Theoretical questions about the evolution of symbionts, 
selfish genetic elements, and the parliament of genes
Peter Hammerstein
Innovationskolleg Theoretische Biologie
Humboldt-Universität zu Berlin
Invalidenstr. 43, 10115 Berlin
  p.hammerstein@itb.biologie.hu-berlin.de

        
Competition for survival and reproduction is the key idea in Darwin's  theory of natural selection. Therefore, it seems easy to understand why conflict frequently occurs in the world of organisms. In contrast, the central role of competition often makes it difficult to understand cooperative phenomena in the living world. Yet, the mere existence of organisms demonstrates that evolution can lead to very powerful forms 
of cooperation. Cells cooperate with one another and within cells endosymbionts act to the benefit of the cell. Major obstacles probably had to be overcome in order to achieve this high degree of organismic integration. In order to study these obstacles, it pays to study bacteria with cytoplasmic transmission like that of mitochondria. The genus Wolbachia demonstrates in various ways that there can be major conflict between symbiont and host despite vertical transmission of the symbiont. Different variants of this bacterium induce sex change, parthenogenesis and cytoplasmic incompatibility in the host. Some of the manipulations resemble that of extremely selfish genetic elements, such as the B-chromosome PSR. Unlike PSR, however, Wolbachia is a very 
common passenger of arthropod hosts. This poses a difficult question: what explains this overwhelming success of Wolbachia as a manipulator and why have hosts not developed the means to counter its manipulation? Why does the so called parliament of genes fail to efficiently "outvote" the genes of this cytoplasmic agent?

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