Nucleolus size varies with sex, ploidy and gene dosage in insects

The nucleolus constitutes a cytologically visible phenotype for ribosomal DNA (rDNA). Nucleolar size, as determined by silver staining, is a good indicator of cell proliferation rate and biosynthetic activity. Nevertheless, the relationship between rDNA content and sexual dimorphism for nucleolar size is not well documented. In the present study, the impact of sex and ploidy level on nucleolar size is investigated in three haplo/diploid and three diplo/diploid species of insect. Nucleolar sizes are found to be proportional to ploidy level in the haplo/diploid hymenopterans Trypoxylon albitarse and Nasonia vitripennis. Conversely, in the ant Messor barbarus, nucleolar sizes are larger in haploid males (winged) than diploid females (apterous). Among the diplo/diploid species, evidence for gene dosage compensation on nucleolar activity is suggested by the absence of sex differences in Drosophila simulans, a species in which rDNA is limited to the X chromosome. By contrast, in the grasshopper Stenobothrus festivus, another species with rRNA genes restricted to the X chromosome, the size of the nucleolus is significantly larger in females than in males. Additionally, in the grasshopper Chorthippus parallelus, where rDNA is distributed evenly on several autosomes of males and females, the females also show larger nucleoli than males. In both grasshopper species, the magnitude of the female/male ratio for nucleolus area is very similar to the body size ratio, suggesting that body size, as well as sex, ploidy, gene dosage and physiological activity, may be an important determinant of nucleolus area.