Biology Reference
In-Depth Information
fact that the reverse will never happen seems to manifest rightly the nature of this mutualism
between aphids and
. Apparently, the insect host proÝts much more from this association.
Psyllids, or jumping plant lice, differ from aphids in many aspects of life history and bioge-
ography but are similar in that they feed on phloem sap and harbor maternally transmitted endo-
symbionts (Fukatsu and Nikoh, 1998; Spaulding and von Dohlen, 2001). The primary endosym-
bionts of psyllids are housed in bacteriocytes, where they are enclosed by host-derived membrane
vesicles. The phylogenetic tree derived from the 16S-23S rDNA of these symbionts agrees with
the tree derived from a host gene, a result consistent with a single infection of the psyllid ancestor
and subsequent cospeciation of endosymbionts and hosts (Thao et al., 2000). Thus, these symbionts
are quite similar to
Buchnera
Buchnera
in many respects but differ from the latter in that they constitute a
-3 subdivision of the proteobacteria (Spaulding and von Dohlen, 1998).
According to a recent report on 37 kb of its DNA sequenced so far, the genome of the endosymbionts
of psyllids has been undergoing an evolution that is reminiscent of
unique lineage within the
h
Buchnera
, but its degeneration
seems even more extreme than in the
genome. Among the unusual properties are an
exceptionally low G+C content (19.9%), almost complete absence of intergenic spaces and operon
fusion, and a lack of usual promoter sequence upstream of 16S rDNA (Clark et al., 2001). To
further validate these Ýndings and to indicate which genes are missing from the genome, analysis
of its entire sequence is prerequisite.
Buchnera
S
S
ECONDARY
YMBIONTS
Many insects, homopterans in particular, do not seem so ascetic as to restrict themselves to
mutualistic association with only one class of endosymbiont. They harbor additional inhabitants
that are collectively called secondary symbionts. Many aphids also harbor secondary symbionts
(Buchner, 1965; Fukatsu and Ishikawa, 1993). Whereas
, the primary symbionts, are
usually globular in shape with a diameter of 2 to 4 nm, secondary symbionts vary in shape.
Secondary symbionts in aphids have usually been located in syncytia adjacent to the bacteriome
housing
Buchnera
, but they may occupy other locations (Buchner, 1965; Hinde, 1971; Fukatsu
et al., 2000). Occurrence of the secondary symbionts has no clear correlation to the phylogeny of
aphids, suggesting that they were acquired many times in various lineages independently. Even
within a single aphid species the presence of secondary symbionts is not consistent and varies due
to its biotype or habitat. These facts suggest that the association of aphids with these symbionts is
labile and that their acquisitions by aphids are much more recent than that of
Buchnera
. The
secondary symbionts also vary phylogenetically. Whereas most of them are, like
Buchnera
, members
of the h-proteobacteria (Munson et al., 1991), some may be a
Rickettsia
(Chen et al., 1996) or a
Spiroplasma
(Fukatsu et al., 2001). Roles played by secondary symbionts in aphid biology are
largely unknown, although a recent study demonstrated condition-dependent effects on host growth
and reproduction (Chen et al., 2000). Some secondary symbionts of the pea aphid,
Acyrthosiphon
pisum
, are known to harbor a bacteriophage referred to as APSE-1 (after
A. pisum
secondary
endosymbiont), which belong to the Podoviridae and are related to phage P22 of
Salmonella enterica
(van der Wilk et al., 1999).
Buchnera
Y
EAST
-L
IKE
S
YMBIONTS
Planthoppers differ from aphids in that they harbor eukaryotic yeast-like symbionts (YLSs)
instead of
Buchnera
in the mycetocyte. YLSs are transmitted directly from mothers to their
offspring by transovarial infection and are found in the host at every developmental stage (Chen
et al., 1981). Many lines of evidence suggest that YLSs are essential for normal development
and growth of the host. Molecular phylogenetic analysis placed YLSs in the class Pyrenomycetes
in the subphylum Ascomycotina (Noda et al., 1995). Nutritional roles played by YLSs were
studied extensively using the Asian rice brown planthopper
Nilaparvata lugens
. It turned out
