Figure 1. Phylogeny of Wolbachia and related groups derived from 1.3 kb of 16S rDNA sequence. Parsimony tree. All branches had 100% bootstrap support, except where indicated by numbers on branches. The red lines indicate representative Wolbachia lineages that have specialized in manipulating host reproduction. Wolbachia host genera are shown in parentheses. Wolbachia sequences used in this analysis were from infections in coleopteran (Diabrotica), dipteran (Drosophila), hymenopteran (Muscidifurax) and filarial worm (Brugia) hosts. Two closely related groups, the Ehrlichia and Rickettsia, are infectious disease-causing microbes of mammals (including humans). Crossed lines indicate two key biological differences: (1) Wolbachia have lost the ability to infect mammals and (2) have acquired the ability to alter host reproduction.

Figure 2. Diagrammatic representation of cytoplasmic incompatibility (CI). Two types are shown, uni- and bidirectional CI. In bidirectional CI one of two different strains of Wolbachia are present in two populations of Drosophila. In its most extreme form, bidirectional CI could result in complete reproductive isolation and subsequent speciation of the two populations. Note that removal of Wolbachia by antibiotic treatment results in the loss of CI showing that the infected populations are indeed the same species.

Figure 3. Fertilization and early diploid development in compatible crosses.

Figure 4. Fertilization and early embryonic defects in incompatible crosses. Sperm fertilize the egg apparently normally, but then cannot participate in the formation of the diploid embryo. Often the female nucleus will form a haploid nucleus that attempts to divide and populate the egg. Almost invariably these eggs die.