ABSTRACT
Chromosome complements in avian species include both constant and variable features that must be considered in studies to map genes, identify sites of integration of foreign DNA, and track donor cells to tissue sites in chimeras. Detailed cytogenetic studies in the domestic chicken are restricted primarily to the first 10 chromosome pairs, each of which is morphologically distinct. High-resolution banding permits analyses of yet smaller chromosomes (e.g., chromosomes 1 – 15). The primary constitutive features of the avian genome have been revealed by G banding for structural landmarks, C banding for constitutive heterochromatin, RBG banding for DNA replication patterns, and in situ hybridization for localizing highly repeated sequences to centromeric and telomeric regions. Variable features of the chromosome complement and of gene expression in selected repeated gene families provide convenient metaphase and interphase phenotypes for studies in vitro and in vivo. A significant amount of genomic variability has been observed among early chick embryos (1 – 15% aberration frequency among genetic lines), including haploidy, triploidy, trisomy, and mosaicism. The inadvertent selection of an aberrant embryo as a source of cells for chimeric production could yield surprising or confusing results if cells are not karyotyped. Variability at the ribosomal RNA gene cluster generates polymorphic nucleolar (PNU) patterns in interphase cells. Polymorphic cells have one macro- and one micronucleolus per cell (Pp). This phenotype is easily diagnosed in cytological preparations. We have developed chicken genetic strains with defined PNU patterns. A particular nucleolar variant is expressed in the embryo and at all other stages including adult. We have detected the PNU phenotype in stage X embryos, in embryonic tissues representing ectoderm, mesoderm, and endoderm, and in feather pulp cells from chickens. Heterozygous PNU cells should be easy to track to tissue sites when transferred to nonpolymorphic recipient embryos. While development of homozygous PNU embryos (pp) is arrested at early primitive streak formation, heterozygotes (Pp) develop normally. Studies with rDNA variants should be useful for investigating the relative contributions of maternal ribosomes versus embryo-derived rRNA in early developmental processes.
