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Question

Difficulty: HardIdentifying Hypotheses and Beliefs

Passage

Two biologists present competing hypotheses regarding the evolutionary origin of hydrogenosomes, double-membraned organelles found in certain anaerobic eukaryotes that produce molecular hydrogen (H2H_2).

Biologist 1
Hydrogenosomes evolved directly from an ancestral anaerobic endosymbiotic bacterium that was distinct from the aerobic α\alpha-proteobacterium that gave rise to mitochondria. This ancestral endosymbiont belonged to an anaerobic lineage of bacteria that possessed hydrogenase enzymes. Over evolutionary time, this bacterium transferred most of its genome to the host nucleus, leaving an organelle specialized for anaerobic ATP production. Consequently, hydrogenosomes and mitochondria represent completely independent endosymbiotic events.

Biologist 2
Hydrogenosomes and mitochondria share a common endosymbiotic ancestor: a facultatively anaerobic α\alpha-proteobacterium. Depending on the environmental pressures faced by the host eukaryotic lineage, this single ancestral organelle diverged. In aerobic environments, it evolved into mitochondria, retaining the electron transport chain. In strictly anaerobic environments, it lost the electron transport chain and evolved into hydrogenosomes. Thus, hydrogenosomes are highly modified mitochondria rather than the product of a separate endosymbiotic event.

Based on Biologist 1's hypothesis, if a newly discovered anaerobic eukaryote is found to contain hydrogenosomes that still retain a small genome, the genes in this organellar genome would be expected to share the greatest sequence similarity with the genes of which of the following groups?

  1. Free-living anaerobic bacteria that are distinct from α\alpha-proteobacteriaAnswer
  2. B
    Modern aerobic α\alpha-proteobacteria that are closely related to mitochondrial ancestors
  3. C
    The nuclear genome of the host eukaryote prior to any endosymbiotic events
  4. D
    Archaea that currently inhabit extreme anaerobic environments

Answer

Free-living anaerobic bacteria that are distinct from α\alpha-proteobacteria
Under Biologist 1's model, hydrogenosomes are derived from a distinct anaerobic bacterial endosymbiont that is completely independent of the α\alpha-proteobacterial lineage that gave rise to mitochondria. Consequently, any remaining genes in the organelle's genome would share the highest sequence similarity with modern free-living anaerobic bacteria that are not α\alpha-proteobacteria.

Step-by-Step Solution

1
Locate Biologist 1's claim regarding the evolutionary lineage of hydrogenosomes.
Biologist 1 states that hydrogenosomes evolved from an anaerobic endosymbiotic bacterium distinct from the α\alpha-proteobacterium that gave rise to mitochondria.
This establishes the ancestral source of the hydrogenosome under Biologist 1's hypothesis.
2
Deduce the genetic similarity of residual organellar DNA based on this lineage.
Residual organellar DNA reflects the genome of the ancestral bacterium. Since this ancestor was a distinct anaerobic bacterium and not an α\alpha-proteobacterium, its genes should be most similar to other anaerobic bacteria outside of the α\alpha-proteobacteria group.
This connects the proposed evolutionary ancestor to expected genetic sequence relations.
3
Evaluate the choices to match the deduction.
The option specifying free-living anaerobic bacteria distinct from α\alpha-proteobacteria is the correct match.
This identifies the option that logically represents the evolutionary lineage described by Biologist 1.

Key Concept

Identifying specific predictions and core assumptions within conflicting scientific hypotheses.
Estimated Time:1m 30s
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Topics

Identifying Hypotheses and Beliefs

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