Why Do Obligate Symbionts Often Contain Lower G+C Content When

question 18

Multiple Choice

Why do obligate symbionts often contain lower G+C content when compared to free-living organisms?

• A) Ancestral symbionts had low GC content genomes by chance, and these low GC content genomes are passed onto their progeny.
• B) Free-living organisms must have a more stable genome to survive; low G+C content organisms are often selected against in nature.
• C) Replicating high G+C content genomes requires several additional enzymes, to break apart the strong triple bonds formed from GC pairs, which are rarely found in symbionts with relatively small genome sizes.
• D) Symbionts usually do not have as many DNA repair mechanisms as free-living organisms, and two common spontaneous mutations change GC pairs into AT pairs.

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