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

G+C Content at 4D sites (GC4D) and d_S in the Monodelphis and Homo X chromosomes. (A) GC4D values of Monodelphis genes in 1:1 orthology relationships with Homo genes. The cumulative distribution of GC4D for orthologs on the Monodelphis X chromosome is shifted to higher values compared with that for orthologs on Monodelphis autosomes, and genes on the Monodelphis X chromosome have a much increased median GC4D. This is exactly the opposite of the situation among Homo orthologs, where X chromosome genes have reduced GC4D. (B) The increased GC4D on the Monodelphis X chromosome is accompanied by an increase in median d_S and a rightward shift of the d_S cumulative distribution. This effect is even more pronounced when comparing orthologs between Monodelphis and Trichosurus, suggesting that much of the increase in d_S has taken place along the marsupial lineage. (C) The d_S for Homo–Monodelphis orthologs that lie on the Homo X chromosome and that are syntenic to the Monodelphis chromosome X or to chromosomes 4 and 7. Among orthologs on the Homo X chromosome, only those that are also found on the Monodelphis X chromosome have elevated d_S, confirming that this is largely a marsupial-specific phenomenon.