Preliminary Study On The Molecular Mechanism For The Change Of Food Habit In Cetaceans, Based On Adaptive Evolution At 7 Digestive Enzyme Genes And RNase1 Gene Family

The feeding habit of cetaceans shifted from herbivority to carnivority in the process of returning to the water from the land. The ancestor of cetaceans fed on grasses like the extant cloven-hoofed animals, while extant cetaceans are all carnivores. Fossil records indicated that along with the change of lifestyle from terrestrial to semiaquatic and finally aquatic, the feeding habits changed from herbivory to omnivory and then to sarcophagy in cetaceans. However, the molecular mechanism for such a change has not been well addressed so far. To explore the possible molecular mechanism of cetaceans diet switch, seven digestive enzyme genes were chosen for evolutionary analysis in 10 representative cetaceans, in combination with other mammalian lineages. First, phylogenetic trees were reconstructed for all the digestive enzymes in this study in order to investigate the evolutionary history of these genes. Second, a selective pressure analysis was conducted to detect if the proteinase and lipase genes were under positive selection because of the high protein and lipid diet, and what selective pressure was on the amylase genes. Third, we detected if there were some specific sites or domains in the digestive genes of cetaceans, and their association with dietary switch in cetaceans. Finally, it was detected if there were parallel or convergent sites between cetaceans and the carnivoras, another mammalian lineage having similar diet with high protein and lipid. It was showed that all the gene trees were similar to the well accepted species trees, and thus supported the artiodactyla as the nearest ancestor of cetaceans. Of all the seven genes examined, four (PNLIP, LIPF, CTRC and MGAM) were under positive selection in cetaceans. A total of 21 parallel amino acid substitutions and 4 convergent substitutions were identified between cetaceans and carnivores. These findings could provide some new insights into molecular mechanism for the dietary switch along with the cetacean evolution.In addition, ribonuclease1 (RNase1) gene family related to the feeding habit was examined in this study. We annotated the RNasel gene family of 6 cetacean species and 13 species from other 9 orders of mammals. And then the phylogenetic analysis was constructed for all the RNase1 genes to study the evolution history of RNase1. The results showed that there was only one copy of RNasel in cetaceans, but there were at least two copies in herbivores such as cow, camel and snob-nosed monkey. The RNase1 was duplicated in Ruminantia and Tylopoda respectively. Gene duplication did not occur in cetaceans, which might be related to the dietary switch of cetaceans from herbivority to carnivority.