Biology Reference
In-Depth Information
Wolbachia
-free status determines in females a net decrease (17.7%) in realized fecundity, which
combines offspring production and longevity of females and also reduces longevity in males
(Dobson et al., 2002). All individuals of the naturally infected line were infected with two distinct
Wolbachia
strains, which also induce CI (Sinkins et al., 1995). BeneÝcial
Wolbachia
strains are
here also able to induce CI, which is considered a reproductive alteration, and for the Ýrst time
Wolbachia
infection was shown to be involved in both mutualism and reproductive parasitism. This
case perfectly illustrates the hypothesis that microorganisms simultaneously exhibiting both strat-
egies can be expected. However, we do not know so far whether the two co-occurring
Wolbachia
strains are each specialized or are equally involved in both effects.
Other less clear or controversial examples of facultative beneÝcial
Wolbachia
have been
reported. In the pteromalid wasp
Nasonia vitripennis
, removal of infection presumably reduced
fecundity of uninfected females, suggesting a positive effect of infection on host Ýtness (Stolk and
Stouthamer, 1996). However, the signiÝcance of this result was later refuted due to possible
confusion of infection effects with differences in genetic background of strains (Bordenstein and
Werren, 2000). In
Drosophila simulans
, a decrease in female fecundity following elimination of
Wolbachia
was equally observed by Poinsot and Merot (1997), but the effect was not permanent
in the course of generations, making conclusions difÝcult. Finally, two intriguing cases were
independently found where
Wolbachia
infection increased fertility in males. In the Þour beetle
Tribolium confusum
,
Wolbachia
enhanced male fertility at the expense of other Ýtness components,
including viability and female fecundity (Wade and Chang, 1995). Because
Wolbachia
also induces
CI in this species, the male fertility effect was interpreted as a way of accelerating
Wolbachia
spread
through the host population. Such an advantage for male fertility was also reported by Hariri et al.
(1998) in the Þy
Sphyracephala beccarii
(Diopsidae), but in this case
Wolbachia
did not induce
CI, and interpretations are unclear.
O
BLIGATORY
W
OLBACHIA
IN
I
NSECTS
Obligatory
Wolbachia
-Induced Parthenogenesis
Among reproductive manipulations caused by
Wolbachia
, thelytokous parthenogenesis has been
observed in certain haplodiploid parasitic wasps (Stouthamer, 1997; Huigens and Stouthamer,
Chapter 15
of this topic). In these species, infection permits females to produce all-female
offspring without being fertilized. Because
Wolbachia
are maternally inherited, elimination of
males procures a high selective advantage for
Wolbachia
. In some species, this effect is so strong
that males do not exist in natural populations and can be induced only by antibiotic treatments
that remove
Wolbachia
infection (Stouthamer et al., 1990). Except for species of the genus
Trichogramma
, these males fail to produce offspring with conspeciÝc females and consequently
cannot transmit their genes. Gottlieb and Zchori-Fein (2001) reviewed different reproductive
barriers that could occur in crosses between antibiotic-induced males with conspeciÝc females.
Disorders of sexual functionality vary among species. In
Encarsia formosa
(Aphelinidae), anti-
biotic-induced males produce sperm and sometimes mate with conspeciÝc females, but insemi-
nation fails (Zchori-Fein et al., 1992). A more dramatically degenerate state occurs in
Muscidi-
furax uniraptor
(Pteromalidae), where males are incapable of producing sperm and females are
not sexually receptive. Moreover, females are totally devoid of a muscle in the spermathecae,
which plays a major role both in drawing sperm into the spermathecal reservoir after copulation
and in egg fertilization (Gottlieb and Zchori-Fein, 2001).
In all cases, the reproductive barrier between sexes is complete and irreversible, and establish-
ment of stable sexual lines from asexual ones is impossible. Thus,
Wolbachia
-induced partheno-
genesis is the only possible mode of reproduction, making
Wolbachia
an obligatory symbiotic
partner. Such host dependence could have arisen as follows: in infected wasps, parthenogenesis
has made sexual traits useless, and the absence of selective pressures on them has permitted
