Fig. 5.

PKS evolution in algae. a Occurrence of PKS in different groups of algae. The scheme of endosymbiotic history was adopted from ref. [12]. b Working model for the molecular evolution of algal PKSs. Like bacterial type I PKSs, algal type I PKS evolved by the fusion of ancestral type II genes, which first gave rise to iterative (unimodular) type I PKS. Subsequently, AT domains were displaced to be encoded by separate genes, partially reversing the previous fusion step. Duplications of modules and domains then resulted in noniterative (multimodular) type I PKS. Information on the order of AT displacements and duplications is currently lacking (see Discussion for details). Note that early steps of this scenario may have occurred in bacteria