Biology Reference
In-Depth Information
segregate 1:1 during mitosis and therefore effectively behave like chromosomes. In embryos in
which centrosomes are separated from nuclei (i.e., via inhibition of DNA replication using aphid-
icolin or mutations that block nuclear division),
Wolbachia
remains clustered around the cen-
trosomes (Glover
et al., 1990). The association of
Wolbachia
and astral microtubules is further
suggested by experiments in which microtubule disruption with colchicine was observed to cause
the partial disassociation of
Wolbachia
from the spindle poles, whereas cytochalasin B was observed
to leave bacterial clusters intact (Callaini
et al., 1994; Braeckman et al., 1997).
WOLBACHIA
EFFECT ON HOST POPULATION SIZE
As described above, the reproductive advantage afforded to females by
Wolbachia
-induced CI
allows for the spread of infections into the host population. Recently, an additional model has been
developed to examine the effect of
Wolbachia
infection on host population size during natural
invasions and artiÝcial releases of
Wolbachia
-infected hosts (Dobson
et al., 2002a). The model
extends prior models by adding a density-dependent population-growth model and examining
bidirectional CI with up to three different infection types and an uninfected cytotype. This model
has been used to examine events occurring during both natural
Wolbachia
invasions and artiÝcial
releases conducted as part of applied population-suppression and replacement strategies. The model
predicts a transient reduction in the host population size as a unidirectionally incompatible infection
invades
(
Figure 13.2A)
.
The temporary reduction results primarily from the reduced brood hatch
in incompatible crosses. Therefore, the level of host-population reduction is greatest when the
population harbors similar frequencies of both infected and uninfected individuals. As the frequency
of one cytotype increases (i.e., due to invasion or elimination of the infection), the frequency of
incompatible crosses decreases, and the host-population size recovers.
Once an infection has invaded and reached equilibrium, the carrying capacity of the infected
host population may differ from the carrying capacity of the original uninfected population due to
Ýtness effects associated with
Wolbachia
infection (Figure 13.2A). With increasing CI survivorship
(1 Ï
s
h
), a reduced number of infected hosts is compensated by an increase in uninfected hosts
such that the total host-population size at equilibrium remains constant (Dobson
et al., 2002a). In
contrast, increased fecundity costs associated with infection (1 Ï
s
f
) and number of uninfected eggs
produced by infected females (n) typically result in a reduced frequency of infected hosts at
equilibrium and a lower carrying capacity. Variation in the parameters that affect density-dependent
population growth and the initial host-population size have no effect on the initial
Wolbachia
infection frequency required for population replacement, cytoplasmic drive rates, or the equilibrium
prevalence of the infection following invasion (Dobson
et al., 2002a).
Interestingly, the type of intraspeciÝc host competition has a signiÝcant effect on host-popula-
tion dynamics during
Wolbachia
invasion (Dobson
et al., 2002a). In host populations with high
reproductive rates and scramble-type competition, the reduced brood hatch that results from incom-
patible matings yields fewer immatures. Due to reduced intraspeciÝc competition, however, dis-
proportionately more of the immatures survive. The increase in adult number that results from male
releases is less pronounced in insect populations with lower reproductive rates and does not occur
in insect populations with contest-type competition. Empirical tests are required to validate the
predicted effects of
Wolbachia
invasion on host-population size.
APPLIED STRATEGIES EMPLOYING
WOLBACHIA
INFECTIONS
Applied strategies based on
Wolbachia
-induced CI and parthenogenesis have been proposed. As
Wol-
bachia
infections occur within a broad range of invertebrates, these strategies are potentially applicable
to a variety of medically and economically important insects. Alternative strategies for controlling
medically and economically important insects are of increasing interest due to environmental and
