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

[Amanda0628]

A)

Ancestral symbionts had low GC content genomes by chance, and these low GC content genomes are passed onto their progeny.

B)

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.

C)

Free-living organisms must have a more stable genome to survive; low G+C content organisms are often selected against in nature.

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.