NUCLEOLAR SIZE POLYMORPHISMS IN COMMERCIAL LAYER CHICKENS - DETERMINATION OF INCIDENCE, INHERITANCE, AND NUCLEOLAR SIZES

In the chicken, one chromosome pair encodes the ribosomal (r)RNA genes and two nucleoli are formed in interphase cells. The nucleolus is the site of rRNA synthesis and ribosome biogenesis. Nucleolar size polymo rphisms have been detected in research strains of chickens and found t o represent heterozygosity for rRNA gene copy number. The objectives o f the present study were to determine whether nucleolar size polymorph isms exist in commercial chicken flocks and whether such polymorphisms are under genetic control. The occurrence of nucleolar size polymorph isms was studied in three lines of commercial layer chickens (designat ed A, B, and C). Nucleolar size polymorphisms were found in all three lines. However, the lines differed in the proportion of individuals ex hibiting the polymorphic phenotype of two unequal-sized nucleoli (2P). The 2P phenotype, determined in successive years, was found in 10 to 14% of Line A birds, 18 to 23% of Line B birds, and 41 to 63% of Line C birds. The inheritance pattern of the nucleolar size polymorphism an d nucleoli sizes were studied in Line C birds. The 2P phenotype was fo und to be inherited in Mendelian fashion. The large nucleolus was 1.7x and 1.6x that of the smaller nucleolus in 2P males and females, respe ctively, and was larger than the nucleoli of 2E birds (2E = nonpolymor phic phenotype of two equal-sized nucleoli). Total nucleolar size was greater in cells of 2P birds than in cells of 2E birds. These data sug gest that the genetic basis for the large nucleolus in Line C 2P cells is a ''larger than normal'' rDNA cluster.