Observations on the micro‐organisms occurring in the gut of the pea aphid Acyrthosiphon pisum

Abstract

Most aphids possess symbiotic bacteria in specialised argans, the bacteriacytes occurring in tbeir haemocoele (Buchner, 1965). These Gramnegative bacteria, recently identified by malecular techniques as belanging to the y-3 subgraup af the Proteabacteria and named Buchnera aphidicola (Munsan et al., 1991b), are called the aphid primary symbiotes and have not been cultivated to date (Hauk, 1974; Munson et al., 1991b). They are cansidered as obligate symbiates of the insect, although viable but unfertile aposymbiotic aphids can be obtained in some species of aphids (Prosser & Douglas, 1991; Sasaki etai., 1991; Rahbe et al., 1992). Their association with Aphididae seems widespread, and is considered to be monophyletic since the origin of aphids, mare than 80 million years ago (Mun son et al., 199Ia). It is speculated that they canfer to their aphid hasts many metabolic advantages, leading to a beller adaptation to their ecolagical niche, i.e. phloem feeding (Houk & Griffiths, 1980; Campbell, 1989; Ishikawa, 1989; Douglas, 1990; Nardon & Grenier, 1993). The study of aphid primary symbiosis implies a characterisation of the micro-organisms involved, and their differentiation From ather microorganisms harboured by aphids. Actually, many aphid species also harbaur secondary symbiotic bacteria, maternally transmitted and located in sheath cells surrounding the primary bacteriomes (Houk & Griffiths, 1980). Molecular characterisation assigns them to the Enterobacteriaceae, a group which included E. coli and not Buehnera spp. (Unterman et al., 1989). Their function in aphid physiology is considered la be le crucial since they are absent in many aphid species, such as Myzus persieae and Schizaphis graminum. Early reports also mentioned aphids with more than twa symbiotes (Buchner, 1965), and a recent paper describes an eukaryotic symbiosis in a species of gall-forming, social aphid belonging to the Hormaphidinae (Fukatsu & Ishikawa, 1992). Aside From their haemocoele symbiotes, sorne aphids have been reported to harbaur gut microorganisms (Schander! el al., 1949; Srivastava & Rouat!, 1963). Nothing is known on how widespread the infection s are, or on the function and origin of these micro-organisms. In the course of a study on the primary symbiosis of the pea aphid Acyrthosiphon pisum, we thought it useful ta further characterise the micro-organisms occurring in the aphid gut. We report here the observations and comment an their possible origin and their presence at different ages and in two biotypes currently used in our laboratory.