Genetic analysis of copper resistance in Paramecium aurelia syngen 4.

Measurement of the median tolerance limit to cupric ion of 28 wild stocks of Paramecium aurelia syngen 4 revealed a phenotypic polymorphism: two stocks showed a much greater resistance than the others. Genetic analysis showed the resistant phenotype was produced by a recessive allele, cur, identical in both resistant wild stocks. No modification of the phenotypic expression of this allele could be detected after backcrossing into different genetic backgrounds. The results are interpreted to support the importance of mutational adaptation in this inbreeding species. Possible patterns of internal genetic organization of species which rely on mutation for adaptation are discussed.