Abstract:
In Locusta migratoria, vitellogenin (Vg) synthesis is normally induced by juvenile hormone (JH) only in adult females. Larval and adult female and male locusts were tested for inducibility of Vg synthesis by the synthetic JH analog, methoprene. While fourth and fifth larval stages of both sexes could be induced to synthesize Vg, adult males could not. Quant i tati ve assays showed the relative response in the order: adult female > fifth instar female > fifth instar male. Dur i ng the fif t h stadium, maximal vitel logenic response was obt ained in mid-instar; in the female, t his was preceded by stimulated DNA synthesis. Experiments involving female-male fat body tranplantations and precociously induced adults indicated that sex- limited gene programming is determined in the fat body cells early in development and is partially expressed by the fifth instar but fully manifest only af t er the metamorphic molt. Fat bodies from adult females were cultured for up to 72 hours in synthetic medium without loss of viability. With high levels of methoprene in vitro, successful secondary and a very weak, variable primary stimulation of Vg synthesis was obtained. otal RNA was prepared from fat bodies in different states and the content of mRNAVg assayed by dot hybridization with a cloned Vg gene probe. During primary stimulation of adult and fifth instar females, mRNAvg was first detectable after 24 hours, then increased slowly for about 12 hours, and then much more rapidly. In secondary stimulation of allatectomized normal vitellogenic females, the lag was reduced to about 12 hours and initial mRN~g accumulation was more rapid. By 48 hours, the mRNg level was only about 50% higher in secondary than in primary stimulation, but the rates of Vg synthesis, assayed in the same fat bodies , was about 3-fold higher. This suggests increased translational efficiency in the secondary response, possibly due to availability of ribosomes. After withdrawal of JH by allatectomy of vitellogenic females, mRNg decayed with a half-life of about 24 hours, but the ini t ial decline in Vg synthesis was much more rapid. These results suggest that JR-dependent Vg synthesis is regulated at the translational as well as the transcriptional level.