Residues contribute to water pollution and pose a chal lenge to microalgal populations because numerous contami nants are toxic to microalgae, even in the micromolar range. Adaptation of microalgae from herbicide sensitivity to herbicide resistance was analysed by an ecological genetic approach, using the unice llu lar alga Dictyosphaerium chlorelloides (Chl orophyceae) as an experimental model. A dose-effect study showed that the Malthusian parameter under conditions of r selection in an uncrowded envi ronment and the carrying capacity under conditions of K selection in an crowded environment were both restricted even by low concentrations « I flM) of 3-(3 ,4-dichl orophenyl)-I,I -dimethyl urea (DCMU) herbicide. When a culture was treated with 50 flM DCMU, it cleared after a few days, as a result of destruction of sensitive cells by the herbicide. However, after further incubation for several days, the cul ture sometimes regained colour, owing to the growth of cell variants resistant to the herbicide. A fluctuation analysis was carried out to distinguish between ( 1 ) herbicide-resistant cells arising by direct and spec ific acquired adaptation in response to the herbicide and (2) herbici de-resistant cells arising by rare spontaneous mutations occurring randomly during replication of organisms prior to the incorporation of the herbicide. The fluctuation analysis unequivocally demonstrated that DCMU is not facilitating the development of DCMU -resistant cells; rather, we found that DCMU-resistant cells occur spontaneously by mutation in nonselective conditions prior to the incorporation of the herbicide (preselective mutations). The rate of spontaneous mutation from DCMU sensitivity to DCMU resistance was 2.2 X 1 0-6 mutants per cell division. Mutation was recurrent from a normal wild-type DCMU-sensitive allele to an DCMU-resistant allele, but such herbicide-resistant alleles were detrimental in terms of fitness in the absence of the herbicide. A competition experiment between wi ld-type DCMU-sensitive cells and DCMU-resistant mutants showed that, in small populations, the DCMU-resistant mutants are driven to extinction. The resistant variants are maintai ned in the absence of the herbicide as the res ult of a balance between new resistant cells arising by rare spontaneous mutation and resistant cells eliminated by natural selection. Tn our case, the average frequency of DCMU-resistant mutants in the absence of the DCMU is about five DCMU-resistant mutants per million cells. The results of our experimental model suggest that spontaneous mutation from herbicide sensitivity to herbicide resistance is sufficient in itself to assure the survival of mi­ croalgae populations in herbicide-contaminated envi ronments when the population size is large.

Peer reviewed