How Does Recombination Influence GC-Content?
Three hypotheses have been proposed to explain how recombination might influence base composition: (1) recombination promotes the creation of new GC-alleles via mutation; (2) recombination favors the spread of GC-alleles when both AT and GC alleles are present in a population (i.e., there is a fixation bias toward GC alleles), via biased gene conversion (BGC) (Galtier et al. 2001); (3) there is a selective pressure in favor of a high GC-content, and GC* increases with high crossover rates because selection is more efficient (Charlesworth 1994). Among these three models, the selectionist one appears very unlikely. Indeed, the increase in GC-content induced by a single AT GC mutation, occurring in megabase-long noncoding sequences, is extremely small. Given the limited population sizes of mammals, it is totally unrealistic that this tiny increase in GC-content might be gripped by natural selection. Conversely, various observations support the BGC model. Notably, there is a fixation bias
