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The maintenance of alcohol dehydrogenase polymorphism in Bromus mollis L.

Microgeographic differentiation was studied in a natural population of soft bromegrass (B. mollis) located in an environment that was conspicuously variable for soil moisture. In particular, we examined the effect of this environmental variable on the distribution of alleles that specified electro-phoreticaIIy distinct forms, or aIIozymes, of alcohol dehydrogenase (EC 1.1.1.1). The major features of subpopulation structure found were (1) phenotypic differentiation among sites for measurement characters, (2) a consistent and large difference in plant height between wet and dry sites, (3) no significant differentiation for alcohol dehydrogenase aIIozyme frequencies, (4) substantial differences in allozyme frequencies at three other loci affecting enzymes that are apparently unrelated to anaerobiosis, (5) higher levels of outcrossing in the wetter sites, and (6) an excess of heterozygotes at each of the six aIIozyme loci, but this excess was restricted to the dry sites. These findings are discussed in relation to the problem of determining selective forces which act on individual aIIozyme variants, and which, because of the coherent genetic system in this species, may be confounded with other modes of selection.