The case
The mealybug Planococcus citri has two bacterial symbionts nested one inside the other. The gammaproteobacterium Moranella endobia lives in the cytoplasm of the betaproteobacterium Tremblaya princeps.
- A metabolic patchwork (LIT-1565). The two symbionts’ nutrient genes barely overlap. Several essential amino-acid pathways need interspersed gene products from Tremblaya, Moranella and possibly the insect.
- Genes from elsewhere (LIT-1569). Tremblaya’s 139-kb genome likely degenerated after it acquired Moranella. At least 22 expressed genes transferred to the insect’s genome from several other bacteria fill symbiont gaps, and none of them come from Tremblaya. The system depends on genes from at least six lineages. The authors call this a path to intimacy distinct from the organelles’.
- Turnover (LIT-1572). Across five mealybug species, Tremblaya goes back to one infection. The gammaproteobacteria inside it come from several replacements by related lineages. The insect’s transferred genes stayed stable through the turnover.
What it can show
- Gene loss without transfer to a controlling genome. Reduction here was compensated by genes from third parties. It was not compensated by moving the reduced symbiont’s genes to the host (QUESTION-277, G).
- Organization that outlasts its constituents. The arrangement persisted while one of its members was replaced. This bears on whether a partnership is one individual or a succession, and on THEORY-418’s test of whether complementation alone gives closure.
Documented in works the record has not read. What it shows rests on their abstracts until they are read.
Note of 2026-10-27 (later): Tremblaya past the tipping point
McCutcheon et al. (LIT-1571, read in NOTE-1213) use Tremblaya princeps from Planococcus citri as their electron micrograph of a “tipping-point endosymbiont” (Figure 4). It has three membranes: two of its own, inherited, and an outer host vacuolar membrane. Inside are a few ribosomes. Its genome is below the 200 kb at which translation genes begin to go (THEORY-1049), so on that paper’s account its envelope, transport and some translation must be supplied by the host. The paper also cites a host-encoded protein identified in the cytoplasm of a mealybug endosymbiont (one protein, mechanism unknown). This adds a second kind of compensation to the case: alongside genes from third parties, the host supplies cell biology the symbiont can no longer make.
Note of 2026-10-27 (batch 2): the imported protein is in Moranella
Sørensen et al. (LIT-1560, read in NOTE-1241) identify the host-encoded protein seen in a mealybug endosymbiont. It is MurF, in Moranella (genome 0.54 Mbp), not in Tremblaya. The note above mentioned it beside Tremblaya’s tipping-point envelope, and could be read as placing it there. So the host supplies Tremblaya’s envelope, transport and translation on THEORY-1049’s account only. No protein import into Tremblaya, or into any symbiont below 200 kb, has been shown. The nearest case the review names is Nardonella (0.20–0.23 Mbp), where import is unconfirmed.