A NOVEL EUKARYOTIC EXTRACELLULAR SYMBIONT IN AN APHID, ASTEGOPTERYX-STYRACI (HOMOPTERA, APHIDIDAE, HORMAPHIDINAE)

In studies on the endosymbiotic system of various aphids, immunoblotting with an antiserum against symbionin, an aphid endosymbiont-specific protein, revealed that Astegopteryx styraci does not have symbionin while all the other aphids examined do. Histological studies showed that A. styraci possesses neither bacteriocyte nor endosymbiont but harbours budding microbes in the haemocoel and fat body. By stepwise density-gradient centrifugation, the microbes were successfully isolated from the aphid tissues. When the protein synthesized by isolated microbes was analysed in vitro, it showed antibiotic sensitivity of the eukaryotic type. By electron microscopy, cell organelles such as nuclei and mitochondria were found in the microbial cells. Thus it is concluded that A. styraci harbours eukaryotic extracellular symbionts instead of the prokaryotic endosymbionts found in most other aphids. The possible function and evolutionary origin of these eukaryotic microbes, and implications for the classification and phylogeny of aphids are discussed.