Biology Reference
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have evidence that suggests that this is not the case. First, the phylogeny of
Wolbachia
matches
that of Ýlarial nematodes (Casiraghi et al., 2001). This indicates that
cospeciated with
the host during the evolutionary radiation of Ýlarial nematodes. Second, arthropod and nematode
Wolbachia
Wolbachia
tree, the
internal branches are quite short compared to the length of the subtrees of arthropod and nematode
appear to have originated through a star-like radiation: in the global
Wolbachia
Wolbachia
(Lo et al., 2002). This suggests that the time elapsed between the split of arthropod and
in Ýlarial nematodes
was quite short. This suggests that there has not been enough time for nematode
nematode
Wolbachia
and the start of the evolutionary radiation of
Wolbachia
to have
a signiÝcant evolutionary radiation in nematodes other than the Ýlariae after its separation from
arthropod
Wolbachia
Wolbachia
. In other words, based on the genes thus far used for global phylogenetics of
Wolbachia
appear approximately equidistant
at the nucleotide level. If we assume that the rate of molecular evolution has been even roughly
constant in
(
ftsZ
and 16S rDNA), the six supergroups of
Wolbachia
moved from an arthropod to a close
ancestor of the Ýlariae or, conversely, from this hypothetical close ancestor of the Ýlariae to some
ancestral arthropod. The widespread presence in arthropods of bacteria phylogenetically related to
Wolbachia
, we should conclude that
Wolbachia
Wolbachia
(i.e., the various alpha-proteobacteria) makes more realistic the hypothesis that
Wolba-
chia
was originally present in arthropods and then moved to Ýlarial nematodes. That genome size
of arthropod
Wolbachia
is around 1.5 megabases (Mb), compared to around 1 Mb in the nematode
Wolbachia
(Sun et al., 2001), also points to the ancestral status of the former to the latter. In
summary, there is overall evidence that suggests that the ÑcharacterÒ
was acquired during
the Ýrst steps of the evolutionary radiation of Ýlarial nematodes. Consistent with this hypothesis
is the absence of
Wolbachia
in nematodes closely related to the Ýlariae, the Thelazidae (Bandi,
unpublished observation). Furthermore,
Wolbachia
is absent in those Ýlarial species that appear to
represent deep branches in the evolution of the Ýlariae (Casiraghi et al., 2001; C. Bandi, unpublished
observation). Should
Wolbachia
be found in nematodes phylogenetically distant from the Ýlariae,
this could represent the result of an infection event independent of the one that led to Ýlarial
infection or from horizontal transmission from a Ýlaria to another nematode (but, of course, future
phylogenetic evidence could prove otherwise).
Wolbachia
V
. H
T
; S
EX
VS
. N
O
S
EX
ERTICAL
VS
ORIZONTAL
RANSMISSION
There is no evidence for horizontal transmission of
Wolbachia
in Ýlarial nematodes. There is no
evidence for polymorphism in the presence/absence of
Wolbachia
in a given species of Ýlarial (see
above and Bandi et al., 2001b; Casiraghi et al., 2001). Lack of evidence for something does not,
of course, rule it out. However, based on the biology of Ýlariae, it would be difÝcult to hypothesize
how horizontal transmission of
Wolbachia
could occur in these nematodes. In arthropods are found
a wealth of parasites, parasitoids, and predators of other arthropods, members of different species
living in close contact, with chances for individuals to be mechanically injured (which perhaps
made the transmission of microorganisms through the hemolymph possible), and detritivory and
coprophagy. In summary, among arthropods can be found all the kinds of Ñdangerous liaisonsÒ and
Ñunsanitary behaviorsÒ that provide a Ñroad networkÒ for the transmission of microorganisms
including the intracellular ones like
Wolbachia
. In Ýlarial nematodes, life is probably more Ñclean
and protectedÒ: localization of different species of Ýlariae is usually quite different in the vertebrate
host; chances for mechanical injuries are low; in the case of injuries within the host, fragments of
the nematode (including
Wolbachia
) are likely to be quickly removed by the host immune system;
parasites or parasitoids of Ýlarial nematodes that could transmit microorganisms among individuals
are not known. Microorganisms are thus unlikely to have many chances for horizontal transmission
in Ýlarial nematodes, in particular among members of different species.
The lack of evidence for recombination in Ýlarial
Wolbachia
further argues against frequent
occurrence of horizontal transmission. In arthropod
Wolbachia
, disagreement among phylogenies
based on different genes (or on different portions of the same gene) provides evidence for the
