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 work, 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.
