As genes encoding cyclin-dependent kinases had been cloned from Schizosaccharomyces pombe, the fission yeast, Saccharomyces cerevisae, the budding yeast, and human cells, investigators working in plant systems designed experiments to determine whether plants also possessed CDKs that functioned as the catalysts for cell cycle progression. To isolate a cDNA encoding a CDK in maize, investigators aligned the predicted amino acid sequences of human cdc2, S. pombe cdc2, and S. cerevisiae cdc28 genes. Degenerate oligonucleotide PCR primers corresponding to the boxed amino acids were generated and a DNA fragment of the expected size was synthesized by PCR using these primers and maize cDNA library as a template.
The PCR product was then used as a probe to screen the maize cDNA library, and a full-length cDNA clone was isolated. The clone was sequenced, and the predicted amino acid sequence was 64% identical to human cdc2 and 63% identical to S. pombe cdc2 and to S. cerevisiae cdc28.
What does the level of sequence similarity suggest about the evolution of cdc2 genes?