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problem. Theoretically, the following factors favor the maintenance of maternally inherited endo-
symbionts in host populations: (1) efÝcient vertical transmission, (2) positive Ýtness effect on the
host, (3) reproductive manipulation such as CI, and (4) mechanism for horizontal transmission
(Caspari and Watson, 1959; Fine, 1978; Turelli, 1994). Which of these factors can be responsible
for the maintenance of wBruCon, wBruOri, and wBruAus in populations of
C. chinensis
?
STRATEGIES OF
W
B
RU
C
ON
AND
W
B
RU
O
RI
TO PERSIST IN HOST
POPULATIONS
We could speculate as to how wBruCon and wBruOri strains are maintained in the host populations.
Considering that wBruCon caused complete CI and that its bacterial titer in eggs was small,
maintenance of wBruCon may principally be realized through an efÝcient mechanism for CI. In
contrast, wBruOri caused only partial CI, and its bacterial titer in eggs was very high, suggesting
that a sophisticated mechanism for vertical transmission may compensate for the incomplete CI of
wBruOri. On the other hand, it was puzzling why wBruAus, which caused no CI and accounted
for a very small fraction of the bacteria in eggs, can be stably maintained in host populations.
PECULIAR PROPERTIES OF
W
B
RU
A
US
Among the three
Wolbachia
strains in
C. chinensis
, wBruAus exhibited peculiar properties. First,
the titer of wBruAus (around 10
7
wsp
copies equivalent per adult insect) was smaller by an order
of magnitude than that of wBruCon and wBruOri (around 10
8
wsp
copies equivalent per adult
insect)
(Figure 18.5)
.
Second, infection with wBruAus showed no detectable reproductive symptoms
of the host insect, whereas infections with wBruCon and wBruOri caused signiÝcant levels of CI
(Table 18.1).
Third, although the titer of wBruAus in oocytes and unfertilized eggs was extremely
low (less than 10
2
wsp
copies equivalent per egg)
(
Figure 18.6),
the
Wolbachia
was stably inherited
through host generations in the laboratory, and the infection rate in natural populations was over
97% on average
(
Figure 18.4)
.
In addition to them, we identiÝed further unusual properties of
wBruAus atypical of endosymbiotic bacteria.
ANTIBIOTIC RESISTANCE OF
W
B
RU
A
US
In an attempt to obtain
Wolbachia
-free
C. chinensis
, a triple-infected strain, jC, was reared with artiÝcial
beans containing either tetracycline or rifampicin. Tetracycline treatment of only one generation was
sufÝcient to eliminate wBruCon and wBruOri. On the other hand, wBruAus persisted throughout Ýve
generations of tetracycline treatment and could not be eliminated. After the treatment, the insects were
transferred to and maintained on normal beans. The insects, named strain jC
Aus
, contained wBruAus,
but it was completely free of wBruCon and wBruOri. The same patterns of tetracycline sensitivity,
resistant wBruAus, and sensitive wBruCon and wBruOri were found in other strains of
C. chinensis
(data not shown). The same results were obtained from rifampicin treatments (data not shown). Thus,
wBruAus was shown to be exceptionally resistant to tetracycline and rifampicin, although
Wolbachia
and other endosymbiotic bacteria are, in general, susceptible to these antibiotics.
SEX-LINKED INHERITANCE OF
W
B
RU
A
US
In mating experiments between
C. chinensis
strains of different infection status, we observed the
inheritance of the three
Wolbachia
strains. As expected, wBruCon and wBruOri were maternally
inherited to the offspring. Unexpectedly, however, we found that wBruAus was passed to the
offspring not only maternally but also paternally. The crosses between COA fathers and CO mothers
produced all CO males and, surprisingly, all COA females. When these COA females were mated
