How are secondary endosymbiotic events different from primary endosymbiotic events?
The Correct Answer and Explanation is :
Endosymbiosis refers to a symbiotic relationship where one organism lives inside another. Primary and secondary endosymbiotic events are related processes but differ in their origins and the complexity of the organisms involved.
Primary endosymbiosis refers to the process in which a eukaryotic host cell engulfs a prokaryotic cell (usually a bacterium) that eventually becomes a permanent part of the host. This is believed to be the origin of organelles such as mitochondria and chloroplasts in eukaryotic cells. The engulfed bacterium becomes an endosymbiont, eventually evolving into an organelle. The best-known examples are the origin of mitochondria from aerobic bacteria (alpha-proteobacteria) and chloroplasts from cyanobacteria in plant cells. In primary endosymbiosis, the engulfed organism’s DNA is transferred to the host cell’s nucleus over time, and the symbiont evolves into an organelle that carries out specific functions like energy production (mitochondria) or photosynthesis (chloroplasts).
Secondary endosymbiosis, on the other hand, involves a more complex process where a eukaryotic cell (already containing an organelle from primary endosymbiosis) is engulfed by another eukaryotic cell. This results in the acquisition of a new organelle that originally came from a prokaryote, but the host is not directly engulfing a prokaryote. For example, some algae and protists, like the Euglenozoa and certain dinoflagellates, have acquired plastids through secondary endosymbiosis. In these cases, the host cell already has mitochondria and possibly a chloroplast, but it engulfs another eukaryotic cell that contains photosynthetic plastids, which are then retained and further adapted to the new host’s needs.
The main difference between primary and secondary endosymbiosis is the origin of the engulfed cell: primary endosymbiosis involves the direct engulfment of prokaryotic cells, while secondary endosymbiosis involves the engulfment of a eukaryotic cell that has already undergone primary endosymbiosis. This distinction results in more complex evolutionary processes in secondary endosymbiosis due to the involvement of eukaryotic organisms.