The replication of the rDNA complement of only one nucleolus organizer region during polytene chromosome formation (replicative dominance) was initially observed in Drosophila melanogaster. Here we demonstrate replicative dominance in Drosophila simulans and D. melanogaster/D. simulans interspecific hybrids. A second nucleolar phenomenon, nucleolar dominance, is observed in the diploid tissue o...

In Xenopus, one of the properties defining Spemann's organizer is its ability to dorsalise the mesoderm. When placed ajacent to prospective lateral/ventral mesoderm (blood, mesenchyme), the organizer causes these cells to adopt a more axial/dorsal fate (muscle). It seems likely that a similar property patterns the primitive streak of higher vertebrate embryos, but this has not yet been demonstr...

A karyotype analysis of the electric eel, Electrophorus electricus (Teleostei, Gymnotiformes), a strongly electric fish from northern South America, is presented. Two female specimens were analyzed, one from the Amazon River and one from the Araguaia River. The specimens had a chromosomal number of 2n = 52 (42M-SM + 10A). C-bands were present in a centromeric and pericentromeric position on par...

The isthmic organizer, which patterns the anterior hindbrain and midbrain, is one of the most studied secondary organizers. In recent years, new insights have been reported on the molecular nature of its morphogenetic activity. Studies in chick, mouse and zebrafish have converged to show that mutually repressive interactions between the homeoproteins encoded by Otx and Gbx genes position this o...

Theories concerning the mode of origin of peripheral nucleoli in amphibian oocytes have been examined and tested. In Triturus cristatus the giant fusing loops of the 3 shortest lampbrush bivalents resemble nucleoli when viewed in phase contrast and may be considered as possible sites of production of nucleoli. Giant fusing loops, however, differ from peripheral nucleoli in certain important res...

I N this paper evidence will be presented that crossing over at the bobbed (bb) locus of Drosophih melanogaster generates a variety of mutant alleles. Some of these alleles are distinguishable from one another and their parental alleles in the degree of their effect as judged by bristle size, abdominal etching and viability. These observations are in harmony with the genetic organization of the...

In this study, the karyotype, C-heterochromatin blocks, and nucleolar organizer regions (NORs) of four specimens of Lepus europaeus from Turkey were examined. The diploid number of chromosomes (2n), the fundamental number of chromosome arms (FN), and the number of autosomal arms (FNa) were determined as 48, 90, and 86, respectively. All of the chromosomes have been determined to be possessed of...

Oral Squamous cell carcinoma (OSCC) accounts for over 90% of oral cancer cases and a small percentage of these cases are thought to develop from potentially malignant disorders. Main etiological factor associated with OSCC is smoking. It’s a well known fact that in chronic smokers nuclear alterations are evident. There is a correlation between nucleolar function, size and cell doubling time in ...

Active nucleolar organizer regions (NORs) are affine to silver (Ag) and therefore visible in silver stained cell specimens. These so called AgNORs are of diagnostic relevance for early cancer diagnosis. Therefore we seek to automatically detect and segment these dark, spot-like regions. This is challenging, since the contrast between the AgNORs and the nucleus background varies heavily from cel...

The present study was undertaken to provide more information on the incidence of satellite nucleoli in developmental stages of the megakaryocytic lineage. Satellite nucleoli representing solitary silver stained nucleolus organizer regions (AgNORs) present in nuclei in addition to other nucleolar types were observed in all stages of megakaryocytic development. However, the incidence of satellite...