The molecular evolution of the haustorium formation genes <i>Pirin</i> and <i>QR1</i> in parasitic Orobanchaceae

Abstract Parasitic plants use invasive haustoria to penetrate the tissues of host plants and steal resources. For many root parasites in the family Orobanchaceae, haustorium formation closely follows seed germination, and detection of nearby hosts is important for both of these developmental processes. While KAI2, a parasite gene involved in host-responsive seed germination, has been heavily studied, comparatively less is known about haustorium formation genes. Therefore, we investigated the molecular evolution of two such genes in parasitic Orobanchaceae, Pirin and QR1, and compared their copy number and patterns of natural selection to those of KAI2. In contrast to KAI2, we detected a lower average copy number for both Pirin and QR1 in parasitic plants relative to nonparasites. As with KAI2 proteins, we found an elevated rate of molecular evolution for some parasitic Pirin proteins; however, we detected higher conservation of QR1 in parasites and relatives than in other plants. These results complement gene expression and population genetics studies that have begun to uncover the evolutionary and functional details of haustorium formation genes. Furthermore, they shed light on the ways in which parasites evolve to detect and exploit hosts.