Factors affecting the codon usage bias of SRY gene across mammals.

Codon usage bias (CUB) is extensively found in a wide variety genomes and it is mostly affected by mutation pressure and natural selection. Analysis of CUB helps in studying the evolutionary features of a genome. The SRY gene plays an important role in male reproductive organ and a good candidate to study the evolutionary forces, since little work was reported earlier on this gene. We used bioinformatic methods to analyze the protein-coding sequences of SRY gene in 172 different mammalian species to understand the patterns of codon usage and the evolutionary forces acting on it. We found that the codon bias of SRY gene varies widely across mammals. Relative synonymous codon usage (RSCU) value revealed that the codons such as TCG, CCG, CAT, ATT, ACT, GCT, GTT, GCG, GGG and GGT were over-represented. Correspondence analysis indicated that the distribution of codons was more close to the axes indicating that compositional constraints might correlate to codon bias. Z-score analysis on RSCU values of codons identified a set of 11 codons viz. TCT, TTT, CTA, CTC, TAT, CAG, CGT, ATA, ACC, AAT and GTA which differed significantly at p<0.01 between 5% high and low gene expression datasets. Further, it was evident from the neutrality plot that GC12 was influenced by both mutation pressure and natural selection. From the study we concluded that natural selection played a dominant role, but mutational pressure played a minor role in the codon usage pattern of SRY gene across mammals.