Fig. 4.

Red Queen effects in the evolutionary diversification of glycans. Each arrowed circle represents a potential evolutionary vicious cycle, driven by a Red Queen effect, in which hosts are constantly trying to evade the more rapidly evolving pathogens that infect them. Hosts require glycans for critical cellular functions but must constantly change them to evade glycan-binding pathogens, and yet do so without impairing their own fitness. Hosts also produce soluble glycans such as mucins, which act as decoys to divert pathogens from cell surfaces; but pathogens are constantly adjusting to these defenses. Hosts recognize pathogen-specific glycans as markers of “non-self,” but pathogens can modify their glycans to more closely mimic host glycans. There are also possible secondary Red Queen effects involving host glycan-binding proteins that recognize “self”. In each of these cycles, hosts with altered glycans that can still carry out adequate cellular functions are most likely to survive. Reproduced with permission from Varki A. 2006. Cell. 126:841–845. Copyright Elsevier.