Lampbrush chromosomes are giant meiotic chromosomes supporting a widespread transcription of RNA from many thousands of promoters distributed throughout the lengths of all chromosomes (Callan, 1986; Macgregor, 1987). The explorations into lampbrush chromosomes represent very specific and update field in both cell biology and genetics. They are an excellent model for studying many important aspects of vertebrate genome structure and function, although until recently the amphibians were the only group in which the chromosomes of such type were under investigation.
A new object has been introduced into this field of cell biology and animal genetics by Dr. Nancy Hutchison from the Fred Hutchinson CRC of Seattle (Hutchison, 1987) and our research group at the University of Saint-Petersburg (Kropotova, Gaginskaya, 1984; Chelysheva et al., 1990; Solovei et al., 1990, 1992). The techniques and general methodology of amphibian lampbrushology were used for carrying out studies on lampbrush chromosomes from avian growing oocytes.
Due to their size (about 30 times as long as corresponding mitotic chromosomes) and clearly defined morphological differentiation, avian lampbrush chromosomes have been found to present an excellent model for super-high resolution chromosome mapping. This aspect is especially important for animal genetics and poultry science.
General descriptions of lampbrush chromosomes in a few species of birds have been done with a special attention to site-specific chromosome markers' ultrastructure and cytochemistry. We have found avian lampbrushes to have got certain structural peculiarities related to avian genome and karyotype organisation. There are several types of landmark lateral loops, their formation in heterochromatic regions being the characteristic of avian lampbrush chromosomes. A novel type of site-specific structural marker of protein nature has been discovered and examined on the chicken lampbrush chromosome 2. Snurposome-like (Gall, 1991) bodies are found to mark the centromeric regions of lampbrushes in birds other than Galliformes. Other site-specific properties of lampbrush structure, such as lateral loop of normal morphology but different sizes, regions depleted of lateral loops, and structures known as double-loop bridges can serve as useful morphological markers in lampbrush chromosome mapping.
We have constructed detailed cytological maps of all chicken lampbrush macrobivalents including the asymmetric ZW sex bivalent. Chromosome mapping of the Japanese quail and turkey is in progress. These maps provide a basis for physical mapping of genes and gene transcription on avian lampbrush chromosomes by in situ hybridisation technique. They give a good possibility to align molecular and morphological markers with a high resolution. We were able to localize certain conserved repeated sequences of avian genomes in relation to chromosome morphological markers on chicken lampbrush chromosomes.
Cytological marker mapping and physical mapping of genes by in situ hybridisation can be brought into coincidence with genetic maps constructed on the ground of chiasma frequency analysis in the lampbrush chromosomes. We calculated genetic lengths of the macrochromosomes 1-5 and of the ZW pseudoautosomic region in the chicken oocytes and the linkage between the certain landmarks along the lampbrushes. The total genetic length of the domestic fowl genome has been estimated as 2950-3200 cM. The mean genetic distance per 1,000 kb was found in the chicken to be 2.4-2.6 cM that is three fold as higher as in the human genome. The telomeric expansion of the chicken genetic maps have been demonstrated.