Biology Reference
In-Depth Information
A
B
Uninfected
taxa
C
D
E
Wolbachia
-infected
taxa
F
G
H
FIGURE 17.2
The hypothesized effect of
Wolbachia
on species phylogeny. If
Wolbachia
accelerate the
speciation process, then groups of organisms infected with
Wolbachia
(gray branches) may be more speciose
than related, uninfected groups (black branches).
Wolbachia
manipulate host sex ratios and reproductive strategies to gain a transmission advantage
into the next generation. By doing so, they may spread more rapidly through populations than does
the average incompatibility gene. Therefore, reproductive isolation associated with a selÝsh genetic
element like
may require fewer generations on average to evolve than genetically based
reproductive isolation, which is thought to arise through the gradual accumulation of several
isolating barriers (Coyne and Orr, 1997). The rapidity with which reproductive isolation evolves
is important when considering rates of speciation and the amount of time two allopatric populations
have before coming back into contact in sympatry, where they could potentially interbreed. If
Wolbachia
Wolbachia
indeed accelerate the speciation process, then groups of organisms harboring
Wolbachia
are predicted to be more speciose than closely related uninfected groups (Werren, 1998; Figure
17.2). Data are currently not complete enough to address this prediction.
CYTOPLASMIC INCOMPATIBILITY AND SPECIATION
. CI is a
post-fertilization incompatibility that typically leads to F1 inviability between infected males and
uninfected females, or females harboring a strain of
Most of the recent progress on
Wolbachia
and speciation has centered on CI-
Wolbachia
different than that of the male
(Figure 17.1).
Because CI reduces or eliminates the production of F1 hybrids, it can hinder gene
Þow among hybridizing populations. The phenomenon was Ýrst described, without knowledge of
its infectious nature, in the mosquito
Wolbachia
by the German scientist Hans Laven (1951). He
fortuitously came upon CI in intraspeciÝc crosses among geographic races (some only miles apart)
that showed complex incompatibility relationships. In fact, he found approximately 15 different
Ñcrossing typesÒ within the
C. pipiens
complex (Laven, 1959). He viewed these crossing types as
evidence that species could arise without morphological divergence. Laven also found that the
causal factor of this incompatibility was not nuclear but cytoplasmic, suggesting that the cytoplasm
may play a more important role in animal speciation than previously thought. Thus, the Ýrst work
to link CI and speciation coincided with the discovery of the CI phenomenon. It would only be
realized some 12 years later that the causative agent of CI was
C. pipiens
Wolbachia
(Yen and Barr, 1971).
T
F
CI
WO
ORMS
OF
In his early experiments, Laven observed the two major forms of CI: unidirectional and bidirectional
(Figure 17.1). Unidirectional CI results in crosses between an infected male and uninfected female.
Although the precise molecular mechanisms underlying CI are unknown, the cytological effects of
CI are clear and occur in various insect species. For example, in both
Drosophila
and the parasitic
wasp
, sperm from an infected male fertilize the uninfected egg, but the paternal chromo-
somes do not undergo proper condensation during early mitotic divisions of the egg and are not
Nasonia
