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horizontally, the Ýrst stage of infection is represented by their invasion into the epithelium of the
gastric caeca, from which they invade oenocytes. In newly infected mosquitoes, the microsporidians
typically develop in a benign manner in both sexes and are vertically transmitted to the next
generation through the eggs. Only in this next generation will male killing arise (Bechnel and
Sweeney, 1990).
Here the death of the male is associated with the rupturing of the cuticle during the release of
infective spores in its later instars. The potential for horizontal transmission may explain why males
are killed late in development. After the fourth instar, mosquitoes pupate in the water, after which
adult mosquitoes are only rarely associated with the water. In addition, the hard pupal case can be
a barrier for the guest microbes to escape from their host to water to seek a new host. Thus, the
fourth instar would be the last opportunity for them to escape from the male host, through which
no vertical transmission is possible. The question then arises of why they do not escape from males
earlier. This is probably because of an optimizing strategy. Given that vertical transmission is not
possible in males, the guest microbes wait until the host is as large as possible before entering the
full-blown infectious state, thereby maximizing spore number (Hurst, 1991). This strategy some-
what resembles that of certain parasitoid wasps in which wasps manipulate the development of
lepidopteran larvae and escape from them before pupation after fully exploiting the constituents of
larval bodies.
W OLBACHIA
Of the several known guest microbes, the one that now commands the attention in almost all areas
of the life sciences, from sociobiology to molecular biology, is Wolbachia . Wolbachia are strictly
intracellular bacteria infecting a number of invertebrates including mites, crustaceans, Ýlarial
nematodes, and especially insects (Werren and OÔNeill, 1997). A recent survey showed that about
76% of arbitrarily chosen insect species were infected with these guest microbes, making this group
one of the most ubiquitous endosymbionts described to date (Jeyaprakash and Hoy, 2000). Mater-
nally transmitted through the cytoplasm of eggs, these endosymbionts form a monophyletic group
relative to other b-proteobacteria, particularly to other Rickettsia , causing human diseases such as
typhus, Rocky Mountain spotted fever, and Q fever. In arthropods, they are distinguished by their
ability to modify host reproduction in a variety of ways, such as cytoplasmic incompatibility (CI)
in most species (Hoffmann and Turelli, 1997), thelytokous parthenogenesis in haplodiploid species
(Stouthamer et al., 1990), male killing in several insects (Hurst et al., 1999), and feminization of
genetic males in isopod crustaceans (Rigaud, 1997). All these effects are advantageous to Wolbachia
and allow them to persist and spread their share in host populations (Hoffmann and Turelli, 1997;
Rigaud, 1997). The present discussion is limited to a brief introduction of CI and the evolution and
molecular biology of Wolbachia ; other interesting phenomena caused by Wolbachia are described
in detail in later chapters of this topic.
CI occurs when crosses between Wolbachia -infected males and uninfected females fail to
produce progeny, whereas both crosses between uninfected males and infected females and those
between infected males and females are fertile (Yen and Barr, 1971). This suggests that the
Wolbachia infection interferes with the function of sperm or the male chromosomes and that an
infected oocyte is essential to rescuing proper function of the male gamete. In some insect species,
chromosomes from the sperm of Wolbachia -infected males fail to condense in the cytoplasm of an
uninfected egg but do condense in that of an infected egg (Breeuwer and Werren, 1990; OÔNeill
and Karr, 1990; Montchamp-Moreau et al., 1991). In infected insects, Wolbachia are detected in
nearly all the tissues but not in the sperm. The molecular mechanisms by which Wolbachia modify
chromosomes of the sperm of the infected males and how the modiÝed chromosomes are rescued
in the egg cytoplasm harboring Wolbachia are still largely unknown.
From an evolutionary point of view, incompatibility infections of Wolbachia in Culex mosqui-
toes are much more interesting. In Culex mosquitoes, no naturally occurring uninfected individuals
 
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