Cytogenetic data are presented for Astyanax altiparanae populations from three Brazilian hydrographic systems. The chromosomal data obtained in A. altiparanae support the hypothesis of diploid number conservation. However, small differences in the karyotype formula and number of nucleolar organizer regions were observed in these populations. The apparent karyotypical similarity among the studie...

Scaptotrigona xanthotricha has a wide geographic distribution in the Brazilian Atlantic rainforest. One population from southeast and two from northeast Brazil were analyzed and were found to have chromosome polymorphisms. Although the chromosome number 2n = 34 is conserved in this species, karyotypic analysis revealed clear differences between the three populations. Congruent and ubiquitous mu...

In genetic hybrids, nucleolus formation on chromosomes inherited from only one parent is the epigenetic phenomenon, nucleolar dominance. By using Arabidopsis suecica, the allotetraploid hybrid of Arabidopsis thaliana and Arabidopsis arenosa, natural variation in nucleolar dominance was found to occur, providing a unique opportunity to examine homologous nucleolus organizer regions (NORs) in the...

The genus Mesoplodon (Cetacea: Odontoceti: Ziphiidae) is one of the few cetacean genera with the karyotype 2n = 42. The 2n = 42 karyotype of M. europaeus and M. carlhubbsi is largely consistent with the general cetacean karyotype 2n = 44, although other 2n = 42 karyotypes do not exhibit clear homologies with the general cetacean karyotype. Therefore, the chromosomes of Mesoplodon species may be...

The rDNA arrays in Drosophila contain the cis-acting nucleolus organizer regions responsible for forming the nucleolus and the genes for the 28S, 18S, and 5.8S/2S RNA components of the ribosomes and so serve a central role in protein synthesis. Mutations or alterations that affect the nucleolus organizer region have pleiotropic effects on genome regulation and development and may play a role in...

We discuss the evolutionary significance of four aberrant 18S rDNA clones that were obtained from human, chimpanzee, and gorilla DNA libraries. We show that these clones carry representatives of a small 18S rDNA pseudogene family that arose in a common ancestor of these species. Aspects of their structure and phylogenetic distribution suggest that the 18S pseudogenes no longer interact genetica...

The interphase nucleolus in Allhim porrum meristematic cells is characterized by the presence of 1-4 dense fibrillar zones of rather complex organization. Each such zone appears to consist essentially of a convoluted, evacuolated, filamentous structure approximately 1-5 /*m in diameter. At the ultrastructural level, these structures exhibit an intricate array of lacunar spaces each of which is ...

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...

In both pIants and animals the mechanism of ribo-somal R N A synthesis is thought to be the same, yet genetic and biochemical evidence points to a dispersed origin along chromosomal D N A in plants (1, 2, 3) and a strict localization of genes coding for ribosomal R N A in the nucleolar organizer regions of chromosomes in animals (4, 5). A precise means of locahzing ribosomal cistrons has been m...

Typically, Drosophila have nucleolar organizer regions (NORs) confined to the sex chromosomes. Salivary gland cells of hybrids between Drosophila mulleri females and D. arizonensis males exhibit features in nucleolar organizer regulation that differentiate the species on one hand, and which show an interplay between the X and the microchromosome on the other hand.In the hybrid females only the ...