Biology Reference
In-Depth Information
infection now sweep into the susceptible uninfected population? Or would individuals from the
two populations not exchange genes, at least in part due to unidirectional CI, where an infected
male and uninfected female fail to produce hybrids? The answers partly depend on the amount of
genetically based reproductive isolation that has accrued since the split of these populations. For
example, considerable gene Þow will clearly not allow stable coexistence of infected and uninfected
populations because the infected cytoplasm will sweep through (Turelli, 1994). Additionally, it also
depends on how strong natural selection (e.g., disruptive selection) opposes migration and the
spread of ÑforeignÒ genes (Telschow et al., 2002a). While theoretical and experimental evolution
studies are poised to signiÝcantly enhance our understanding of these issues, empirical studies of
natural systems are now motivating the questions and offering us a glimpse into how unidirectional
CI can promote the origin of new species.
One recent study is largely responsible for this new outlook on CI-assisted speciation.
Shoemaker et al. (1999) characterized pre- and postmating isolation in the mushroom-feeding
species pair
D. recens
and
D. subquinaria
.
D. recens
is infected with
Wolbachia
, while the closely
related
D. subquinaria
is uninfected. Levels of mitochondrial diversity in
D. recens
are reduced
but consistent with a sweep of
Wolbachia
through this species in the distant evolutionary past.
What is most intriguing about these species is that their ranges are likely to overlap or have
overlapped in the past in the north-central part of the United States or central Canada. Areas of
overlap create opportunities for interspeciÝc hybridization, and genetic evidence indeed suggests
that hybridization has occurred between these species (Shoemaker et al., 1999, and unpublished).
But as
Table 17.3
shows, interspeciÝc gene Þow could be severely reduced due to the comple-
mentary action of three main isolating barriers: unidirectional CI, sexual isolation, and hybrid
male sterility, the latter two being genetically based. One key feature in this species pair is that
unidirectional CI and sexual isolation act asymmetrically but in opposite directions. Thus, instead
of gene Þow being conÝned to one cross direction, as would be expected with the existence of
just one of these isolating barriers (or if they both operated in the same direction), gene Þow is
limited in both directions. This pattern provides just the right Ýt for a more stable coexistence
of the infected and uninfected species. And if that is not enough, hybrid male sterility awaits the
surviving males produced in the F1 generation. The assortment of symbiotic and genetically
based isolating barriers provides clear evidence that
Wolbachia
can act in concert with other
barriers and still be essential to the speciation process. Whether it is essential in this
Drosophila
TABLE 17.3
The Percent of Fit Hybrids between
Drosophila recens
and
D. subquinaria
Is Successively
Reduced When Multiple Isolating Barriers Are Considered
Sexual Isolation +
Unidirectional
CI (%)
Sexual Isolation +
Unidirectional CI + Hybrid
Male Sterility (%)
Cross
(male x female)
No Isolation
(%)
Sexual
Isolation (%)
D. recens
D. subquinaria
100.0
69.3
10.1
5.1
D. subquinaria
D. recens
100.0
28.9
28.9
14.5
Data are modiÝed from laboratory measurements of reproductive isolation in Shoemaker et al. (1999). Percent Ýt hybrids
is estimated by multiplying the strength of each isolating barrier considered (sexual isolation is calculated by dividing
the interspeciÝc mating frequency by the intraspeciÝc mating frequency with relation to the same species female;
unidirectional CI between an infected
D. recens
male and uninfected
D. subquinaria
female reduces hybrid production
to 14.6%; hybrid male sterility is complete and therefore reduces the percent of surviving, fertile F1 offspring by 50%,
assuming a 1:1 sex ratio).
