An analysis of the gene complement of a marsupial, Monodelphis domestica: Evolution of lineage-specific genes and giant chromosomes

d_N, d_S, d_N/d_S, and intron length vary with G+C content at 4D sites among Monodelphis–Homo and Monodelphis–Trichosurus 1:1 orthologs. (A,C) The variation of the median values for d_N, d_S, d_N/d_S, and Monodelphis intron lengths in Monodelphis–Homo 1:1 orthologs. (B,D) The same relationships for 1:1 orthologs between the two marsupials Monodelphis and Trichosurus. The orthologs were divided into five equally populated classes according to Monodelphis G+C content at 4D sites (GC4D). We found that median d_N/d_S dropped from 0.12 for Trichosurus orthologs and 0.11 for Homo orthologs in the lowest G+C class (G+C <31.5% and <34.5%, respectively) to a median d_N/d_S of 0.10 and 0.058 in the highest G+C class (G+C >56.1% and >63.5%, respectively). The higher median d_N/d_S values for orthologs from the two marsupial species is likely to stem, in part, from Trichosurus EST sequencing errors that disproportionately affect d_N over d_S. Genes with high GC4D also exhibit higher median d_S and d_N values and have reduced intron lengths.