Experimental evolution in Chlamydomonas II. Genetic variation in strongly contrasted environments

Abstract

Experimental populations of Chiamydomonas were selected in Light (photoautotrophic) or Dark (heterotrophic) environments. Each population was a clone, founded by a single spore and propagated vegetatively thereafter. A heterogeneous environment was simulated by mixing Light and Dark lines in each growth cycle and redistributing them between the two environments in the next cycle. Some lines maintained permanently in the Dark evolved greatly increased growth within fewer than 300 generations, at the expense of reduced growth in the Light. Lines maintained in both Light and Dark environments evolved a negative genetic correlation between Light and Dark growth, and displayed more genetic variance of fitness than lines maintained in either environment exclusively. It is possible that genetic variance near mutation-selection balance is greater in heterogeneous environments because selection is weaker. However, the evolution of distinctly specialized lineages in these experiments suggests that in the conditions of batch culture a cost of adaptation creates negative frequency-dependent selection that maintains genetic variance. Genetic variance was greater in the more permissive environment (Light) than in the more restrictive environment (Dark).

Experimental evolution in Chlamydomonas. III. Evolution of specialist and generalist types in environments that vary in space and time

“…We used lines of Chiamydomonas reinhardtii that had been maintained asexually on minimal medium in the Light or on acetate-supplemented medium in the Dark, as a continuation of the experiment reported by Bell & Reboud (1997). These base lines were transferred to each of four experimental treatments.…”

“…All three measures gave similar results; for simplicity, we report only P0 in this paper. The selection and assay conditions are different (flasks and culture tubes), but growth in the two environments is positively correlated (Bell & Reboud, 1997).…”

“…We have previously demonstrated a cost of adaptation in the base populations by comparing the selection lines with their founders: improvement in the environment of selection was associated with regress in the other environment (Bell & Reboud, 1997).…”

“…When populations were cultured in heterogeneous environments, comprising a range of media differing quantitatively in macronutrient concentration, genetic variance of fitness was maintained, or at least eliminated more slowly than in a uniform environment (Bell, 1997). Moreover, clonal populations cultured in a heterogeneous environment that comprised two qualitatively different growth conditions, autotrophic growth in the light and heterotrophic growth in the dark, evolved substantial genetic diversity within a few hundred generations (Bell & Reboud, 1997). …”

“…The Dark line was propagated in exactly the same way, except that the medium was supplemented with 1.2 g L sodium acetate, the flasks were wrapped with aluminium foil to exclude light, and the growth period was 7 days. Further details of how the base populations evolved from the founders are given by Bell & Reboud (1997). Routine laboratory procedures are described by Harris (1989).…”