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infection, coadaptation, and reciprocal dependence between partners (i.e., obligatory symbiosis)
(Lipsitch et al., 1995). Inversely, in reproductive parasitism the symbiont can spread, even though
it has a detrimental effect on infected females. Moreover, the ÑmanipulationsÒ performed by
reproductive parasites are detrimental to those individuals that do not transmit them; this obviously
implies a decrease of the Ýtness of individuals in the host population. For these reasons, selective
pressures acting on each partner can be antagonistic and lead to conÞict, unstable association, and
sometimes loss of infection (Hurst and McVean, 1996; Rigaud, 1997; Vavre et al., in press).
Interestingly, the strictly intracellular
bacteria have adopted both strategies in
their invertebrate hosts. In Ýlarial nematodes, the hosts seem to be dependent on the infection.
Indeed, the removal of bacteria has detrimental effects on development, survival, and reproduc-
tion (Bandi et al., 2001b). Inversely, in arthropods,
Wolbachia
infections most often induce
reproductive alterations, including cytoplasmic incompatibility (CI), male killing, feminization,
and parthenogenesis (Werren and OÔNeill, 1997). Assuming that
Wolbachia
in nematodes and
arthropods share a common symbiotic ancestor, the evolutionary interpretation of such diversity
in
Wolbachia
lifestyles is not clearly understood. Moreover, obligatory symbiosis was recently
found in insects, suggesting that host differences between arthropods and nematodes are not
sufÝcient to explain this diversity. The goal of this chapter is to underline those differences
between obligatory and
Wolbachia
infections to try to answer the following two
questions: (1) What are the main conditions required for a
facultative
Wolbachia
infection to evolve toward
obligatory symbiosis? (2) What are the evolutionary mechanisms capable of generating such
evolutionary transitions?
In the Ýrst part of the chapter, we summarize basic information and focus on those aspects of
symbiosis between
Wolbachia
and Ýlarial nematodes that could help understand how an association
with these bacteria could evolve to obligatory symbiosis.
In the second part, we present those cases in insects where
Wolbachia
infection is either
beneÝcial (but facultative) or obligatory for host reproduction. Examples of these associations are
few and poorly documented, but they raise important evolutionary questions.
In the Ýnal part, we propose possible hypotheses on the conditions required for a
Wolbachia
Wolbachia
infection to evolve in an obligatory symbiosis. In particular, we focus on the roles of horizontal
transfer, multiple
Wolbachia
infections, and bacterial virulence in this evolution. Finally, we spec-
ulate on the signiÝcance of
Wolbachia
obligatory symbiosis in the mutualism concept.
OBLIGATORY
WOLBACHIA
IN FILARIAL NEMATODES
Several recent reviews have been published on the
endosymbionts of Ýlarial nema-
todes (Taylor and Hoerauf, 1999; Bandi et al., 1999, 2001b). Owing to space constraints, not
all the original work up to the beginning of 2001 will be cited, and the reader is referred to the
above reviews.
Wolbachia
A S
C
I
UMMARY
OF
URRENT
NFORMATION
(AÏF) have thus far been described on the basis of branching and
clustering patterns in unrooted phylogenetic trees derived from 16S rDNA and
Six supergroups of
Wolbachia
ftsZ
gene sequences
(Lo et al., 2002). The majority of nematode
Wolbachia
have been assigned to supergoups C and
D. There is, however, one species of Ýlaria (
has not been
precisely assigned to any of the six supergroups (Lo et al., 2002). Supergroups A, B, E, and F
encompass the
Mansonella ozzardi
) whose
Wolbachia
Wolbachia
of arthropods. The branching order of these six supergroups of
Wolbachia
is not clear, and some of these groups might be paraphyletic. It is, however, clear that all the
Wolbachia
thus far found in nematodes are phylogenetically very distant from any of the
Wolbachia
thus far found in arthropods. There is therefore no evidence for current transmission of
Wolbachia
from arthropods to nematodes, or vice versa.
 
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