Evidence for a new type of endosymbiotic organization in a population of the ciliate Mesodinium rubrum from British Columbia

The ciliate Mesodinium rubrum has excited considerable interest because it has been reported to contain cryptophycean endoxymbionts which lack nuclei (Taylor et al., 1969, 1971). Hibberd (1977), however, has recently shown that in Mesodinium from the United Kingdom the cryptophycean chloroplasts and mitochondria share a common cytoplasm with an apparent cryptophycean nucleus. This cytoplasm is separated from the ciliate cytoplasm by a single membrane. Here we confirm, through extensive observation including serial sectioning, the presence of multiple, discrete, anucleate chloroplast-mitochondrial complexes in a population of Mesodinium from British Columbia. In addition to the normal ciliate macro- and micronuclei, however, these cells contain a third type of nucleus, possibly of cryptophycean origin, which is apparently in its own membrane-bound cytoplasm. This suggests the possibility of a new type of endosymbiotic organization in which endosymbiont organelles have become separated into discrete cytoplasmic compartments.