Biology Reference
In-Depth Information
could not be identiÝed and the relationship could not be manipulated. Until about 10 years ago,
the primary symbionts were described as Ñmycoplasma-likeÒ or Ñrickettsia-likeÒ (e.g., Hinde,
1971a) simply because of their shared trait of living within animal cells. In 1991, Baumann et al.
identiÝed the primary symbionts from their 16S ribosomal RNA gene sequence. The bacteria were
not mycoplasmas or rickettsias but
h
-proteobacteria allied to
E. coli
(Munson et al., 1991a). Bau-
mann et al. assigned the bacteria to a novel genus,
, in recognition of the remarkable
achievements of Paul Buchner (Munson et al., 1991b). Subsequent research has revealed that the
accessory symbionts of Buchner include Ýve taxa, none of which has been brought into axenic
culture. None has been described formally, and most are known by acronyms. These Ýve bacteria
are PASS (= R type), PABS (= T type) and U type in
Buchnera
h
-proteobacteria, PAR (= S type) in
b
-
proteobacteria
species (Chen et al., 1996; Chen and Purcell, 1997; Fukatsu
et al., 2000, 2001; Darby et al., 2001; Sandstrm et al., 2001).
Buchner and colleagues believed that the aphids required their complement of primary symbionts.
The evidence was slight. Some aphid species included morphs that lacked primary symbionts, and
these were invariably small, e.g., the dwarf males of
,
and a
Spiroplasma
spp. The obligate nature of the
symbiosis was not demonstrated deÝnitively until 1971 when Mittler showed that aphids treated with
the antibiotic auromycin (= tetracycline) grew very poorly and produced no offspring (Mittler, 1971).
It has also been accepted widely that the accessory/secondary symbionts are not essential to
the aphid. This issue is currently being researched actively and is reviewed below. However, the
term ÑsymbiontÒ has led to the unfortunate assumption that aphids ÑshouldÒ beneÝt from all the
bacteria they contain. To avoid any preconceived notions as to the signiÝcance of these bacteria to
the aphid, I prefer to call them Ñaccessory bacteria.Ò
This article reviews our current understanding of
Stomaphis
and other bacteria associated with
aphids. It records the dramatic increase in knowledge and understanding of the symbiosis over the
last 10 years, achieved largely through the application of molecular approaches including the public
availability of the complete genome sequence of
Buchnera
Buchnera
in the pea aphid,
Acyrthosiphon pisum
(Shigenobu et al., 2000). Some important areas of current ignorance are also identiÝed.
LOCALIZATION OF BACTERIA
B
UCHNERA
cells are restricted to mycetocytes (also known
as bacteriocytes), cells whose sole function appears to be to house the bacteria. In most aphid
species studied, the mycetocytes do not divide after the birth of the aphid and accommodate the
proliferating
Through most of the aphid lifespan, the
Buchnera
population entirely by increasing in size. Mycetocytes are often very large
cells. For example, a young adult pea aphid,
Buchnera
Acyrthosiphon pisum
, weighing 4 mg may bear ca.
100 mycetocytes, of mean diameter 85
n
m, each of which contains 23,500
Buchnera
cells (calcu-
lated from data in Wilkinson and Douglas, 1998).
Within the mycetocytes, each
cell is separated from the cytoplasmic contents by a
membrane of insect origin, known as the symbiosomal membrane. Two or more
Buchnera
Buchnera
cells
are virtually never observed in a single symbiosome, suggesting that when each
cell
divides and its daughter cells separate, the surrounding insect membrane pinches off to form two
separate symbiosomes. The insect cell cytoskeleton probably mediates and controls both these
processes and the regular distribution of
Buchnera
cells across the cell cytoplasm, but this has not
been studied. Also unknown is whether the conditions experienced by
Buchnera
Buchnera
cells vary between
the periphery and central regions of each mycetocyte.
The mycetocytes of aphids lie in the hemocoel (body cavity) of the insect. In embryos and
young larvae, they are aggregated together as a coherent
V
-shaped organ, lying dorsal to the gut,
with the base of the
to the posterior and bounded by a single layer of cells known as sheath
cells. As the aphids develop and the ovarioles increase in volume through the production of eggs
V
