Biology Reference
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elevated levels of free amino acids. For example, the
Buchnera
-free
pea aphids
A. pisum
studied
by Prosser and Douglas (1991) had protein content depressed by 20% and free-amino-acid content
increased by 40% relative to untreated controls. These data suggest that protein synthesis by the
Buchnera
-free aphids is limited by a shortfall of certain amino acids, resulting in an accumulation
of other amino acids in the free amino acid pool. The key amino acids with low concentrations
in the free-amino acid pool of
Buchnera
-free aphids have been identiÝed as phenylalanine and
methionine in
A. pisum
(Prosser and Douglas 1991; Douglas, 1996). These are among the nine
amino acids that aphids and other animals cannot synthesize
de novo
, known as the essential
amino acids. The implication that these essential amino acids are provided to the aphid by
Buchnera
Ýts with the Ýnding that the amino acid composition of plant phloem sap, the diet of
aphids, is grossly deÝcient in these nine essential amino acids. Whereas animal protein contains
on average 50% essential amino acids, plant phloem sap contains 10 to 25% essential amino
acids (Douglas, 1993).
The direct evidence that
Buchnera
provides aphids with essential amino acids is threefold. The
Ýrst line of evidence derives from the development of methods to raise aphids on a chemically
deÝned liquid diet that mimics phloem sap. When individual essential amino acids are deleted from
these diets, aphid growth and reproduction are generally not affected, but the growth of experi-
mentally generated
Buchnera
-free aphids is depressed by elimination of some or all of these
nutrients, varying among aphid species (e.g., Mittler, 1971; Douglas et al., 2001).
Second, the intact aphidÏ
Buchnera
association and isolated
Buchnera
cells can synthesize
essential amino acids, but
Buchnera
-free aphids (as expected) have no essential-amino-acid
biosynthetic capability (e.g., Douglas, 1988; Sasaki and Ishikawa, 1995; Febvay et al., 1999;
Douglas et al., 2001). Essential amino acids synthesized by
Buchnera
are recovered from aphid
proteins, which indicates that they are made available to aphid tissues. Recently, Douglas et al.
(2001) quantiÝed the rates of essential amino acid production by the
Buchnera
population in
A. fabae
. When these data are compared against the rate of increase in
Buchnera
protein content
(as calculated from the estimated proliferation rate of 0.3 cells/day and amino acid composition
of
Buchnera
protein), the amount of each essential amino acid made available to the aphid
tissues is estimated at 50 to 75% for all essential amino acids except tryptophan (Table 2.1).
TABLE 2.1
Essential Amino Acid Production by
Buchnera
in
Aphis
fabae
Production per
Buchnera
Cells per Day (fmol)
a
Amount Released to
Aphid (%)
b
Amino Acid
Phenylalanine
0.28
76
Isoleucine
0.19
75
Leucine
0.33
74
Lysine
0.26
74
Valine
0.22
70
Methionine
0.04
68
Threonine
0.19
56
Histidine
0.02
54
Tryptophan
0.02
19
a
Data from Douglas, A.E. et al. (2001).
b
Calculation assumes that
Buchnera
cells proliferate at rate of 0.3 cells/day
and that their protein has the same amino acid composition as that in
Escher-
ichia coli
(Schaechter, 1992).
