Symbiosis and the Origin of Species - Insect Symbiosis - page 225

the host to restore Wolbachia -altered phenotypes. In particular, the host-selection pressure to
accommodate or control the presence of Wolbachia may accelerate host genetic substitutions and
lead to genetic divergence between populations and potentially the evolution of genetically based
isolation. This outcome may be especially true for a bacterium like Wolbachia , which can alter
important aspects of host Ýtness, including gametogenesis, mitosis, sex determination, sex ratios,
and cytonuclear genetic interactions. The genes underlying these host accommodations within
species could end up causing postmating incompatibilities between species.
This model makes the prediction that Wolbachia will leave a genetic footprint of its presence
through host genetic substitutions in the nuclear or mitochondrial genomes, some of which may
be maladaptive in hybrids. Testing this prediction is not simple. Virtually nothing is known about
genetic substitutions (or actual genes, for that matter) that interact with Wolbachia . However, there
has been a recent burst of work describing hostÏ Wolbachia interactions in insects. These studies
can be seen as part of the Ýrst phase in characterizing the nuclear genes involved in host accom-
modation. In these studies, Wolbachia are typically ÑmovedÒ from the resident species background
into a nave or foreign genetic background, either by microinjection or backcrossing methods.
Wolbachia -induced phenotypes are then characterized in this new genetic background. The most
common effect described in these studies is a change in CI level and bacterial densities when the
Wolbachia are in a foreign genetic background (Boyle et al., 1993; Bordenstein and Werren, 1998;
McGraw et al., 2001; Poinsot and Merot, 2001). Other WolbachiaÏ host genotypic interactions
include rescue of a Drosophila lethal mutation (Karr, 2000) and a phenotypic switch from femini-
zation to male killing when a Wolbachia strain is experimentally transferred from one Lepidopteran
species to another (Fujii et al., 2001).
Some of these interactions between the Wolbachia and host genomes may be due to selection
on the host to modify Wolbachia . For example, in Nasonia , CI type differs between N. vitripennis
and the sister species N. giraulti and N. longicornis , and this difference is genetically controlled
(Bordenstein et al., in press). CI type has apparently been modiÝed by the N. vitripennis nuclear
genome to increase host Ýtness. The nuclear genetic divergence underlying this difference could
possibly contribute to the genetically based postmating incompatibilities observed between these
young species, though this notion remains speculative.
Taken together, the collection of studies across diverse insects provides strong support for
ongoing hostÏ Wolbachia coevolutionary interactions. Because developmental processes such as
gametogenesis and embyrogenesis are likely to be altered by Wolbachia and subsequently accom-
modated by the host, it seems probable that evidence will eventually mount in favor of this model.
More detailed genetic experiments are now needed to dissect the host genes involved and their
potential association with hybrid incompatibilities.
Model 4 (Reinforcement)
The expression of CI can select for additional forms of reproductive isolation through reinforcement
Ð the process by which postmating isolation acts as a direct selective pressure for the evolution
of premating isolation in areas of sympatry (Dobzhansky, 1937; Noor, 1999). Reinforcement is
historically viewed as a means by which speciation will be completed (e.g., premating isolation
seals off any remaining gene Þow from incomplete postmating isolation). Premating isolation is
selected for because postmating isolation is a ÑwastelandÒ for parental gametes: since hybrid
offspring are dead or sterile, they cannot pass on genes themselves. Selection will consequently
favor parents that mate preferentially with homospeciÝc mates, thereby maximizing their potential
to pass genes into future generations. Despite the elegance of this reasoning, the reinforcement
hypothesis has been the subject of some controversy (Coyne and Orr, 1998; Noor, 1999). How
does Wolbachia Ýt into all this and perhaps provide new insight into reinforcement?
Consider two allopatric populations that have accrued independent genetic substitutions and
harbor different CI- Wolbachia strains. What would the fate of these incipient species be if they