24. Which one of the following statements regarding ‘Endosymbiotic hypothesis of origin of eukaryotes’ is INCORRECT?
    (1) Mitochondria arose from an α-proteobacterium and plastids arose from cyanobacteria.
    (2) The event of engulfment of a photosynthetic cyanobacterium by a host cell was primitive to
    engulfment of an α- proteobacterium during the eukaryotic origin.
    (3) Protists chlorarachniophytes, likely evolved when a heterotrophic eukaryote engulfed green alga, exemplifying secondary endo-symbiosis.
    (4) One of the membranes of the engulfed double- membraned cyanobacteria was lost in some of the hosts that eventually led to red and green algae descendants.

The Endosymbiotic Hypothesis: An Overview

The endosymbiotic hypothesis posits that eukaryotic cells evolved through a series of symbiotic events in which one prokaryotic cell engulfed another. Over time, the engulfed cells lost their independence and became organelles, specifically mitochondria and plastids (such as chloroplasts in plants and algae).

Key Points of the Endosymbiotic Hypothesis

• Mitochondria:

  • Evolved from an α-proteobacterium that was engulfed by an archaeal host cell.

  • This event likely occurred first, giving rise to the ancestral eukaryotic cell.

• Plastids (Chloroplasts):

  • Evolved from a photosynthetic cyanobacterium that was engulfed by a eukaryotic host.

  • This event occurred later, in a lineage that gave rise to plants and algae.

• Evidence:

  • Mitochondria and plastids have their own DNA (circular, like bacteria), reproduce by binary fission, and have ribosomes similar to those of prokaryotes.

  • The host cell and symbiont became so interdependent that neither could survive alone.

Evaluating the Statements

Let’s analyze each statement to determine which is incorrect regarding the endosymbiotic hypothesis:

1. Mitochondria arose from an α-proteobacterium and plastids arose from cyanobacteria.

• Correct.

  • This is a core tenet of the endosymbiotic hypothesis. Mitochondria are derived from α-proteobacteria, and plastids (chloroplasts) from cyanobacteria.

• Evidence:

  • Mitochondrial DNA is most closely related to that of α-proteobacteria.

  • Plastid DNA is most closely related to that of cyanobacteria.

2. The event of engulfment of a photosynthetic cyanobacterium by a host cell was primitive to engulfment of an α-proteobacterium during the eukaryotic origin.

• Incorrect.

  • This statement reverses the order of events. The engulfment of an α-proteobacterium (leading to mitochondria) occurred first, giving rise to the ancestral eukaryotic cell.

  • The engulfment of a cyanobacterium (leading to plastids) occurred later, in a lineage that already had mitochondria.

• Evidence:

  • All eukaryotes have mitochondria (or descend from ancestors that did), but only photosynthetic eukaryotes have plastids.

  • This supports the idea that the mitochondrial endosymbiosis was the first, primary event.

3. Protists chlorarachniophytes, likely evolved when a heterotrophic eukaryote engulfed green alga, exemplifying secondary endosymbiosis.

• Correct.

  • Chlorarachniophytes are a group of algae that acquired their chloroplasts through secondary endosymbiosis—a process in which a eukaryotic host cell engulfed another eukaryotic cell that already had a plastid (from a primary endosymbiosis event).

• Evidence:

  • Chlorarachniophyte chloroplasts are surrounded by more than two membranes, indicating a history of multiple engulfment events.

4. One of the membranes of the engulfed double-membraned cyanobacteria was lost in some of the hosts that eventually led to red and green algae descendants.

• Correct.

  • Plastids (chloroplasts) are typically surrounded by two membranes, which are thought to correspond to the inner membrane of the original cyanobacterium and the membrane of the host’s phagocytic vacuole.

  • However, in some lineages, one of these membranes may have been lost or modified, but this is not universally accepted or well-documented.

  • More commonly, additional membranes are added (not lost) in secondary endosymbiosis, but primary plastids (as in red and green algae) usually retain two membranes. The statement is somewhat misleading, but the most common and supported view is that the two membranes are retained, not lost, in primary plastids. However, the phrasing is ambiguous—if it suggests that one membrane was lost in primary endosymbiosis, this is not supported by current evidence and is not the standard interpretation.

• Clarification:

  • In primary endosymbiosis (leading to red and green algae), the plastid is bounded by two membranes—the inner membrane from the cyanobacterium and the outer membrane from the host’s phagocytic vacuole. Loss of one membrane is not typical and not widely supported in the literature.

  • If the statement is interpreted as referring to secondary endosymbiosis (where additional membranes are added, not lost), it is also not correct for the specified lineages (red and green algae, which are products of primary endosymbiosis).

• Conclusion:

  • The most incorrect statement is actually (2), as it reverses the order of events. However, (4) is also problematic if taken literally for primary endosymbiosis, but it is less clearly incorrect than (2), which is definitively wrong.

However, based on the phrasing of the question and the options, the most clearly incorrect statement is:

(2) The event of engulfment of a photosynthetic cyanobacterium by a host cell was primitive to engulfment of an α-proteobacterium during the eukaryotic origin.

This is incorrect because the engulfment of the α-proteobacterium (mitochondria) occurred before the engulfment of the cyanobacterium (plastids).

Statement (4) is ambiguous and may be misleading, but it is not as directly incorrect as statement (2), which is unambiguously wrong.

Key Takeaways

• Mitochondria arose from an α-proteobacterium, and plastids arose from cyanobacteria.

• The engulfment of the α-proteobacterium (leading to mitochondria) occurred before the engulfment of the cyanobacterium (leading to plastids).

• Chlorarachniophytes are an example of secondary endosymbiosis, where a eukaryote engulfed another eukaryote with a plastid.

• Primary plastids (as in red and green algae) are typically surrounded by two membranes, not one.

• The incorrect statement is:

  (2) The event of engulfment of a photosynthetic cyanobacterium by a host cell was primitive to engulfment of an α-proteobacterium during the eukaryotic origin.

Summary Table

Conclusion

The endosymbiotic hypothesis is well-supported by molecular, genetic, and structural evidence. The most clearly incorrect statement among those provided is that the engulfment of a photosynthetic cyanobacterium by a host cell was primitive to the engulfment of an α-proteobacterium. In fact, the mitochondrial (α-proteobacterial) endosymbiosis occurred first, and the plastid (cyanobacterial) endosymbiosis occurred later in a lineage that already had mitochondria