Increased prey choice is associated with higher kleptoplastidic reliance in Mesodinium ciliates

Kleptoplastidic organisms display varying levels of reliance on stolen plastids, with some lineages evolving complex strategies for plastid (and other organelle) integration into metabolism. More complex methods of plastid integration typically coincide with greater prey specialization, but it remains unclear whether or how kleptoplastidic lineages preferentially obtain organelles from compatible prey types. Here, we used the Mesodinium genus of ciliates, which span a gradient of reliance on kleptoplasty, to test for prey specialization during ingestion. We used two Mesodinum species: a highly kleptoplastidic species, M. rubrum, and a more mixotrophic species, M. chamaeleon. We conducted a series of experiments offering Mesodinium two different cryptophyte prey species simultaneously. Mesodinium and cryptophyte prey populations were measured across six different prey combinations. We observed that M. rubrum showed greater prey selectivity than M. chamaeleon at the time of ingestion. Despite this greater selectivity, M. rubrum was still unable to completely select for compatible prey types during feeding. Growth of M. rubrum was not affected by prey type over the course of this study, while limited effects on the growth of M. chamaeleon were seen. Preferential feeding by M. rubrum suggests the presence of a prey identification system that is not present in M. chamaeleon, consistent with our hypothesis that prey selectivity increases with increased reliance on kleptoplasty.