Intra‐ and interspecies transfer of endosymbionts in Euplotes

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Abstract
Fresh water Euplotes species with a “9 type 1 pattern” of frontoventral cirri (E. patella type) depend upon symbiotic bacteria that live in their cytoplasm. Aposymbiotic cells of these species are unable to divide and eventually die if not reinfected in time. We have investigated 1) whether two types of bacterial symbionts found in stocks of E. aediculatus can replace each other and 2) whether the ability to divide can be restored for aposymbiotic cells, not only by their own endosymbionts, but also by endosymbionts of other Euplotes species. We reinfected aposymbiotic cells by injecting symbiont‐containing cytoplasm into them. The results show that the two types of symbionts found in E. aediculatus stocks can replace each other. This allows us to interpret the type 7 symbiont as a secondary invader that has displaced the original omikron‐like symbiont in some E. aediculatus stocks. Finding that the symbionts of E. woodruffi are able to support growth of E. aediculatus and that those of E. plumipes support growth of E. eurystomus suggests that all Euplotes species with a 9 type 1 cirrus pattern have the same deficiency with respect to cell division. The deficiency is best explained by assuming that it is caused by a hereditary defect that arose long ago in the common ancestor of this group of species. That many of the donor‐recipient combinations we tried did not lead to a resumption of growth and cell division of the recipients probably reflects secondary adaptations of the endosymbionts to the cytoplasm of their host species, so that they no longer have the ability to grow in other environments.