocharax saladensis and Steindachnerina biornata were observed, confirming that the presence of this type of chromosome is a striking characteristic of this group of fish.

No abstract

The family Curimatidae is a fish group usually considered chromosomally conserved in their diploid number. However, some studies show small changes in the karyotype microstructure, and the presence of B chromosomes, indicating a chromosomal diversification within the group, even if structural changes in the karyotypes are not visible. Few studies associate this trait with an evolutionary pattern within the family. This study aimed to characterize the karyotype, (NORs)nucleolus organizer regions , and heterochromatin distribution of six species of Curimatidae of the genera Cyphocharax Fowler, 1906 and Steindachnerina Fowler, 1906: Cyphocharax voga (Hensel, 1870), Cyphocharax spilotus (Vari, 1987), Cyphocharax saladensis (Meinken, 1933), Cyphocharax modestus (Fernández-Yépez, 1948), Steindachnerina biornata (Braga et Azpelicueta, 1987) and Steindachnerina insculpta (Fernández-Yépez, 1948) and contribute data to a better understanding of the mechanisms involved in the chromosomal evolution of this group of fish. All specimens had 2n=54, m-sm, and B microchromosomes. Five species exhibited single NORs, except for Steindachnerina biornata, which showed a multiple pattern of ribosomal sites. NORs were chromomycin A3 positive (CMA3+) and 4’-6-diamino-2-phenylindole (DAPI-) negative, exhibiting differences in the pair and chromosomal location of each individual of the species. FISH with 5S rDNA probe revealed sites in the pericentrometic position of a pair of chromosomes of five species. However, another site was detected on a metacentric chromosome of Cyphocharax spilotus. Heterochromatin distributed both in the pericentromeric and some terminal regions was revealed to be CMA3+/DAPI-. These data associated with the previously existing ones confirm that, although Curimatidae have a very conservative karyotype macrostructure, NORs and heterochromatin variability are caused by mechanisms of chromosome alterations, such as translocations and/or inversions, leading to the evolution and diversification of this group of fish.