SPECIALIZATION OF ENDOPLASMIC RETICULUM ARCHITECTURE IN RESPONSE TO A BACTERIAL SYMBIOSIS IN PERIDINIVM BALTICUM (PYRRHOPHYTA)1

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Abstract
Transmission elrctron microscopy (TEM) and rapid freeze‐fracture analyses were used to provide data for a more accurate description of the relationship of Peridinium balticum (Levander) Lemmermann to endosymbiotic bacteria present in the dinoflagellate's cytoplasm. Bacteria were found in all of the cells observed and were present in both gamete and zygote life cycle stages. Analysis of micrographs showed that bacteria were tightly clustered in the posterior portion of the dinoflagellate cell in numbers of 50‐200/ section. Rapid freeze‐fracture replicas re‐vealed a specialization of the endoplasmic reticulum (ER). ER surrounded each bacterium and consisted of a series of infoldings which contacted the bacterial plasma membrane in several places. Extensions of ER were common between adjacent bacteria. We have interpreted the development and specialization of endoplasmic reticulum of the dinoflagellate host as an example of a cellular response of one organism to another, supportive of the serial endosymbiotic theory. Intramembrane particles (IMP) were seen on both protoplasmic (P) and exoplasmic (E) faces of the bacterial plasma membrane. Aquisition of the bacterial symbiont and the effects of long term culturing are discussed.