Phenotypic evolution in microalgae: a dramatic morphological shift in Dictyosphaerium chlorelloides (Chlorophyta) after exposure to TNT

Abstract

The occurrence of rapid morphological evolution in the microalga Dictyosphaerium chlorelloides (Chlorophyta) was induced after exposure of the wild strains of the alga to the potent algal poison 2,4,6-trinitrotoluene (TNT). After exposing a wild- type population (consisted of representative spherical-ellipsoidal cells) to doses of TNT that were lethal for most of the cells, a TNT-resistant spindle-shaped mutant was able to proliferate. This spindle- shaped mutant appeared spontaneously by rare mutations before the selective treatment: in deed, it was already present in the wild population. The frequency of the mutants within the ancestral population seemed to be determined by the balance between the rate of accumulation by recurrent mutation and the rate of elimination by selection (ranging 10-102 spindle-shaped mutants per 106 cells). We hypothesize that clone selection could take place in asexual populations of microalgae by selection of rare, pre-selective genotypes, driven by a drastic selective pressure subsequent to a catastrophic environmental change.