Preferential Plastid Retention by the Acquired Phototroph <italic>Mesodinium chamaeleon</italic>.

Abstract: The ciliate genus <italic>Mesodinium</italic> contains species that rely to varying degrees on photosynthetic machinery stolen from cryptophyte algal prey. Prey specificity appears to scales inversely with this reliance: The predominantly phototrophic <italic>M. major/rubrum</italic> species complex exhibits high prey specificity, while the heterotrophic lineages <italic>M. pulex</italic> and <italic>pupula</italic> are generalists. Here, we test the hypothesis that the recently described mixotroph <italic>M. chamaeleon</italic>, which is phylogenetically intermediate between <italic>M. major/rubrum</italic> and <italic>M. pulex/pupula</italic>, exhibits intermediate prey preferences. Using a series of feeding and starvation experiments, we demonstrate that <italic>M. chamaeleon</italic> grazes and retains plastids at rates which often exceed those observed in <italic>M. rubrum</italic>, and retains plastids from at least five genera of cryptophyte algae. Despite this relative generality, <italic>M. chamaeleon</italic> exhibits distinct prey preferences, with higher plastid retention, mixotrophic growth rates and efficiencies, and starvation tolerance when offered <italic>Storeatula major</italic>, a cryptophyte that <italic>M. rubrum</italic> does not appear to ingest. These results suggest that niche partitioning between the two acquired phototrophs may be mediated by prey identity. <italic>M. chamaeleon</italic> appears to represent an intermediate step in the transition to strict reliance on acquired phototrophy, indicating that prey specificity may evolve alongside degree of phototrophy. [ABSTRACT FROM AUTHOR]