Rapid GC content evolution in rice through GC-biased gene conversion and selection for translation efficiency

GC-biased gene conversion (gBGC), which favors GC over AT alleles during recombination, is responsible for the elevation of GC content in yeast and animals and is more effective in outcrossing species, yet its role in plants is less well understood. Here, we report empirical evidence for the presence of gBGC in rice (Oryza sativa). While the GC content of noncoding sequences is well correlated with the recombination frequency, the GC content of coding regions is shaped by both recombination and selection for GC-rich codons, which is linked to high translation efficiency. Surprisingly, GC content has declined in both rice and its partially outcrossing wild progenitor, and there is no evidence that full outcrossing results in a net increase of GC content. This suggests that the mutations are more robust than gBGC in altering GC content, implying that GC content will continue to decline in rice and its close relatives.