Q29 - Paulinella Chloroplast Endosymbiosis

Theoretical 2 Real exam question - full text reproduced under IBO's CC BY-NC-SA 4.0 license

Mitochondria & Chloroplasts: Structure & Endosymbiotic Origin History of Life: Origin & Major Transitions

Paulinella is a genus of small aerobic amoeba from the Rhizaria clade. This genus includes a monophyletic clade of photosynthetic species. The photosynthetic species of Paulinella have oval theca, long filamentous pseudopodia and two curved chloroplasts.

The genome of Paulinella chloroplasts is extremely large, about one million base pairs, and its sequence is relatively close to the free-living cyanobacteria Synechococcus genome. The Synechococcus genome contains about three million base pairs.

The last common ancestor of Paulinella chloroplast and Synechococcus lived approximately 500 million years ago. The last common ancestor of all known photosynthetic species in the Paulinella genus lived approximately 50 million years ago. The photosynthetic clade of Paulinella diverged from its closest non-photosynthetic relative approximately 120 million years ago.

True or false?

Q29.1. During Paulinella chloroplast genome evolution, many genes were lost.

Q29.2. Paulinella probably obtained chloroplasts as a result of secondary endosymbiosis.

Q29.3. The endosymbiosis event which made Paulinella a photosynthetic amoeba is estimated to have occurred 50-120 million years ago.

Q29.4. Paulinella cells contain ribosomal RNA from two different sources.

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Question reproduced from IBO 2023, Theoretical Paper 2, licensed under CC BY-NC-SA 4.0 - attributed to the International Biology Olympiad. Open the full exam PDF