Whole-genome duplications contributed to the expansion of cytochrome b5 genes in Paramecium tetraurelia.

Cytochrome b5 (cyt b5) genes encode ubiquitous electron transport hemoproteins found in animals, plants, fungi, and purple bacteria. However, little is known about their evolutionary history in genomes so far. Here, we conducted an extensive genome-wide survey of cyt b5 genes in 20 representative model species and identified 310 of these genes. Both the absolute number and relative proportion of cyt b5 genes in Paramecium tetraurelia were significantly higher than those in other genomes. Our data also showed that whole-genome duplications (WGDs), especially the recent WGD, contributed to the species-specific expansion of cyt b5 genes in the Paramecium genome. Furthermore, 24 cyt b5 genes were identified as the minimal number of ancestral cyt b5 in the ancestral Paramecium genome, which is also the largest number of these genes encountered in an organism. These results suggest that an excess of cyt b5 genes were selectively retained in this species even before the three WGDs took place. Although more cyt b5 genes were retained in P. tetraurelia than in other genomes, more cyt b5 losses were also observed in the P. tetraurelia genome, suggesting that the balance of gene retention and loss maintained an optimum dosage of cyt b5 genes.