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FIG. 12. Models of pathway evolution in endosymbionts. Several theories of pathway evolution (left side) and their endosymbiontic modification (right side) are shown. Pathways may have evolved (bold arrows) in different ways: (a) de novo (all reactions evolved independently from an unknown origin), (b) backwards (retroevolution), (c) by specialization of a multifunctional enzyme (left side, with multiple substrates in blue), (d) duplication of the complete pathway, or (e) by recruitment of enzymes from different pathways. The scenarios shown on the right evolve with specific modifications in endosymbionts; selection pressure is sometimes in the opposite direction, pathway reductions occur to a large extent, and selected pathways evolve, e.g., to provide metabolites useful for the host.
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