A Preliminary Study On The Evolutionary Molecular Basis Of Cetacean Body Size Differences

Cetaceans(whales,dolphins and porpoises)is a group of marine mammals living a complete aquatic life.Its ancestors returned from the land to the ocean approximately 53 million years ago,and gradually adapted to the aquatic environment.During the long-term course of transition,their morphological and physiological traits have undergone a series of adaptive changes,among which body size is one of the most significant ones.The size difference of the cetaceans is huge,of which the smallest size is Phocoena sinus(1.4 m,42 kg)and the largest size is the blue whale(Balaenoptera musculus)(the body length is about 27m,the weight is about 136,000 kg),and thus the difference in body weight between the two is more than 3200 times.In addition,like primates,species from the family Delphinidae has experienced a course of brain capacity enlargement,and its relative brain capacity is larger than that in other cetaceans and terrestrial mammals,next to humans,to be second largest.However,the molecular basis underlying the diversifying evolution in body and brain size have not been well explored so far.Therefore,in this study,we performed a genome-wide scanning in an attempt to determine the molecular basis of diversifying in the body size and brain capacity among cetaceans.Through measurement and literature retrieval,we collected 8 morphological indexes for 7 cetacean species,including 6 body size indexes(body length,body weight,axillary girth,flipper leading to edge length,tail width,dorsal fin height)and 2 brain capacity indexes(absolute brain weight and relative brain weight),and genome assemblies for 7 cetacean species and a relative of them living in land-cow(Bos taurus)were obtained from databases or de novo assembled in our lab.6509 1:1 orthologous genes were identified from the genome sequences of these 8 species by the OrthoMCL software,and the molecular evolution rate for each of the 1:1 orthologous gene in each species was calculated using PAML software.Then,the 8 morphological index data were correlated with the evolution rate of 1:1 orthologous genes based on the PGLS regression analysis.The analysis revealed 606 genes(body length related genes and body weight related genes)correlated to body size indexes.Functional enrichment analysis shows that these genes are mainly enriched in metabolism,thyroid synthesis,cell cycle,suggesting that these pathways may play a key role in controlling the body size during the course of cetacean evolution.At the same time,genes correlated to body size are also enriched in the immune system and cancer related pathways,combined with the evolution rates of these genes in different types of cetaceans,which,to a certain extent,could explain the "peto’s paradox" in cetaceans,that the larger the animal body size does not mean a higher risk of cancer in theory.In analysis relating genes to brain capacity,248 genes were correlated to absolute brain weight index,and 897 to relative brain weight index.Interestingly,these two types of genes are all enriched in pathways associated with the chemical signal transduction between neurons,and the former gene set is also enriched in energy metabolism pathways.These results suggest that,in the process of brain enlargement or diversifying in brain capacity,genes involved in synaptic transmission and energy metabolism may play a key role.In this study,genes correlated to the body size and brain capacity between different cetaceans were screened at a whole genome level,and the genes functions were enriched in pathways,which suggested the molecular basis for the body size and brain capacity diversifying during the evolutionary course of cetaceans.