ECDYSTEROID BIOSYNTHESIS IN LARVAL TESTES OF SPODOPTERA-LITTORALIS

The ability of testes of the last larval instar of the cotton leafworm, Spodoptera littoralis to synthesise ecdysteroids was investigated. The endogenous ecdysteroid titres of both the testes and the haemolymph differed in their composition and times of peak concentration during this instar. At two developmental stages, one at the start of the rising titre concentration and one in the middle of the rising concentration (65 and 89h into the instar, respectively; the final instar lasts 5 days under our conditions), the ratios of 20-hydroxyecdysone to ecdysone were similar (1.8.1) in the testes. However, at the same stages, the ratios in the haemolymph were 5:1 (65h) and 11:1 (89h). Incubation in vitro of [H-3]2-deoxyecdysone or [H-3]2,22,25-trideoxyecdysone with various larval tissues (prothoracic glands, testes, gut, carcass, fat body, Malpighian tubules) from last instar larvae (65 and 89h into the instar for [H-3]2-deoxyecdysone, 57h for [H-3]2,22,25-trideoxyecdysone) showed that the testes were capable of the greatest rates of conversion of the precursors into ecdysone and 20-hydroxyecdysone. Further investigation of the conversion of [H-3]2,22,25-trideoxyecdysone in testes throughout the final instar showed that the production of ecdysteroids varied with time. The significance of these results is discussed in relation to the likelihood of testicular ecdysteroid synthesis.