In the chicken, the nucleolus organizer regions, or sites of the genes encoding 18S, 5.8S, and 28S ribosomal RNA (rRNA), map to one pair of microchromosomes that can be identified by silver nitrate cytochemistry. This nucleolar organizer chromosome also contains the major histocompatibility complex. Chickens aneuploid for this chromosome have been identified and reproduced for over seven generations. Crossing two trisomic parents results in the production of viable disomic, trisomic, and tetrasomic progeny, showing two, three, and four nucleoli and nucleolar organizers per cell, respectively. A molecular analysis of rRNA genes was undertaken to establish the gene copy numbers in the aneuploid genotypes, and to determine if elevated numbers of rRNA genes are stably maintained and inherited over multiple generations. Gene copy numbers were determined using hybridization analysis of erythrocyte DNA obtained from individuals comprising a family which segregated disomic, trisomic, and tetrasomic genotypes. The values obtained were 290, 420, and 570 rDNA repeats per cell for disomic, trisomic, and tetrasomic animals, respectively. These results provide molecular confirmation of the two aneuploid states and show that elevated gene copy numbers have been maintained over multiple generations. Fibroblasts derived from disomic and tetrasomic embryos were found to grow at similar rates in culture, and mature rRNA levels in chicken embryo fibroblasts from disomic, trisomic and tetrasomic embryos were also found to have similar levels of mature rRNA. Therefore, despite the increase in rDNA content, the level of rRNA is regulated to diploid amounts in aneuploid fibroblasts.

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