PROTEIN EVOLUTION IN COMPLEX BACTERIAL ENVIRONMENTS

How does environmental complexity affect protein evolution? We measured the effects of a set of mutants of Bacillus subtilis glutamate dehydrogenase across 19 different environments – from homogenous single cell populations in liquid media to heterogeneous biofilms, plant roots and soil. The effects of protein mutations on organismal fitness were reproducible in liquid cultures. However, 84% of the tested alleles showed opposing effects under different carbon and nitrogen sources (antagonistic pleiotropy). In biofilms and soil, drift followed selection resulting in different protein alleles fixating in replica experiments. Thus, per individual condition, by the combined action of selection and chance, a wide range of protein mutations persisted and sometimes fixated. However, due to pleiotropy, across longer periods and multiple environments nearly all this diversity would be lost.