Cytoplasmic Incompatibility - Insect Symbiosis - page 158

FIGURE 14.1 (Color Ýgure follows p. 206.) Crossing types of Wolbachia infections. Red, blue, and violet
represent different Wolbachia strains. All infected strains are unidirectionally incompatible with uninfected
strains; this is shown only for the red strain. The red and blue strains are unidirectionally incompatible with
each other. Females of the red strain cannot rescue the modiÝcation of the blue strain, whereas females of
the blue strain can rescue the modiÝcation of the red strain. The violet strain is bidirectionally incompatible
with both the red and the blue strain; neither strain can rescue the otherÔs modiÝcation. Females of the double-
infected redÏblue strain can rescue sperm from double-infected, both single-infected and the uninfected strains.
given strain failed to produce progeny when mated with females from other strains, while the
reverse cross was compatible. Laven further found that in some cases, this reproductive isolation
occurred in crosses of both directions. They both concluded that a maternally inherited cytoplas-
mic factor was responsible for these incompatibilities. CI results in embryonic mortality in crosses
between insects with different Wolbachia infection status (Figure 14.1). It can be either unidi-
rectional or bidirectional. Unidirectional CI is typically expressed when an infected male is
crossed with an uninfected female. The reciprocal cross (infected female and uninfected male)
is fully compatible, as are crosses between infected individuals. Bidirectional CI usually occurs
in crosses between infected individuals harboring different strains of Wolbachia . In most insects,
the expression of CI is lethal to the developing embryo, but in insects with haplodiploid sex
determination (Hymenoptera) the end result of CI can be a sex-ratio shift to the haploid sex,
which is male. As a consequence of CI, Wolbachia infections can spread and remain in nature
by replacing uninfected populations, because infected females can successfully mate with both
infected and uninfected males, while uninfected females can only successfully mate with unin-
fected males. Several factors other than CI can affect invasion dynamics and are discussed
elsewhere (see Chapter 13).
D ISTRIBUTION
Wolbachia -induced CI has been reported in almost all major insect orders (Diptera, Coleoptera,
Hemiptera, Hymenoptera, Orthoptera, and Lepidoptera) as well as in Arachnida and Isopoda. As
mentioned above, Wolbachia -induced CI was Ýrst reported in the order Diptera, in C. pipiens in
the family Culicidae (e.g., Ghelelovitch, 1952; Yen and Barr 1973, 1974; Trpis et al., 1981;