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specialized epithelial cells (bacteriocytes) in the bacteriome organ located in the anterior midgut.
In contrast,
is harbored both inter- and intracellularly, principally in the midgut, but has
also been detected in the hemolymph (Cheng and Aksoy, 1999). The third symbiont present in
some tsetse species is related to
Sodalis
, which in many insects is conÝned primarily
to the reproductive organs. A phylogenetic analysis of the
Wolbachia pipientis
strains infecting various
species of tsetse has shown that they are unique and as such represent independent acquisitions by
each species (Cheng et al., 2000).
Tsetse has a viviparous reproductive biology Ð an adult female produces a single egg at a
time that hatches and develops
Wolbachia
. After a period of maturation and sequential molting within
the mother, a third instar larva is deposited and pupates shortly thereafter. Each female can deposit
Ýve to seven offspring during her 3- to 4-month life span. During its intrauterine life, progeny
receives nutrients along with the tsetse symbionts (
in utero
) through the
motherÔs milk gland secretions (Ma and Denlinger, 1974; Cheng and Aksoy, 1999). It is not known,
however, whether whole bacteriocytes
Sodalis
and
Wigglesworthia
or free-living
Wigglesworthia
cells are transferred from the
mother to her larva. Unlike the gut symbionts,
is transovarially transmitted through
maternal lineages. Given the unique reproductive biology of tsetse, all three symbionts are in essence
vertically acquired by the progeny.
Wolbachia
EVOLUTIONARY HISTORIES OF SYMBIONTS
Because it has been difÝcult to cultivate many of these fastidious and often intracellular insect
symbionts
, their physiological characterization and correct taxonomic positioning have been
controversial. Recent advances in polymerase-chain-reaction (PCR)-based technologies as well as
the use of nucleic-acid sequences in phylogenetic reconstruction have provided additional insight
into the evolutionary relationships among bacteria (Woese, 1987). Based on the 16S rDNA sequence
comparison, many insect symbionts (including the tsetse gut organisms) belong to the
in vitro
h
-subdivision
suggesting that microorganisms originating from
this group have been able to enter into and fulÝll symbiotic roles in various insect hosts.
Characterization of different members of genus
of Proteobacteria in the family Enterobacteriaceae
,
Wigglesworthia
from the four species groups
of tsetse Ð
Ð has shown that they form a distinct lineage
(Aksoy et al., 1995). The evolutionary relationship of the various tsetse species has been
independently determined based on observed variation in the internal transcribed spacer (ITS-
2) regions of rDNA (Chen et al., 1999). This analysis has shown similarity between the phy-
logeny of genus
fusca, morsitans, palpalis
and
austeni
and the phylogeny of its bacterial symbionts, implying that a tsetse
ancestor had been infected with a bacterium some 50 to 80 million years ago and from this
ancestral pair, species of tsetse and their associated
Glossina
strains radiated without
horizontal transfer events among species. The bacteriome-associated symbionts of distant insect
orders, such as
Wigglesworthia
from aphids (Hemiptera) (Munson et al., 1991), whiteÞy (Hemiptera)
(Campbell, 1993), Candidatus
Buchnera
Carsonella
from psyllids (Hemiptera) (Thao et al., 2000b), and
Candidatus
from carpenter ants (Hymenoptera) (Schrder et al., 1996), have each
been found to form distinct lineages closely related to
Blochmania
from tsetse (Diptera)
(Figure 4.1)
.
Similar concordant evolutionary history of bacteriome symbionts with their host
insect species has been reported for Candidatus
Wigglesworthia
Blochmania
,
Buchnera
, whiteÞy symbionts,
and Candidatus
.
In addition to the obligate P-symbionts, whose mutualistic biology is intimately linked with
their host physiology, many insects harbor one or more secondary microorganisms (S-symbionts)
that often display commensal relationships with their host. These organisms confer no known
beneÝts to hosts, do not appear to inÞuence the nutritional and reproductive biology of their
host, and often infect various host tissues. Observations in psyllids have indicated independent
acquisitions of closely related but different S-symbionts (Thao et al., 2000a). In contrast,
members of genus
Carsonella
Sodalis
characterized from distant tsetse species do not exhibit any signiÝcant
