Juvenile hormone controls previtellogenic proliferation of ribosomal RNA in the mosquito fat body.

During the previtellogenic development of mosquito fat body cells, the nucleolus, the organelle responsible for producing ribosomes, enlarges threefold, reaching maximal size between 2 and 3 days after eclosion. The granular component of the nucleolus containing ribosomal precursors increases considerably as well. These signs of nucleolar activation correlate with the synthetic rate and accumulation of poly(A)- RNA (predominantly ribosomal RNA) in the fat body cells. The amount of poly(A)- RNA in fat body cells increases during the first 2 days after eclosion and then declines gradually. The rate of RNA synthesis exhibits similar kinetics, but both the rise and the decline are sharper than for the accumulation of RNA. All the characteristics of nucleolar activation, its enlargement, accumulation of poly(A)- RNA, and the increased rate of RNA synthesis, are blocked by removal of the corpora allata (CA) in newly eclosed adult females but could be restored by either implantation of CA or topical application of juvenile hormone III or its analog, 7-S-methoprene, to allatectomized females. Thus, previtellogenic activation of fat body nucleoli for ribosomal RNA production is controlled by juvenile hormone from the corpora allata.