The case
Paulinella chromatophora is a thecate amoeba with photosynthetic inclusions, chromatophores. They descend from a cyanobacterium acquired in a primary endosymbiosis independent of, and much later than, the one that gave plants their plastids: about 60 million years ago, by LIT-1557’s estimate.
- Reduction (LIT-1621). The chromatophore genome is 1.02 Mb with 867 protein-coding genes, 26% of a free-living relative’s. Pseudogenes show reduction still under way. It keeps the photosynthesis genes and lacks essential amino-acid and cofactor pathways, so it depends wholly on its host.
- Transfer and import (LIT-1557). More than thirty expressed genes moved to the amoeba’s nucleus. Three photosystem I subunits encoded by transferred genes are made in the host cytoplasm, imported into the chromatophores, apparently via the Golgi, and assembled with chromatophore-encoded subunits.
- In context (LIT-1560). A 2024 review counts hundreds of imported proteins in such recently evolved organelles. It asks whether import still separates organelles from endosymbionts.
What it can show
- The organelle route, recent and separate. Gene transfer plus import gives the host regulatory control (C) over a former organism. This is the route the mealybug (CASE-171) and aphid (CASE-173) cases do not take (QUESTION-277).
- A second independent run. Since it is independent of the plastids, it shows which steps recur when a photosynthetic organelle is made.
Documented in works the record has not read. What it shows rests on their abstracts until they are read.
Note of 2026-10-27 (batch 2): the review read
Sørensen et al. (LIT-1560, read in NOTE-1241) update the figures this case takes from its unread sources:
- Acquisition date. ”~ 90–140 million years ago”. LIT-1557’s 60 million years is an older estimate.
- Genome. About 1 Mbp, with “878–860” protein-coding genes and about two-thirds of the original coding capacity lost. The case’s “26% kept” uses LIT-1621’s comparator.
- Nuclear genes. About 60 come from the chromatophore and about 170 from other bacteria.
- Imported proteins. They number “hundreds”, “mostly from the host itself”. The review says transfer played “only a circumstantial role” (THEORY-1100). So the previous title’s “more than thirty genes moved to the host nucleus and their proteins imported back” overstated transfer, and the title is revised.
- The subunits. The review names two transferred photosystem I subunits shown to be imported, PsaE and PsaK. The case’s “three” comes from LIT-1557’s unread abstract and is unchecked.
- The route. Import runs through the Golgi, with a bipartite N-terminal targeting peptide on long proteins. About 20% of long imported proteins serve the chromatophore’s DNA replication and gene expression, which gives the host control of both (C).