Biology Reference
In-Depth Information
Unidirectional:
Bidirectional:
x
x
=
=
C
C
x
x
=
=
C
I
x
x
=
=
I
I
x
x
=
=
C
C
Gene flow reduced in one cross direction
Gene flow reduced in both cross directions
FIGURE 17.1
-induced CI. C and I denote compatible and incompatible crosses,
respectively. Unidirectional CI occurs when sperm from infected males (black Ýll) fertilize eggs from unin-
fected females (no Ýll). Bidirectional CI occurs in both cross directions when sperm from a male infected
with one
The dynamics of
Wolbachia
Wolbachia
variant (black Ýll) fertilize infected eggs with a different
Wolbachia
variant (gray Ýll).
asexually produce all female offspring (Stouthamer et al., 1990, 1993). Unlike male-killing
Wolbachia
,
PI-
can give rise to all female populations that can persist. Finally, CI is the most common
alteration and occurs in all the major insect orders as well as in mites and isopods (Hoffmann and
Turelli, 1997). This effect is typically characterized by a sperm modiÝcation that leads to abnormalities
in post-fertilization paternal chromosome behavior and, ultimately, embryonic mortality (OÔNeill and
Karr, 1990; Reed and Werren, 1995; Presgraves, 2000). It is typically expressed in crosses between an
infected male and uninfected female, thereby reducing the Ýtness of uninfected females (Figure 17.1).
The key point here is that
Wolbachia
are in various ways in the business of modifying repro-
duction, the central element of speciation. If reproduction is barred between two populations by
whatever means, then reproductive isolation has evolved and speciation is under way. This feature
makes
Wolbachia
a more likely symbiont for promoting speciation than other symbionts that, for
example, impart a novel biochemical ability to their hosts.
Wolbachia
R
I
EPRODUCTIVE
SOLATION
within species can quite easily be asso-
ciated with post- and premating isolation among species (Werren, 1998). For example, CI can play
a direct role in postmating isolation by causing F1 hybrid inviability among populations infected with
different CI-
Some of the reproductive alterations induced by
Wolbachia
strains. It is worth noting here that, like an F1-dominant genetic incompati-
bility, CI could have a severe effect on gene-Þow reduction, more so than typical recessive genetic
incompatibilities. Such recessive incompatibilities are often expressed in F2 hybrids, backcross
hybrids, or the heterogametic sex (Orr, 1997), thereby allowing gene Þow through certain hybrid
combinations. In addition to postmating isolation,
Wolbachia
can also be associated with premating
isolation through the induction of parthenogenesis. Gene Þow between infected parthenogenetic and
uninfected sexual populations could be reduced due to these differences in reproductive strategies.
Thus, it is not farfetched to imagine cases where
Wolbachia
could accelerate the speciation process
alone or in conjunction with other genetically based reproductive isolation barriers.
Wolbachia
R
S
APID
PECIATION
Mendelian nuclear genes and selÝsh genetic elements may promote speciation at different rates.
This possibility is important to consider because
Wolbachia
fall into the latter class of elements.
