Production and utilization of farnesoic acid in the juvenile hormone biosynthetic pathway by corpora allata of larval Diploptera punctata.

The regulation of juvenile hormone III (JH III) biosynthesis and release by the corpora allata (CA) was studied in penultimate and final instar male and female larvae of Diploptera punctata using an in vitro radiochemical assay. Final instar female CA produced JH III (2 to 6 pmol.hr-1 per pair) only during the first 8 days of the 20-day stadium, but produced and released farnesoic acid (FA), a JH III precursor, (0.5 to 2.0 pmol.hr-1 per pair), during the entire stadium. FA was identified by cochromatography on TLC of both the product from CA incubations with [3H]FA standard as well as by esterification of biosynthesized FA to methyl farnesoate (MF). Biosynthesis of MF and JH III acid occurred at less than 0.25 and less than 2 pmol.hr-1 per pair, respectively. Because regression analysis of the time course of JH III release over 6 hr showed a constant rate of release after a lag period lasting up to 1.7 hr, rates of JH release of untreated CA (spontaneous) and FA-treated CA (stimulated) were determined by 2-hr sequential incubations of CA, following a 2-hr preincubation. FA-treated rates of JH III release were higher than spontaneous rates. In penultimate instars, the decline in the spontaneous rates during the latter half of the stadium was accompanied by a reduced degree of FA stimulation. In final instars, as the spontaneous rates declined between Days 2 and 6, the magnitude of the stimulation of JH III biosynthesis by FA increased, whereas in the second half of the stadium, the degree of stimulation was very low. Addition of exogenous FA to CA from late final instars increased FA production significantly more than JH III production. This apparent stimulation of FA biosynthesis is evident in CA with both high and low spontaneous rates of JH release and suggests that rate limitation of JH III biosynthesis in the final stadium occurs in part at the level of the o-methyl transferase which converts FA into MF.