Biology Reference
In-Depth Information
to the aphidÔs physiology has been studied exten-
sively, mainly using chemically deÝned diets containing labeled compounds and aposymbiotic
insects produced by injection of antibiotics, such as rifampicin and tetracyclin, as well as heat
treatment. Among the Ýndings from these studies, the most conspicuous is amino acid metabolism
in aphids, in which
The nutritional contribution of
Buchnera
plays a crucial role. Phloem sap, the sole diet of aphids, is rich in
carbohydrates but notoriously poor in nitrogenous compounds (Douglas, 1993). Thus, it had long
been an enigma that aphids were so prosperous with extraordinarily high fecundity, notwith-
standing their poor diet. Sasaki et al. (1990) showed that, unlike many other insects, aphids do
not excrete nitrogenous wastes as uric acid but recycle the amino group by producing the amino
acid glutamine, which is taken up by
Buchnera
. It is now evident that in aphids most, if not all
(Douglas, 1998), essential amino acids are synthesized by
Buchnera
using glutamine, both
ingested and recycled, as a substrate (Sasaki and Ishikawa, 1995). Also, many lines of evidence
strongly suggest that the vitamin riboÞavin is synthesized and supplied to the host insect by
Buchnera
Buchnera
(Nakabachi and Ishikawa, 1999).
Studies of the primary endosymbiont of aphids later designated
from molecular points
of view were initiated in the early 1980s. An epoch-making Ýnding in earlier studies was that
Buchnera
Buchnera
, when housed in the bacteriocyte, selectively synthesized a single protein species, sym-
bionin, in large quantities (Ishikawa, 1982), which was later demonstrated to be a homolog of
GroEL, a stress protein of
that functions as a molecular chaperone (Hara et al.,
1990; Kakeda and Ishikawa, 1991; Ohtaka et al., 1992). These studies led to
Escherichia coli
the Ýnding that
intracellular bacterial symbionts in both insects and eukaryotes at large tended to produce GroEL
homologs selectively (Ahn et al., 1994; Aksoy, 1995; Baumann et al., 1996; Charles et al., 1997a).
It is likely that in addition to chaperone activity, the GroEL homologs produced by bacterial
symbionts have evolutionarily acquired new functions (Morioka et al., 1994; Yoshida et al., 2001).
Recently, Fares et al. (2002) suggested that the GroEL homolog produced by
could be
essential for buffering against deleterious mutations inherent in the endosymbiont, which may also
account for the rampant production of essential amino acids by
Buchnera
Buchnera
. The Ýnding that the
genomic DNA of
is very rich in A and T (Ishikawa, 1987) is now applicable to almost
all known bacterial endosymbionts (Clark et al., 1999).
Recent analysis of the
Buchnera
Buchnera
genome conÝrmed the results of the nutritional studies.
Phylogenetically,
but differs from the latter in that it contains
more than 100 genomic copies per cell (Komaki and Ishikawa, 1999); in addition, its genome size
is only one seventh that of
Buchnera
is a close relative of
E. coli
(Charles and Ishikawa, 1999). There are genes that are used in
the biosynthesis of amino acids essential for the hosts in the
E. coli
genome, but those for
nonessential amino acids are largely absent, indicating complementarity and syntrophy between
the host and
Buchnera
genome contains a gene for the
riboÞavin synthase b chain, which is actively expressed, conÝrming the contribution of
Buchnera
(Shigenobu et al., 2000). The
Buchnera
Buchnera
with respect to the provision of this vitamin (Nakabachi and Ishikawa, 1999). By contrast, it was
shown that the
genome did not contain genes for the sterol synthesis, excluding the long-
suspected possibility that this symbiont should provide aphids with cholesterol, a precursor to the
molting hormone. In addition,
Buchnera
lacks genes for the biosynthesis of cell-surface compo-
nents, including lipopolysaccharides and phospholipids, regulator genes, and genes involved in the
defense of the cell (Shigenobu et al., 2000). Knowing how
Buchnera
makes up for the lack of these
gene products to retain its identity as a cell will be of paramount importance (Shigenobu et al.,
2001; Shimomura et al., 2002).
For all the contributions of
Buchnera
to the host, it is this symbiotic system that is most readily
sacriÝced for the hostÔs survival. While aphids are kept starved, their total bacteriocyte volume
consistently decreases at a much higher rate than that of the decline of their body weight. Upon
resumption of the feeding, their total volume of bacteriocytes immediately restores the original
value. These results strongly suggest that aphids will surmount the nutritionally adverse conditions
by consuming their endosymbionts as nutritional reserves (Kobayashi and Ishikawa, 1993). The
Buchnera
