Heterogeneity in codon usage in the flatworm Schistosoma mansoni.

Synonymous codon choices vary considerably among Schistosoma mansoni genes. Principal components analysis detects a single major trend among genes, which highly correlates with GC content in third codon positions and exons, but does not discriminate among putatively highly and lowly expressed genes. The effective number of codons used in each gene, and its distribution when plotted against GC3, suggests that codon usage is shaped mainly by mutational biases. The GC content of exons, GC3, 5', 3', and flanking (5' + 3' + introns) regions are all correlated among them, suggesting that variations in GC content may exist among different regions of the S. mansoni genome. We propose that this genome structure might be among the most important factors shaping codon usage in this species, although the action of selection on certain sequences cannot be excluded.