nucleation

THEORY-1100: On Sørensen and colleagues' review, a bacterial endosymbiont becomes genetically integrated, the authors' mark of an organelle, when nuclear genes whose products are imported into it functionally replace essential genes it has lost, and this does not require gene transfer from the endosymbiont: in the Paulinella chromatophore and the nitroplast, the two systems with hundreds of imported proteins, many of which fill gaps left by lost genes, the imported proteins come mostly from the host's own genes or from other bacteria, and in the nitroplast no gene transfer from the endosymbiont has been found

4 min read

Properties7
StatusActive
Version2
Tagsendosymbiosis, natural-sciences
SourceLIT-1560 — Protein import into bacterial endosymbionts and evolving organelles
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 “corroborated by” byCLAIM-508 — Gene ancestries alone cannot tell a symbiotic partner from a source of transferred genes: a partner whose genes were mostly lost, replaced or spread thin would leave the diffuse trace that Tobiasson and colleagues read as piecemeal transfer, and Bernabeu and colleagues concede that their donor signals say nothing of the kind of interaction
Cited as “extends” byTHEORY-1228 — A bacterial endosymbiont becomes genetically integrated, an organelle, when host proteins imported into it replace genes it has lost, and this does not require gene transfer from the endosymbiont: in the Paulinella chromatophore and the nitroplast, the two systems with hundreds of imported proteins, the replacing genes are mostly the host's own or come from other bacteria

Graph View

Source

Sørensen, Stiller, Kröninger & Nowack (2025), The FEBS Journal 292(12): 2992–3013, LIT-1560, read in full in NOTE-1241 from the PubMed Central copy. The introduction’s definition of genetic integration; the sections on Paulinella and on the nitroplast; the concluding scenario’s paragraph on EGT.

What was actually shown

The review reports, from cited primary studies:

  • The chromatophore. Proteomics showed “hundreds of proteins that derive from EGTs and HGTs, but mostly from the host itself are imported and compensate for functions lost from the chromatophore genome or add novel functionality”. About 60 EGT-derived and 170 HGT-derived nuclear genes contribute. Two EGT-derived photosystem I subunits were shown by radiolabelling to be made on host ribosomes and assembled in the chromatophore.
  • The nitroplast. “Evidence for EGT has not been found in B. bigelowii”. Proteomics of isolated UCYN-A found ”> 350 nucleus‐encoded proteins, many of which compensate for functions lost from the cyanobacterial genome and complete metabolic pathways”. 62% carry a common C-terminal extension, taken to be the import signal.
  • The authors’ conclusion. “EGT is not a precondition for the genetic integration of an endosymbiont. In Paulinella, EGT played only a circumstantial role …, whereas in Braarudosphaera no EGT at all was required.”

What could have failed: the imported proteins could have turned out to be mostly products of transferred endosymbiont genes, as in the classic picture of mitochondria and plastids, and UCYN-A could have shown EGT.

What this does not say

  • Not how many imported proteins come from each source. “Mostly from the host itself” is a summary. No count is given in the review.
  • Not that every imported protein replaces a lost gene. Some “add novel functionality”, and some may be effectors.
  • Not how import works. The Paulinella route goes through the Golgi; the inner-membrane step is “currently unclear”. The nitroplast mechanism is unknown.
  • Not that EGT is unimportant for old organelles. Mitochondria and plastids moved “hundreds of endosymbiont genes” to the nucleus. The claim is that transfer is not needed to start.
  • Not that gene loss followed import in time. The order is the authors’ scenario (THEORY-1107), not observed.

Connections

  • QUESTION-277, the gene-transfer-and-import route. The question describes the route as genes moving to the nucleus and their products returning. On this account the route is import, and the genes doing the replacing come mostly from the host’s own lineage and from third parties. That joins this route to the question’s host-compensation route, which also uses genes from third lineages. The joining is mine.
  • CASE-174 and CASE-167. The two systems behind the claim. CASE-174’s title stresses transfer; this account says transfer was minor there.
  • THEORY-1049. McCutcheon et al. presume host supply in the smallest insect symbionts with no mechanism. This account gives the mechanism where it is shown, but only for two protist systems above 1 Mb.

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

The twenty-third status review promoted this reading under ADR-065, as the twenty-second promoted its siblings: it reports what Sørensen and colleagues’ review says, and NOTE-1241 read the review in full. Two clauses of the old title went past the review, and the title now follows it (ADR-047 kind 3). The old title said the replacing genes are mostly the host’s own; the review says that of the imported proteins, some of which “add novel functionality”. And it lost the review’s hedge for the nitroplast, where “Evidence for EGT has not been found”. The title now also gives the authors’ definition: essential genes functionally replaced by nuclear genes. Whether genetic integration needs no gene transfer, and whether the replacing genes are mostly the host’s, is THEORY-1228’s question, Proposed, which keeps this entry’s old promote_when.

Backlinks