Psyllids, whiteflies, aphids, and mealybugs are members of the suborder Sternorrhyncha and share a common property, namely the utilization of plant sap as their food source. Each of these insect groups has an obligatory association with a different prokaryotic endosymbiont, and the association is the result of a single infection followed by maternal, vertical transmission of the endosymbionts. The result of this association is the domestication of the free-living bacterium to serve the purposes of the host, namely the synthesis of essential amino acids. This domestication is probably in all cases accompanied by a major reduction in genome size. The different properties of the genomes and fragments of the genomes of these endosymbionts suggest that there are different constraints on the permissible evolutionary changes that are probably a function of the gene repertoire of the endosymbiont ancestor and the gene losses that occurred during the reduction of genome size.
Evolutionary studies suggest that 200-250 million years ago an aphid ancestor was infected with a free-living eubacterium. The latter became established within aphid cells. Host and endosymbiont (genus Buchnera) became interdependent and unable to survive without each other. The growth of Buchnera became integrated with that of the aphids, which acquired the endosymbionts from their mothers before birth. Speciation of host lineages was paralleled by divergence of associated endosymbiont lineages, resulting in parallel evolution of Buchnera and aphids. Present day Buchnera retains many of the properties of its free-living ancestor, containing genes for proteins involved in DNA replication, transcription, and translation, as well as chaperonins and proteins involved in secretion, energy-yielding metabolism, and amino acid biosynthesis. Some of these processes are also observed in isolated endosymbiont cells. Genetic and physiological studies indicate that Buchnera can synthesize methionine, cysteine, and tryptophan and supply these amino acids to the aphid host. In the case of some fast-growing species of aphids, the overproduction of tryptophan by Buchnera involves plasmid-amplification of the gene coding for anthranilate synthase, the first enzyme of the tryptophan biosynthetic pathway. These recent studies provide a beginning in our understanding of Buchnera and its role in the endosymbiosis with aphids.
Psyllids are plant sap-feeding insects that harbor prokaryotic endosymbionts in specialized cells within the body cavity. Four-kilobase DNA fragments containing 16S and 23S ribosomal DNA (rDNA) were amplified from the primary (P) endosymbiont of 32 species of psyllids representing three psyllid families and eight subfamilies. In addition, 0.54-kb fragments of the psyllid nuclear gene wingless were also amplified from 26 species. Phylogenetic trees derived from 16S-23S rDNA and from the host wingless gene are very similar, and tests of compatibility of the data sets show no significant conflict between host and endosymbiont phylogenies. This result is consistent with a single infection of a shared psyllid ancestor and subsequent cospeciation of the host and the endosymbiont. In addition, the phylogenies based on DNA sequences generally agreed with psyllid taxonomy based on morphology. The 3 end of the 16S rDNA of the P endosymbionts differs from that of other members of the domain Bacteria in the lack of a sequence complementary to the mRNA ribosome binding site. The rate of sequence change in the 16S-23S rDNA of the psyllid P endosymbiont was considerably higher than that of other bacteria, including other fast-evolving insect endosymbionts. The lineage consisting of the P endosymbionts of psyllids was given the designation Candidatus Carsonella (gen. nov.) with a single species, Candidatus Carsonella ruddii (sp. nov.).Insects within the families Aphididae (aphids), Psyllidae (psyllids), Aleyrodidae (whiteflies), and Pseudococcidae (mealybugs) feed predominantly or exclusively on plant phloem sap. These insects have a number of common structural properties (8) and constitute separate lineages within the suborder Sternorrhyncha (15, 54). The utilization of plant sap necessitates the penetration of tissue by flexible mouth parts (stylets) and the ingestion of the fluid. This mode of feeding is conducive to the transmission of viruses and other infectious agents, and members of these families are vectors of pathogens of agriculturally important plants (7,28,51). In addition, these insects may reach enormous populations, causing plant nutrient deprivation, leaf curling, and gall formation (8). Plant phloem sap, the diet of these insects, is rich in carbohydrates but deficient in nitrogenous compounds (21,46). Due to this deficiency, a large amount of plant sap is consumed and is excreted as honeydew. This sticky material may cover the plant and serve as a substrate for fungal growth (8).A common feature of organisms that live on diets containing an excess of one class of compounds and a deficiency in essential nutrients is the presence of prokaryotic intracellular symbionts (endosymbionts) that may provide the missing essential nutrients (6, 21, 37). Early histological studies indicated that these endosymbionts are housed within specialized cells (bacteriocytes) that form an aggregate (bacteriome) found within the body cavity (5, 10, 37). Typically, insects of a particular family or superfamily were found to have a ...
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