Data from: Experimental adaptation to marine conditions by a freshwater alga

The marine-freshwater boundary has been suggested as one of the most
difficult to cross for organisms. Salt is a major ecological factor and
provides an unequalled range of ecological opportunity because marine
habitats are much more extensive than freshwater habitats, and because
salt strongly affects the structure of microbial communities. We exposed
experimental populations of the freshwater alga Chlamydomonas reinhardtii
to steadily increasing concentrations of salt. About 98% of the lines went
extinct. The ones that survived now thrive in growth medium with 36 gL−1
NaCl, and in seawater. Our results indicate that adaptation to marine
conditions proceeded first through genetic assimilation of an inducible
response to relatively low salt concentrations that was present in the
ancestors, and subsequently by the evolution of an enhanced inducible
response to high salt concentrations. These changes appear to have evolved
through reversible and irreversible modifications respectively. The
evolution of marine from freshwater lineages is an example that clearly
indicates the possibility of studying certain aspects of major ecological
transitions in the laboratory.