HETEROGENEITY IN CODON USAGE IN THE FLATWORM SCHISTOSOMA-MANSONI

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