nucleation

THEORY-1049: On McCutcheon and colleagues' survey, bacterial endosymbionts lose their own cell-envelope and envelope-transport genes together below about 400 kb, and begin to lose translation genes (ribosomal proteins, tRNAs, aminoacyl-tRNA synthetases) below about 200 kb, so that the smallest-genome symbionts cannot be self-sufficient and their membranes, transport and protein synthesis are presumably supplied by the host, as for organelles

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Properties6
StatusActive
Version3
Tagsendosymbiosis, natural-sciences
SourceLIT-1571 — How do bacterial endosymbionts work with so few genes?
Bears onQUESTION-277 — When formerly autonomous partners come to depend on one another, which of mutual dependence, reproductive cohesion, the distribution of essential functions across genomes, the host's regulatory control and the alignment of selection does each route to dependence bring about, and which of them, alone or together, make the partnership a new evolutionary individual?
Cited as “extends” byTHEORY-1208 — Bacterial endosymbionts lose their cell-envelope and envelope-transport genes together below about 400 kb and begin to lose translation genes below about 200 kb, and the host supplies the membranes, transport and protein synthesis that the smallest-genome symbionts no longer encode, as for organelles

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Source

McCutcheon, Garber, Spencer & Warren (2024), PLOS Biology 22(4): e3002577, LIT-1571, read in full in NOTE-1213 from the PubMed Central copy. Figure 2 (the gene matrix) and its caption; the sections on envelopes, transport and translation.

What was actually shown

  • A gene matrix (Figure 2). It covers representative bacteria under 1 Mb: all such genomes, but only the largest and smallest per genus, some in draft form.
    • Genes for fatty acids, phospholipids, cell wall, the BAM complex, the sec translocon and ATP synthase are “generally lost in concert”.
    • Below about 400 kb they are “almost universally” gone.
    • Below about 200 kb, many ribosomal-protein, tRNA and aaRS genes are also gone. One cicada endosymbiont lacks tRNAs for eight amino acids whose codons it still uses, and up to 20 ribosomal proteins can be missing.
  • Composition (Figure 3). Among 98,966 proteomes, tipping-point endosymbionts sit outside the main cloud, beyond the extremophiles. This is “preliminary”.
  • Indirect evidence of host supply.
    • Electron micrographs show three membranes, one host-derived.
    • Host amino-acid transporters are enriched at the symbiont vacuole.
    • A host-encoded protein has been seen in an aphid endosymbiont, and another in a mealybug endosymbiont (one each).

What could have failed: the losses could have been gradual, with no threshold, or scattered across functions, not concerted.

What this does not say

  • Not where the thresholds are, precisely. The 200 kb cutoff is “somewhat arbitrary”, and no statistical method locates either boundary.
  • Not how the host supplies anything. Membrane contact sites, imported tRNAs, imported aaRSs and host ribosomal proteins are all hypotheses. The paper names four mysteries and solves none.
  • Not that gene transfer to the host is how control arises. No bacterial tRNA gene moved to a nucleus has been seen imported, and no new aaRS transfer has been found. Supply, if it happens, may draw on the host’s existing machinery.
  • Not that the symbionts are organelles. The paper calls them “organelle-adjacent”. Large-scale import is established only in protist endosymbionts above the tipping point.

Connections

  • QUESTION-277, regulatory control (C) and gene distribution (G). Here host control is forced by the symbiont’s losses, not gained by gene transfer. Losses that move G past a threshold make C necessary. So the two dimensions are coupled, and in the insect cases G may come first.
  • CASE-174 and CASE-167. Paulinella and the nitroplast are the pre-tipping-point systems with large-scale import. The insect symbionts here reach dependence on the host by a different road, with less evidence of import.
  • CASE-173 and CASE-171. Aphid and mealybug endosymbionts are where single host proteins have been seen inside the symbiont. Tremblaya, in the mealybug case, is a tipping-point endosymbiont.

Note of 2026-10-27 (batch 2): the imported proteins are above the thresholds

Two readings sharpen what this account rests on.

  • Where the imported proteins are. Sørensen et al. (LIT-1560, read in NOTE-1241) identify the host-encoded proteins seen inside insect symbionts:

    • RlpA4 in Buchnera (0.64 Mbp);
    • MurF in Moranella (0.54 Mbp);
    • ColA and two transaminases elsewhere.

    All of these symbionts are above both thresholds. No import into a symbiont below 200 kb has been shown; Nardonella (0.20–0.23 Mbp) is the nearest candidate, and there it is unconfirmed. So host supply in tipping-point endosymbionts remains inferred, as “What this does not say” states.

  • Envelope erosion starts early. The Buchnera APS genome (LIT-1596, read in NOTE-1236) has already lost phospholipid and LPS synthesis at 641 kb. That is consistent with this account’s remark that envelope erosion begins in larger genomes, and that 400 kb marks complete loss.

Note of 2026-10-30: promoted, and the wider thesis filed beside it

The twenty-second status review promoted this reading under ADR-065: it reports what McCutcheon and colleagues’ survey shows and what they infer from it, and NOTE-1213 read the paper in full. Whether endosymbionts lose these genes at these thresholds and the host supplies what they lack is THEORY-1208’s question, Proposed, which keeps this entry’s old promote_when.

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