Biology Reference
In-Depth Information
A
120
100
80
infected with
PASS + PAR
60
40
uninfected
20
0
0 0 0 0 0100
120
lifetime fecundity
of aphids at 20
°
C
(mean + s.d.)
B
120
100
sweet
pea
80
60
alfalfa
40
20
bur clover
0
0 0 0 0 0100
120
lifetime fecundity of
uninfected aphids
(mean + s.d.)
FIGURE 2.6
Mean lifetime fecundity of pea aphids
Acyrthosiphon pisum,
which naturally contain no acces-
sory bacteria (uninfected) and have been infected experimentally with the accessory bacteria PASS and PAR.
(A) Impact of temperature on aphids reared on bur clover. (B) Impact of rearing plant at 20
p
C. [Redrawn from
Chen, D.-Q., Montllor, C.B., and Purcell, A.H. (2000).
Entomol. Exp. Appl.
95:
315Ï323: Tables 1 and 3.]
(Wilkinson et al., 2001b) subsequently taken up by embryos and used as precursors for essential
amino acid synthesis by the embryo symbiosis.
In contrast to their requirement for
Buchnera
, aphids apparently do not require accessory
bacteria. Aphids lacking any accessory bacteria detectable by microscopy and molecular methods
can be isolated from natural populations, and they generate vigorous parthenogenetic colonies in
the laboratory (unpublished results). These data beg the question whether accessory bacteria are
of any signiÝcance to the insect; perhaps they just Ñhitch a ride.Ò
The experiments of Chen et al. (2000) are directly relevant to this question. When a clone of
A. pisum
naturally lacking accessory bacteria was injected with the bacteria PASS and PAR, stable
vertically transmitted associations were formed. The response of the resultant associations to
temperature and host plants was altered relative to the uninfected controls. Aphid fecundity was
depressed at the standard rearing temperature of 20
p
C, but the thermal range of the aphids was
expanded (Figure 2.6A), and the effective plant range was increased (Figure 2.6B). The underlying
mechanisms are obscure, but these results raise the possibility that accessory bacteria have a greater
role than is currently appreciated in shaping ecologically important traits of aphids.
Other experiments have also implicated accessory bacteria as determinants of the plant range
of aphids.
Aphis fabae
performs relatively poorly on one occasional host plant,
Lamium purpureum
.
Unusually, the performance of aphids treated with antibiotic to eliminate their bacteria is not reduced
(relative to bacteria-free aphids on other plant species), and the reduction in performance is restricted
to aphids bearing their normal complement of bacteria (Adams and Douglas, 1997). This speciÝc
effect is correlated with elevated levels of the accessory bacteria (Wilkinson et al., 2001a), raising
the possibility that these bacteria are acting as opportunistic pathogens that may tend to reduce or
limit the plant range of their aphid hosts.
