Precocenes induce effect of juvenile hormone excess in Locusta migratoria

Abstract

Precocene I and II (PI and PII)¹ and synthetic analogues^2–4 induce symptoms of permanent juvenile hormone (JH) deficiency in certain hemipteroid^1,5,6 and orthopteroid^7–11 insects. Younger nymphs treated with precocenes moult to precocious prothetelic nymph–adult intermediates, termed adultiforms⁸, whereas treatment of older nymphs (too late for obtaining morphogenetic response) or of adults results in lack of oocyte development. The precocenes act as ‘anti-allatins’, causing atrophy of the corpora allata (CA)^8,12–14. Recent findings suggest that the CA specifically metabolize precocenes to cytotoxic 3,4-epoxy-recocenes, possibly due to the epixidases present therein for JH biosynthesis; the accumulating precocene-epoxide seems to poison the glands^15,16. This theory is supported by the claim that active CA are required for the effect of PII in Oncopeltus^6,17. We demonstrate here that 7-ethoxy-PII (‘Precocene III’ = PIII), or PII itself, applied to last instar Locusta nymphs, induce an effect strictly typical of that of JH excess, in spite of eventually exerting the conventional anti-allatin effect. Our findings provide strong circumstantial evidence that supposedly inactive CA of these nymphs are temporarily activated by the precocenes, resulting in an outburst of JH biosynthesis before the glands are inhibited or destroyed.