Evolutionary Dynamics of Two Differentially Expressed Mammalian Proteins Encoded by a Single Gene, CAMK4

Understanding the patterns of genetic variations within fertility-related genes and the evolutionary forces that shape such variations is crucial in predicting the fitness landscapes of subsequent generations. Here, we report distinct evolutionary features of two differentially expressed mammalian proteins (CaMKIV: Ca²⁺/-calmodulin-dependent protein kinase IV and CaS: Calspermin) that are encoded by a single gene, Camk4. While CaMKIV, which expressed in multiple tissues including testis and ovary, is evolving at a relatively low rate (0.46 - 0.64 ×10^-9 nucleotide substitutions/site/year), the testis-specific CaS gene that is predominantly expressed in postmeiotic cells is evolving at least 3 to 4 times faster (1.48 - 1.98 ×10^-9 subs/site/year). The maximum-likelihood based selection analyses revealed that CaMKIV is constrained by intense purifying selection and therefore, remained functionally highly conserved throughout the mammalian evolution, whereas the testis-specific CaS gene is under strong positive selection. The substitution rates of different mammalian lineages within both genes are positively correlated with GC content, indicating the possible influence of GC-biased gene conversion on the estimated substitution rates. Of note, the observation of such unusually high GC content of the CaS gene (≈ 74%), particularly in the lineage that comprised of the bovine species, suggests the possible role of GC-biased gene conversion in the evolution of CaS that mimics positive selection.