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

Question

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) Organisms with a low G+C content organisms are at a selective disadvantage to those with a high G+C content when free-living.
C) It is more difficult to replicate high G+C content genomes than to replicate low G+C content genomes.
D) Two common spontaneous mutations change GC pairs into AT pairs and symbionts usually have fewer DNA repair mechanisms to fix these compared with free-living organisms.

Accepted Answer

Symbionts often have fewer DNA repair mechanisms, leading to a higher rate of mutations that change GC pairs into AT pairs, contributing to a lower G+C content over time.