Juvenile hormone mimics the photoperiodic apterization of the alate gynopara of aphid, Aphis fabae

Abstract

Extensive investigations of the environmental factors influencing the alate–apterous dimorphism of the summer, parthenogenetic, virginoparous generations of aphids have shown that crowding has a major influence on the development of the alate morph, although temperature and nutrition are also important^1,2. These factors may act prenatally, postnatally or both, depending on the species. However, the physiological control of this dimorphism within the individual has not been elucidated. As the apterous adult and the larvae share certain morphological features³ it has been suggested that the hormone controlling larval–adult metamorphosis, juvenile hormone (JH), also influences development of apteriform characteristics^4,5. Indirect evidence from measurements of corpus allatum (CA) volumes and CA nuclei were believed to support this theory⁶ and early experiments with topically applied JH extracts and analogues seemed to divert alate development towards the apterous condition^2,5. Attempts to obtain more direct evidence have been negative or inconclusive⁶. Recently, Lees⁷ has questioned this hypothesis, citing the following evidence: (1) examination of more extreme forms produced by topical application of JH mimics to middle instar alatae shows them to be developing towards a fifth instar, supernumerary larva (that is juvenilization) rather than towards an apterous morph (that is apterization); (2) in an autumn, short-day-induced alate morph of Aphis fabae, namely the gynopara (see below), authentic apterization can be effected by postnatal exposure to long photoperiods, the first larval instar being most sensitive to this treatment. By contrast, it was the late second and the third instars that were most sensitive to juvenilization following topical application of JH analogues. It is shown here that larger doses of a more active hormone (JH1) will closely mimic the photoperiodic apterization of these gynoparae but that apterization is not due to the neotenic action of JH. Evidence is also presented suggesting that the alateapterous dimorphism in summer, long-day generations is not controlled by JH.