Patterns of ribosomal gene variation in elite commercial chicken pure line populations

The nucleolus organizer region (NOR) encodes the tandemly repeated 18S, 5.8S and 28S ribosomal (r) RNA genes. The NORs of broiler and layer commercial chicken pure lines were studied to establish the type and extent of genetic variation at this important locus. The parameters studied were gene copy number, repeat size, and diversity of NOR-types. The populations were organized into three groups for analysis including brown-egg broiler (13 lines), brown-egg layer (six lines), and white-egg layer (eight lines). The ribosomal gene copy number average of the white-egg layer populations was significantly lower (329 genes) than that of the brown-egg layers (372 genes); the brown-egg broiler ribosomal gene average was intermediate (350 genes). The white-egg layer populations exhibited a ribosomal repeat unit average size of 36 kb, significantly different from the brown-egg layer and brown-egg broiler average repeat unit size of 32.5 and 33.9 kb, respectively. NOR array size was similar among the three groups (6 mb). The brown-egg broiler populations exhibited polymorphic NOR patterns, intra- and interline, whereas the white-egg layer populations were essentially monomorphic for NOR-type; brown-egg layers exhibited an intermediate level of NOR diversity. Some NOR array characteristics may be a function of breed origin as brown-egg commercial populations, both broilers and layers, have similar breed origins and exhibited similarities for predominant repeat unit size as compared with white-egg layer populations. However, the finding that brown-egg broiler lines typically exhibit a greater number of segregating NOR-types than brown-egg layer lines suggests that the selection schemes of broiler vs. layer pure line populations may also have influenced the degree of variation at this gene complex.