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Fig. 4. Model of how Wolbachia exploits the unique structure of the gonomeric spindle to produce both embryonic mortality and conversion to haploid male development in Nasonia. In control embryos, the maternal (red) and paternal (blue) genomes align in separate regions of the metaphase plate. Both sets of chromosomes are segregated equally to produce two diploid nuclei, where the maternal and paternal genomes mingle for the first time. In CI embryos, the paternal genome is somehow modified by Wolbachia and this causes its segregation at anaphase to be aberrant. One explanation for the range in segregation behavior of the paternal genome is that the level of Wolbachia modification varies from high to low. When Wolbachia modification is high, the paternal genome is not segregated and two haploid nuclei of maternal origin (two red nuclei) are produced (category 2). These embryos develop into males. When Wolbachia modification is moderate to low, the paternal genome is segregated abnormally. The paternal genome is either wholly segregated to one side of the spindle (category 3), producing one haploid nucleus of maternal origin (red nucleus) and one abnormal nucleus of maternal and paternal origin (red-blue nucleus); or the paternal genome is unequally segregated to both daughter nuclei (category 4), producing two nuclei that contain the maternal genome and unequal quantities of the paternal genome. Often the resulting nuclei are connected by chromatin bridges. Category 3 embryos continue development into a haploid male whereas category 4 embryos arrest development in early embryogenesis.
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