Potential for selection among nearly neutral allozymes of 6-phosphogluconate dehydrogenase in Escherichia coli.

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Abstract
Six gnd alleles coding for naturally occurring allozymes of 6-phosphogluconate dehydrogenase [6-phospho-D-gluconate:NAD(P)+ 2-oxidoreductase, EC 1.1.1.43] were transferred by transduction into the genetic background of E. coli K-12 and examined for their selective effects in chemostats in which gluconate was limiting. Four of the alleles are evidently neutral or nearly neutral, inasmuch as their selective effects, if any, fall below the limit of resolution of the procedure, 0.5%/h or .apprx. 1%/generation. One allele is detrimental in limiting gluconate but not in limiting glucose or fructose. Another allele has a detrimental, density-dependent, epistatic interaction with tonA. All 6 alleles are neutral or nearly neutral in natural populations but they are not functionally equivalent; their functional differences are potentially important because they can become expressed as differences in fitness under the appropriate conditions of environment or genetic background. Under these conditions, otherwise neutral alleles can become subject to selection.