Chromosomal diversification in populations of Characidium cf. gomesi (Teleostei, Crenuchidae)

Comparative cytogenetic studies carried out in two populations of Characidium cf. gomesi from Botucatu region, SP, Brazil, showed a similar karyotypic structure in a diploid number of 50 chromosomes, 32 metacentric and 18 submetacentric chromosomes for males and 31 metacentric and 19 submetacentric chromosomes for females as well as a ZZ-ZW sex chromosome system. Differences between both populations, however, were found in relation to the occurrence of B chromosomes and the distribution of 18S and 5S ribosomal DNA (rDNA) sites. Characidium cf. gomesi from the Alambari Stream, a component of the Tietê River basin, revealed 18S rDNA on Z and W chromosomes, while this gene was located on autosomes in the sample from the Paranapanema River basin. The 5S rDNA sites were observed in a single chromosomal pair (number 25) in the populations from Paranapanema and in two pairs in the specimens from Tietê (numbers 20 and 25). Besides that, in the sample from Paranapanema, both inter and intra-individual variations were found due to the occurrence of up to four heterochromatic supernumerary chromosomes in the cells. The life mode of this fish, restricted to headwaters and subjected to frequent breakdown into sub-populations, may have contributed to the fixation of such chromosomal differences. The karyotypic similarities found in the analysed populations, however, suggest that all are descended from the same ancestor group whereas their differences indicate that they are already existing in reproductively isolated populations.

Molecular cytogenetic study of heterochromatin in Hisonotus leucofrenatus (Teleostei, Loricariidae, Hypoptopomatinae)

The fish species Hisonotus leucofrenatus exhibits a large amount of C-band positive segments with different responses after application of the C-banding technique. Type I class named herein appeared to be heavily stained after C-banding in the terminal position of five chromosome pairs and type II class, weakly stained after C-banding in the interstitial or pericentromeric position in nine chromosome pairs and in the supernumerary chromosomes. No variation was observed in type II C-band positive segments, however, type I segments displayed conspicuous polymorphisms, and six cytotypes were detected among the fish analyzed. Chromosomes were also analyzed by CMA(3) and DAPI staining, which showed that type I C-band positive segments comprised both AT-rich and GC-rich DNA, while type II segments were mainly composed of GC-rich sequences. HindIII-digested genomic DNA exhibits fragments of the ladder-like pattern, characteristic of tandemly arrayed repetitive sequences. Two of those fragments corresponding to monomeric and dimeric units of a 78 bp repetitive DNA sequence were cloned and sequenced. The cloned repetitive DNA was used as probe in fluorescent in situ hybridization experiments. The results revealed that these sequences were located in the same position as the type I C-band positive segments. This satellite DNA did not hybridize with DNA from other species of Hisonotus or from other fish of the family Loricariidae, suggesting that this sequence is specific to H. leucofrenatus. The role of these repetitive sequences in the karyotypic evolution of this species is discussed.

Increased specificity of colloidal silver staining by means of chemical attenuation

A modification of the silver staining procedure of Howell and Black (1980) is reported which makes use of teleostean gelatin as protective colloid and renders a high signal-to-noise ratio. We demonstrate that this ratio can be further increased by subsequent attenuation with a chemical reducer consisting of a mixture of potassium ferricyanide (III) and sodium thiosulphate. It is shown that slight changes of the concentration of the reactive compounds of the chemical reducer make the protocol applicable to human, plant (Aliium cepa, Rhinanthus minor) as well as meiotic insect (Acheta domesticus) chromosome preparations. Due to its broad applicability, the method could find utilization in studies on chromatin and chromosome functions in many species.

The molecular phylogeny of the Sparidae (Pisces, Perciformes) based on two satellite DNA families

In this study, the phylogenetic relationships and which the taxonomic status of the species belonging to the Sparidae family (Pisces: Perciformes) are analysed and revised. This study includes species of this family that are distributed by the North-eastern Atlantic and Mediterranean coasts, is based on the analysis of two satellite DNA families. While one satellite DNA, the centromeric EcoRI family, extends to all the species analysed, the other, the subtelomeric DraI family, is restricted to only six of the 16 species studied. Based on phylogenetic use of these two markers, we conclude that the Sparidae family is composed by two major lineages: one comprising the species of the genera Sparus, Diplodus, Lithognathus, Boops, Sarpa and Spondyliosoma, and one species of Pagellus (P. bogaraveo); and the other lineage is comprised of the species of Pagrus and Dentex, and one species of Pagellus (P. erythrinus). This classification is consistent across the two markers used and clearly contradicts previous morphological phylogenies based mainly on dentition. In addition, the current status and the phylogenetic position of some of the species analysed (i.e. species of Pagrus, Dentex and Pagellus) are not supported by our analyses. Finally, we discuss the value of the morphological characters used until now for the classification of this group of fish.