Figure 1.

Polo-like kinases in different taxons. Polo kinases are found in all eukaryotic lineages other than plants and apicomplexans. Yeast such Saccharomyces cerevisae and Schizosaccharomyces pombe only have one polo-like kinase. Invertebrates, like Caenorhabditis elegans or fruit flies (Drosophila), have Plk genes belonging to the PLK1 and SaK, but not PLK2, subfamilies. The PLK2 subfamily is present in vertebrates as well as in some other bilaterians, such as sea urchin (Strongylocentrotus purpuratus). Plk5 is present in mammals, although amphibians (Xenopus laevis) also contain a divergent but probably true ortholog of this protein (Plx5).