A consideration about the origin of aphid intracellular symbiont in connection with gut bacterial flora.

Abstract

In an attempt to investigate the origin of the intracellular symbiont of the aphid (Buchnera), aphid gut aerobic bacteria were isolated, and their phylogenetic relations to other prokaryotes were examined based on nucleotide sequences of 16S rDNA. It turned out that there are seven aerobic bacterial groups which constitute major flora of the aphid's gut. As three of the isolated bacteria were identified as members of the family Enterobacteriaceae, and share the common ancestor with the intracellular symbiont, the nucleotide sequences of 16S rDNA were determined for 15 representative strains of the family Enterobacteriaceae. One of the gut microbes belonging to the family Enterobacteriaceae was identified as Erwinia herbicola that is found mainly on plant surfaces. This fact may suggest that the intracellular symbiont of aphid is derived from a habitant of plant on which host insects feed.Pea aphid, Acyrthosiphon pisum, harbors prokaryotic intracellular symbionts in the mycetocytes, huge cells in the abdomen, which are differentiated specifically to accommodate the symbionts (1). The date of establishment of the symbiotic association is estimated to be 160-280 million years ago based on 16S rDNA molecular clock calibrated by aphid fossils (12). The aphid and its intracellular symbionts, which are often referred to as Buchnera species (14), are in a closely mutualistic relationship, and neither of them is able to propagate without the other (8,15). Buchnera, essential for the normal growth and reproduction of the aphid, are maternally inherited through generations of the host insect, and have not been successfully cultivated outside the host mycetocyte.

Erwinia aphidicola, a new species isolated from pea aphid, Acyrthosiphon pisum.

“…Strains X 001, X 081, X 151, X 171 and X 181 were isolated from the guts of healthy apterous aphids as previously described (Harada et al, 1996). Whole bodies of the insect, which were surface-sterilized by dipping into 70% ethanol for 5 min, were washed in sterilized Carlson's solution (0.7% NaCI (w/v), 0.02% KCI (w/v), 0.02% CaCl2.2H2O (wlv), 0.01% MgCl2 • 6H2O (w/v), 0.02% NaH2PO4 (w/v), 0.012% NaHCO3 (w/v) and 0.8% glucose (w/v)), and lightly homogenized in the same solution.…”

“…In a previous study, aerobic bacteria were isolated from aphid gut in order to investigate insect-bacteria interaction, and seven groups were identified as major constituents of the gut flora of the pea aphid, Acyrthosiphon pisum (Harada et al, 1996). Based on the molecular phylogenetic analyses of 16S rDNA, three of the seven bacterial groups were identified as members of the family Enterobacteriaceae, and tentatively named bacterium T, W and X. DNA-DNA hybridization testing and a phylogenetic tree of 16S rDNA indicated that bacterium T was identical to Erwinia herb/cola (Harada et al, 1996).…”

“…Based on the molecular phylogenetic analyses of 16S rDNA, three of the seven bacterial groups were identified as members of the family Enterobacteriaceae, and tentatively named bacterium T, W and X. DNA-DNA hybridization testing and a phylogenetic tree of 16S rDNA indicated that bacterium T was identical to Erwinia herb/cola (Harada et al, 1996). Bacterium X was detected from almost all the healthy specimens, and was the largest in number among the seven bacterial groups which constituted the major flora of the insect gut (Harada and Ishikawa, 1993;Harada et al, 1996). In enterobacterial groups we examined, only bacterium X, and Er.…”

Phylogenetic analyses of Klebsiella species delineate Klebsiella and Raoultella gen. nov., with description of Raoultella ornithinolytica comb. nov., Raoultella terrigena comb. nov. and Raoultella planticola comb. nov.

“…While some genera have been investigated in detail, e.g. Yersinia (Ibrahim et al, 1994), Salmonella (Chang et al, 1997 ;Christensen et al, 1998), Serratia (Dauga et al, 1990 ;Harada et al, 1996) and Erwinia (Kwon et al, 1997 ;Hauben et al, 1998), the genus Klebsiella has not been analysed extensively (Carter et al, 1999). The taxonomy of the genus Klebsiella was characterized by a nomenclature reflecting its colourful history.…”

Extreme genome reduction in Buchnera spp.: Toward the minimal genome needed for symbiotic life

“…Besides Buchnera, aphids often harbor additional bacteria that are commonly referred as secondary (S) endosymbionts. These bacteria can be found in the aphid guts, in tissues surrounding the P bacteriocytes (or even invading the P bacteriocytes themselves), and in specialized S bacteriocytes (10)(11)(12). These endosymbionts are also subject to vertical transmission (13), but their patchy distribution among aphid populations implies that they, unlike Buchnera, also undergo horizontal transmission (10).…”