Biology Reference
In-Depth Information
numerous animal group, with the number of described species exceeding 750,000. A majority of
the endosymbionts identiÝed within these species belong to the
-subdivision of the proteobacteria
(von Dohlen et al., 2001). Numerous obligate and facultative intracellular bacteria are found within
the proteobacteria including genera such as
h
Buchnera
,
Francisella,
and
Legionella
in the
h
-proteo-
bacteria and
-proteobacteria.
The Homoptera (Insecta) contains about 4500 aphid species that are known for the presence
of primary bacterial endosymbionts of the genus
Rickettsia
,
Wolbachia
,
Ehrlichia,
and
Bartonella
in the
b
(Blackman and Eastop, 1994). This
particular association represents the best-studied example of animal endosymbionts.
Buchnera
Buchnera
are
Gram-negative cocci with average cell size ranging in diameter from 2 to 5
m (Houk and GrifÝths,
1980). Ultrastructural studies revealed the presence of the bacteria surrounded by host-derived
membrane in the cytoplasm of specialized polyploidic host cells termed bacteriocytes or myceto-
cytes (Buchner, 1965; GrifÝths and Beck, 1973). These are grouped in bilobed structures (bacte-
riomes) within the aphid body cavity (Buchner, 1965).
In a typical case, there are 5.6 million bacterial cells per adult aphid (Baumann and Baumann,
1994). The bacteriomes may be surrounded by a sheath of cells that contains rod-shaped bacteria
related to
n
in young individuals of some aphid species. These bacteria are referred
to as secondary (S-) symbionts. In most cases, the effect of these bacteria on host phenotypes is
not known (Moran and Baumann, 2000). In adult aphids, the bacteriome disintegrates into the
abdomen, and S-endosymbionts can be isolated from a variety of body parts including the
hemolymph (Baumann et al., 1995). Some members of Cerataphidini, a group within Aphidoidea,
do not seem to contain any bacterial endosymbionts but harbor extracellular, yeast-like organisms
(Buchner, 1965).
The association between
Escherichia coli
has
never been cultured on artiÝcial media outside of its aphid, emphasizing the obligate nature of
the association. Likewise, it has been experimentally shown (Houk and GrifÝths, 1980) that
antibiotic treatment of the aphid host, which eliminates the bacteria, reduces aphid growth and
often induces sterility, stressing the need for a bacterial partner. Therefore, aphids are best
considered chimerical or composite organisms consisting of one partner that is an insect and
another partner that is a bacterium.
Buchnera
and its hosts is strictly mutualistic. Thus,
Buchnera
is transmitted vertically, from the mother aphid to the progeny via the infection of
eggs or embryos (Baumann et al., 1995). It has been reported that
Buchnera
cells while transferred
to the progeny are not surrounded by the host-derived membrane (Baumann et al., 1995). This can
be viewed as their last connection to the outer world, perhaps imposing certain limits on the extent
to which the bacterial membrane structure can be degraded and eliminated. Interestingly,
Buchnera
Bloch-
mannia
species, the endosymbionts of ants closely related to Enterobacteriaceae, are found free in
the cytoplasm of bacteriocytes (Schrder et al., 1996).
ENDOSYMBIOSIS: METABOLIC CONSIDERATIONS
From the host point of view, the aphidÏ
association is nutritionally based, while the host
in return provides a stable, nutrient-rich environment to the bacterium. The primary function of the
endosymbionts appears to be the synthesis of essential amino acids that are strictly required by the
aphids. The phloem sap provides a diet rich in carbohydrates but very poor in amino acids (Baumann
et al., 1995; Douglas, 1992, 1998). The concentration of essential amino acids in the sap is very
low, further reducing its already limited nutritional value (Sandstrm and Pettersson, 1994; Sand-
strm et al., 1999). Aphids, like other animals, require ten essential amino acids. Every single
amino acid represents a growth-limiting factor if not provided in adequate quantities (Sandstrm
et al., 1999). Thus, it may not be surprising that as much as 10% of
Buchnera
Buchnera
genomes are dedicated
to amino-acid biosynthesis (Shigenobu et al., 2000; Tamas et al., 2002).
Tryptophan and leucine are present in extremely low concentrations in the phloem sap. With
respect to these amino acids,
Buchnera
plays a particularly important role for the aphid in that the
