Biology Reference
In-Depth Information
A second speciation mechanism associated with
Wolbachia
may be by the
induction of thelytoky (reproduction by females only), as has been found in many
hymenopteran parasitoids, such as
Encarsia formosa
. Populations of
Encarsia
no
longer have males, so that populations essentially become clonal and over time
could differentiate genetically (
Cook and Butcher 1999
).
A third potential
Wolbachia
speciation mechanism is by bidirectional incompat-
ibility; if a population is infected with two different strains of
Wolbachia
that are
incompatible with each other, then the incompatibility could act as a postzygotic
reproductive barrier, as has been suggested for the species complex of
Nasonia
(Hymenoptera) (
Figure 4.2B
). Thus, how
Wolbachia
are maintained in popula-
tions has considerable theoretical and practical importance.
Wolbachia
have been
proposed as mechanisms for driving genes into populations in genetic manipula-
tion projects for improved pest control (for additional discussion of this topic, see
Chapter 14).
Gazla and Carracedo (2011)
reported that females of
D. melanogaster
and
D. simulans
could discriminate between males with and without
Wolbachia
and preferred to mate with males that had the same infection status; this prefer-
ence would influence sexual isolation and also could play a role in speciation.
The interest in the biology and evolution of
Wolbachia
, with its fascinating
effects on reproductive isolation (thus potentially having effects on speciation),
sex ratio, feminization, and male killing, led to the development of a
Wolbachia
genome project (
Bandi et al. 1999
). Genome sizes for six different
Wolbachia
strains were determined by pulsed-field gel electrophoresis (
Sun et al. 2001
). The
Wolbachia
genomes are circular and range from 0.95 to 1.66 Mb, which is consid-
erably smaller than the genomes of free-living bacteria such as
E. coli
(4.7 Mb).
Despite the wealth of information obtained about
Wolbachia
within the past
few years, our understanding of the role of
Wolbachia
in arthropod biology
and evolution probably remains fragmentary. For example, some
Wolbachia
in
arthropods were shown to contain bacteriophages named WO (
Masui et al. 1999,
2000
). A phylogenetic analysis of different WOs from several
Wolbachia
strains
yielded a tree that was not congruent with the phylogeny of the
Wolbachia
, sug-
gesting that the phages were active and horizontally transmitted among the
various
Wolbachia
.
Masui et al. (2000)
speculated that, because all
Wolbachia
strains they examined contain WO, the phage might have been associated with
Wolbachia
for a very long time, conferring some benefit to its microbial hosts.
Wolbachia
strains seem to have a very wide range of effects on their hosts,
ranging from causing male killing, cytoplasmic incompatibility, modification of
immune responses, parthenogenesis, providing nutritional benefits, and toler-
ance to heat shock (
Box 4.2
).
Wolbachia
strains even may block transmission of
disease-causing agents in mosquitoes and
D. melanogaster
(
Box 4.2
).
