Symbiont-mediated Cytoplasmic Incompatibility:What Have We Learned in 50 years?

Cytoplasmic incompatibility (CI) is the most common symbiont-induced reproductive manipulation. Specifically, symbiont-induced sperm modifications cause catastrophic mitotic defects in the fertilized embryo and ensuing lethality in crosses between symbiotic males and either aposymbiotic females or females harboring a different symbiont strain. However, if the female carries the same symbiont strain, then embryos develop properly, which imparts a relative fitness benefit to symbiont-transmitting mothers. Thus, CI drives maternally transmitted bacteria to high frequencies in arthropod species worldwide. In the past two decades, CI has experienced a boom in interest due in part to its (i) deployment in successful, worldwide efforts to reduce the spread of mosquito-borne diseases, (ii) causation by bacteriophage genes, cifA and cifB, that modify animal reproductive processes, and (iii) important impacts on incipient speciation. This review serves as a gateway to experimental, conceptual, and quantitative themes of CI and outlines significant gaps in our understanding of CI’s mechanism that are ripe for investigation from a diversity of subdisciplines in the life sciences.