Biology Reference
In-Depth Information
TABLE 17.1
Some Host Systems Where the Characterization of
Wolbachia
-Induced Bidirectional CI
Has Been Conducted or Seems Imminent
Number of CI Types Acquired
by Horizontal Transfer/
Number Assayed
Number of
CI Types
Host System
Ref.
Culex pipiens
mosquitoes
15
0/4
a
Laven, 1959; Guillemaud et al.,
1997
Nasonia
wasps
6
5/6
b
Breeuwer and Werren, 1990;
Werren et al., 1995b;
Bordenstein and Werren, 1998;
Bordenstein et al., 2001
Drosophila simulans
Þies
5
5/5
OÔNeill and Karr, 1990; Merot and
Poinsot, 1998; James and Ballard,
2000
Coleomegilla maculata
beetles
2
2/2
Jeyaprakash and Hoy, 2000;
Perez and Hoy, 2002
Trichopria drosophilae
wasps
Total
2
2/2
Werren et al., in review
30
14/19
The number of CI types are phenotypically determined by the number of host strains that are uni- or bidirectionally
incompatible with one another. The number of CI types acquired by horizontal transfer is determined by the DNA-
sequence relationships of the
Wolbachia
infections. For example, CI types that are not closely related, based on one to
three different
Wolbachia
gene sequences, are classiÝed into the horizontal transfer group.
Wolbachia
sequences of CI
types that are identical (= a) or share their most recent common ancestor (= b) are classiÝed into the alternative group.
The importance of bidirectional CI to speciation depends critically on how often species or
populations actually harbor multiple incompatibility types (i.e., CI-
strains that are
reciprocally incompatible). Data are limited in this regard (Table 17.1), but there are two good
reasons to think that the number of host systems that harbor mutually incompatible
Wolbachia
has
been grossly underestimated. First, much more is known about overall infection frequencies
throughout major taxonomic groups than intraspecies variation in
Wolbachia
strains (Jeyaprakash
and Hoy, 2000; Werren and Windsor, 2000; Jiggins et al., 2001). Table 17.1 shows those systems
where bidirectional CI has been characterized or where the characterization of bidirectional CI
seems imminent based on indirect evidence. Some of these host systems appear quite vulnerable
to harboring several infections. Second, the systems known to harbor multiple
Wolbachia
Wolbachia
infections
are especially well studied, including the
Nasonia
species complex (
N. vitripennis
,
N. giraulti
, and
N. longicornis
. The number of systems with bidirectional incompat-
ibility will therefore likely increase with the number of systems studied. Given that estimates of
infection frequencies across arthropod species run upwards of 75% (Jeyaprakash and Hoy, 2000),
it appears that we have only scratched the surface of a large mountain of data.
The well-studied system of
),
D. simulans
, and
C. pipiens
stands out in this regard since it single-handedly revived
interest in LavenÔs original ideas about CI and speciation in mosquitoes (Laven, 1959). Some 30
years after LavenÔs work, Breeuwer and Werren (1990) found complete bidirectional CI between
two closely related species of parasitic wasps,
N. vitripennis
and
N. giraulti
. It was the Ýrst such
study to show that bacterial microbes played a direct role in interspeciÝc reproductive isolation
and has remained a hallmark case for
Wolbachia
-based reproductive isolation. Shortly after, a
second study in
D. simulans
showed the same effect within species Ð bidirectional CI between
strains isolated from California and Hawaii (OÔNeill and Karr, 1990). Despite the Ýndings, both
Nasonia
