What are the origins of ecdysteroids in gastropods?

Terrestrial gastropods contain ecdysteroids, the origins of which are unknown. Whether they are synthesized by the animals or they arise from the diet is an open question. To address this problem, labeled cholesterol and various molecules which are efficiently converted into ecdysone by arthropods were tested as possible ecdysone precursors in Stylommatophra (gastropods). None of these experiments led to ecdysone biosynthesis and although snails and slugs were shown to contain some of the enzymes required for a biosynthetic pathway (i.e., a 3-oxoecdysteroid 3 beta-reductase, a 25-hydroxylase, and a 20-hydroxylase), no 2-hydroxylase and 22-hydroxylase activity could be detected. An endogenous origin would imply that Stylommatophora use a biosynthetic pathway different from that of insects. A dietary origin for ecdysteroids is also possible since, when ingested, these molecules remain in animals for several days and undergo limited metabolic conversion.