Biology Reference
In-Depth Information
or embryos, the mycetocytes separate and are distributed to the anterior and posterior ends of the
haemocoel, as either isolated cells or small groups of cells. This breakup of the cells is usually
attributed to the reduced space in the hemocoel, e.g., Buchner (1966), but the consequences for
mycetocyte and
Buchnera
function have not been considered.
cells restricted to mycetocytes? Both limited access to other cells and
poor survival external to mycetocytes may be involved.
Why are
Buchnera
Buchnera
apparently lacks the general
capacity to invade insect cells because the gene inventory of
, as obtained
from the complete genome sequence (Shigenobu et al., 2000), has revealed none of the suite of
genes implicated in adherence to and invasion of animal cells by other bacteria including related
members of the
Buchnera
from
A. pisum
h
-proteobacteria (Hentschel et al., 2000; Goebel and Gross, 2001).
Buchnera
cells injected into the hemocoel of aphids are lysed rapidly (Douglas, unpublished data); it is
not known whether this reÞects insect defense against bacteria or poor
tolerance of
the conditions (ionic composition, osmotic pressure, etc.) in the hemolymph. These data, although
fragmentary, raise the possibility that
Buchnera
have trophic or other requirements met only
within the symbiosomal membrane in mycetocytes. Pertinent to this possibility, the genomic data
also indicate that
Buchnera
have very few membrane transporters to mediate nutrient
exchange and maintain the bacterial metabolic pools. (
Buchnera
cells
cells of necessity derive all their
nutrients from the surrounding mycetocyte cytoplasm.) Perhaps metabolic and signaling inter-
actions between
Buchnera
and the insect are mediated primarily by the transport and receptor
functions of the symbiosomal membrane. If this is correct, then the symbiosomal membrane is
a crucial, but completely unstudied, element in our understanding of the
Buchnera
function of
Buchnera
cells and their interactions with aphids.
A
B
CCESSORY
ACTERIA
Generally, the accessory bacteria are restricted to the sheath cells bounding mycetocytes. They have
also been described free in the hemolymph, where they may attain high densities, and occasionally
in cells of the fat body, in hemocytes, and in mycetocytes that lack
cells (Douglas, 1998;
Fukatsu et al., 2000). One accessory bacterium, PABS, has additionally been reported as associated
with the gut (Darby et al., 2001). Taken together, these data suggest that the accessory bacteria are
less fastidious and more invasive than
Buchnera
Buchnera
.
aphid fresh weight, and
the density of accessory bacteria is about tenfold lower (Humphreys and Douglas, 1997; Wilkinson
et al., 2001a). These values are equivalent to
The density of
Buchnera
cells
in aphids is ca.
1 to 3
10
cells mg
7
Ï1
cells and accessory bacteria, accounting for
8 to 24% and 0.8 to 2.4% of aphid volume, respectively. Limited data suggest that bacterial density
varies with various factors, including temperature, host plant, aphid morph, and developmental age
(e.g., Baumann and Baumann, 1994; Humphreys and Douglas, 1997; Wilkinson et al., 2001a), but
the impact of environmental factors on the symbiosis has not been studied systematically.
Buchnera
MODES OF TRANSMISSION OF BACTERIA
B
UCHNERA
The aphidÏ
Buchnera
symbiosis is a Ñclosed system,Ò i.e., the
Buchnera
cells are obligately vertically
transmitted, such that all the descendants of each
Buchnera
cell are either descendants of the aphid
bearing that
cell or dead; the descendants have no access to either the external environment
or other aphid lineages.
The chief evidence is molecular: the phylogenies of
Buchnera
and aphids are strictly congruent,
i.e., the phylogenetic trees of the two partners map onto each other. This was Ýrst demonstrated
by Moran et al. (1993) for trees constructed for 16S rRNA genes of
Buchnera
and the morphological
characteristics of aphids across all subfamilies of the Aphididae (Figure 2.1) and has since been
Buchnera
 
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