Teratogenic development in chicken embryos associated with a major deletion in the rRNA gene cluster

A new strain of chickens (mPNU) that segregates a severely deleted rDNA cluster was studied. Individuals heterozygous (+/p2 ) and homozygous (p2 /p2 ) for the deletion were found to have 56 and 27%, respectively, of the normal complement of rRNA genes (290 copies/cell). Morphogenesis, cellular rRNA levels, and nucleolar sizes, were investigated and compared in normal +/+, +/p2 , and p2 /p2 embryos. Cellular rRNA contents were similar among the three genotypes at stage X, but subsequently during gastrulation, p2 /p2 levels were reduced to 56% and eventually to 43% of +/+. Viability and morphogenesis were normal in p2 /p2 embryos until the initial primitive streak stage of gastrulation. However, further development was abnormal and characterized by disrupted axis formation. In +/+ and +/p2 embryos, rRNA levels and nucleolar sizes increased during early development; however, the profile of these increases differed temporally and quantitatively between the genotypes. The +/p2 embryos, at the full streak stage of gastrulation, exhibited reduced rRNA levels and nucleolar sizes (80% of +/+), yet the +/p2 embryos developed normally. These studies establish a minimum copy number requirement lower than previously demonstrated, that is, a rDNA genotype with only 56% of the normal gene complement (∼160 genes) is compatible with early embryonic viability. Also, a rRNA threshold was detected: rRNA levels that were 56% of +/+ failed to support normal gastrulation; however, even under the circumstance of reduced rRNA levels (43% of control), some aspects of gastrulation apparently continue (cell migration and invagination). The teratogenic development of p2 /p2 embryos is a biological consequence unique from that found in other metazoan models of rDNA-deficiency, and will be useful as a model to investigate mechanisms of vertebrate gastrulation and axis formation.