Study of steroidogenesis in pupae of the forensically important blow fly Protophormia terraenovae (Robineau-Desvoidy) (Diptera: Calliphoridae)

Abstract

Protophormia terraenovae is a forensically important fly whose development time is studied by forensic entomologists to establish the time elapsed since death (post-mortem interval, PMI). Quantity and nature of ecdysteroid hormones present in P. terraenovae pupae were analysed in order to determine if they could be correlated to the age of pupae found on corpses and thereby could give information on the PMI. Ecdysteroid levels were quantified during the pupal-adult development of synchronised animals using enzyme immunoassay (EIA), a sensitive method allowing acurate quantification in one pupa. Two types of pupae were compared: "fresh" pupae, kept frozen until analysis and "experimentally dried" pupae, which were left for several weeks at ambient temperature. A peak of ecdysteroids was detected between 36 and 96 h after pupariation in fresh animals. It was not observed in "experimentally dried" pupae. High-pressure liquid chromatography (HPLC) analyses combined with EIA showed that 20-hydroxyecdysone (20E) was the major free ecdysteroid at various pupal ages. Enzymatic hydrolysis experiments revealed the presence of apolar conjugates at all ages tested. However, neither qualitative nor quantitative difference was detected between early and late pupae. This study gives precise information on the nature and quantity of ecdysteroids in the course of pupal development of a calliphorid fly. The limits of using ecdysteroid measurement as a tool in forensic entomology are discussed.