Figure 2.

Potential evolutionary consequences of increased host reliance on microbial metabolite production in the gut and liver. These examples are specific to vertebrate physiology, though similar processes may occur in other animal groups. (a) When hosts become more reliant on microbially produced SCFAs as an energy source, they may evolve increased expression of both SCFA transporters in the gut and SCFA metabolizing enzymes in the liver, an example of a collaborative process. (b) When hosts become more reliant on microbially produced vitamins, there may be relaxed selection on host processes that produce these compounds. This would result in an increased concentration of microbially produced vitamins in the gut and a decreased expression of host vitamin producing enzymes in the liver. (Online version in colour.)