The evolution of <i>KAI2</i> and its homologue <i>DLK2</i> in parasitic plants and their non-parasitic relatives

Abstract Parasitic plants in Orobanchaceae attach to host plants and steal resources. These parasites, which vary in photosynthetic status and host dependence, have received considerable attention because they are noxious weeds in agricultural settings, although they are important community members in natural ecosystems. Outside Orobanchaceae, parasitism has arisen 11 times in flowering plants, and convergent morphological and molecular evolution has occurred across phylogenetically distinct groups of parasites. Here, we address whether convergent evolution has occurred in the KAI2 signalling system, which is known to enable host-responsive seed germination in Orobanchaceae. Furthermore, we investigate how selection has shaped KAI2 and its homologue DLK2. Phylogenetic analyses and selection tests show that DLK2 has evolved similarly across angiosperms, although some parasites have experienced modestly elevated evolutionary rates. In contrast, KAI2 has evolved uniquely rapidly in parasitic Orobanchaceae and perhaps been shaped by positive selection on individual codons. A deeper investigation of KAI2 in Asterids reveals a duplication that probably occurred before the divergence of Lamiids and Campanulids. These results shed light on the evolution of germination signalling in parasites and underscore the uniqueness of KAI2 in Orobanchaceae. Furthermore, they enhance our understanding of how molecular evolution contributes to biological innovations such as parasitism.