THE ECOLOGY AND GENETICS OF FITNESS IN CHLAMYDOMONAS .2. THE PROPERTIES OF MIXTURES OF STRAINS

Whether or not mixtures of strains possess special properties depends on whether or not their component genotypes interact with one another. This paper describes a series of experiments designed to investigate genotype-by-genotype interaction among species of the unicellular green alga Chlamydomonas when grown axenically as liquid cultures in chemically defined media. It was shown that strains varied substantially in their average effect on the overall production of mixed cultures, and that specific interactions between pairs of genotypes were also significant, though small. This conclusion was confirmed by experiments in which strains were cultured separately in growth medium that had been conditioned by the prior culture of another strain. However, there was no consistent general tendency for mixtures to be more productive than the average of their components in pure culture; some mixtures in some physical environments were markedly more productive than their component means, but other mixtures in other environments were markedly less productive. The average effect of a strain on the production of mixtures was uncorrelated with its performance in pure culture, showing that fitness measured in pure culture may be a poor predictor of success in mixed populations. However, the average effect of a strain in simple mixtures was correlated with its effect in more complex mixtures. Complex mixtures themselves displayed properties similar to, but more extreme than, those of simple mixtures; thus, in an environment where pairwise mixtures tended to outyield pure cultures, excess production tended to increase with mixture complexity. The results of these experiments seem to be consistent with those of comparable agronomic trials.