| Abstract: | Rod‐shaped bacteria were found together with the “secondary chloroplast” in the endoplasmic reticulum (ER) in the pennate diatom Pinnularia. During the host interphase, bacteria bore cavities into the chloroplast and interacted with thylakoids, forming a functional unit with the plastid of the engulfed and reduced eukaryotic endocytobiont, without perforating the relict cell membrane and the chloroplast envelope. Fluorescently labeled antibodies directed against the large subunit of RUBISCO elicited the same pattern as DAPI–DNA complexes. In pre‐prophase, bacteria started to dissociate from the plastids and left scars at their previous locations. In profile, scars appeared as granular amorphous regions interrupting several stacks of thylakoids, whereas images of glancing sections showed foci of circumscribed spiral domains. The vortex pattern was interpreted as the result of a screwing movement of the bacteria during their withdrawal, initiated while structurally still connected to the thylakoids. Scars became effaced, and thylakoids reoriented during prophasic translocation of the complex plastid from the girdle to the valve. In the transition to prophase, bacteria were found as clusters inside ER cisternae near the nucleus, where the microtubule‐organizing center was accompanied by the spindle precursor. Microtubules were seen to be aligned with ER profiles (containing bacteria). TEM images indicated that intracellular long‐distance transport is controlled by the host cytoskeleton acting upon the ER. Restricted to this stage, bacteria appeared to divide and occasionally bore tentacle‐like appendages. Their intraluminal localization at the nucleus in prophase, near the new plasmalemma after cleavage, and their return to the chloroplasts shortly afterward revealed a cell cycle‐dependent translocation with the ER as circulatory system and, together with the obvious retardation of their division cycle, a domestication by the host. |
