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occurrence of genetic recombination among strains (Jiggins et al., 2001; Werren and Bartos, 2001).
In nematode Wolbachia , phylogenies based on three different genes are congruent (Casiraghi et al.,
2001). For a detectable recombination this must occur between strains showing differences in their
gene sequences; this requires that an individual host become infected with two or more strains.
Where transmission is strictly vertical, and where the bacteria that are vertically transmitted form
a clone, recombination, if it occurs, will not be detectable through the comparison of phylogenies
based on different genes. There are thus three possibilities for nematode Wolbachia : (1) recombi-
nation occurs, but we do not detect it because of the exchange of genetic material among a vertically
transmitted, clonal population of bacteria; (2) the capacity of undergoing recombination has been
lost because of weaker selection for maintaining this capacity among members of a clone; or (3)
recombination simply does not occur, even where chances for exchanging genetic material among
not-too-closely-related bacteria exist. The Ýrst two options imply that the chances for multiple
Wolbachia infection of a single nematode host (and thus of horizontal, in addition to vertical,
transmission to occur) are (or have been) very low. The third option appears the most unlikely:
why should nematode Wolbachia have lost the capacity to exchange genetic material if this can
occur among not-too-closely-related bacteria? We should emphasize that sequence analysis of
cloned polymerase chain reaction (PCR) products of Wolbachia genes, obtained from individual
nematode hosts, has not provided any evidence so far for genetic variation (Bandi et al., 1999; C.
Bandi, unpublished observation).
In conclusion, based on the available information (consistency of host and symbiont phyloge-
nies, no evidence for genetic recombination, and no evidence for Wolbachia polymorphism in
individual nematodes), it is reasonable to assume that horizontal transmission in nematode Wolba-
chia does not occur or, if it does, involves only conspeciÝc nematodes, harboring very closely
related Wolbachia strains.
BENEFICIAL AND OBLIGATORY WOLBACHIA IN INSECTS
BeneÝcial and obligatory Wolbachia infections in insects are poorly documented and have never
been reviewed. Here we present all the cases, to our knowledge, where Wolbachia have either a
positive effect on host fecundity or an obligatory role for insect reproduction.
F ACULTATIVE W OLBACHIA I NFECTIONS T HAT E NHANCE I NSECT F ECUNDITY
The Ýrst beneÝcial Wolbachia was described in a wasp species of the genus Trichogramma
(Hymenoptera: Pteromalidae). Trichogramma are common parasitoids that infest numerous Lepi-
doptera species, including several important pests. Females oviposit into the eggs of their hosts,
within which parasitic larvae feed and develop into adult wasps. Comparing offspring production
in two Moroccan strains of T. bourarachae , Mimouni (1991) established that the higher fecundity
of the ÑhighÒ strain (61.3 ° 1.4 per female during 5 days) was maternally inherited when crossed
and backcrossed with the ÑlowÒ strain, which produced only 25.8 ° 1.8 descent. Based on their
demonstration that antibiotic treatments decreased fecundity of ÑHÒ females and did not affect ÑLÒ
ones and supporting their hypothesis with microscopic observations, Girin and Boultreau (1995)
showed that a maternally inherited microbe infecting the H strain could account for its phenotype.
Using PCR with Wolbachia -speciÝc primers, Vavre et al. (1999a) demonstrated the role of a single
particular Wolbachia located in the A supergroup, which thus appeared as a facultative fecundity
enhancer that did not induce CI.
Another case of beneÝcial Wolbachia infection was recently described in mosquitoes. Mutual-
istic Wolbachia infection was suspected in Culex pipiens (Awahmukalah and Brooks, 1985) and
Aedes albopictus (Dobson et al., 2001; Dobson and Rattanadechkul, 2001) and has been clearly
demonstrated in A. albopictus by Dobson et al. (2002). The authors demonstrated reduced perfor-
mances in an aposymbiotic line derived from a naturally infected one (from Houston, TX). The
 
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