Multinucleate fungi and oomycetes are phylogenetically distant but structurally similar. To address whether they share similar nuclear dynamics, we carried out time-lapse imaging of fluorescently labeled Phytophthora palmivora nuclei. Nuclei underwent coordinated bidirectional movements during plant infection. Within hyphal networks growing in planta or in axenic culture, nuclei either are dragged passively with the cytoplasm or actively become rerouted toward nucleus-depleted hyphal sections and often display a very stretched shape. Benomyl-induced depolymerization of microtubules reduced active movements and the occurrence of stretched nuclei. A centrosome protein localized at the leading end of stretched nuclei, suggesting that, as in fungi, astral microtubule-guided movements contribute to nuclear distribution within oomycete hyphae. The remarkable hydrodynamic shape adaptations of Phytophthora nuclei contrast with those in fungi and likely enable them to migrate over longer distances. Therefore, our work summarizes mechanisms which enable a near-equal nuclear distribution in an oomycete. We provide a basis for computational modeling of hydrodynamic nuclear deformation within branched tubular networks. IMPORTANCE Despite their fungal morphology, oomycetes constitute a distinct group of protists related to brown algae and diatoms. Many oomycetes are pathogens and cause diseases of plants, insects, mammals, and humans. Extensive efforts have been made to understand the molecular basis of oomycete infection, but durable protection against these pathogens is yet to be achieved. We use a plant-pathogenic oomycete to decipher a key physiological aspect of oomycete growth and infection. We show that oomycete nuclei travel actively and over long distances within hyphae and during infection. Such movements require microtubules anchored on the centrosome. Nuclei hydrodynamically adapt their shape to travel in or against the flow. In contrast, fungi lack a centrosome and have much less flexible nuclei. Our findings provide a basis for modeling of flexible nuclear shapes in branched hyphal networks and may help in finding hard-to-evade targets to develop specific antioomycete strategies and achieve durable crop disease protection.

Plant pathogens secrete proteins called effectors that target host cellular processes to promote disease. Recently, structural genomics has identified several families of fungal effectors that share a similar three-dimensional structure despite remarkably variable amino-acid sequences and surface properties. To explore the selective forces that underlie the sequence variability of structurally-analogous effectors, we focused on MAX effectors, a structural family of effectors that are major determinants of virulence in the rice blast fungus Pyricularia oryzae. Using structure-informed gene annotation, we identified 58 to 78 MAX effector genes per genome in a set of 120 isolates representing seven host-associated lineages. The expression of MAX effector genes was primarily restricted to the early biotrophic phase of infection and strongly influenced by the host plant. Pangenome analyses of MAX effectors demonstrated extensive presence/absence polymorphism and identified gene loss events possibly involved in host range adaptation. However, gene knock-in experiments did not reveal a strong effect on virulence phenotypes suggesting that other evolutionary mechanisms are the main drivers of MAX effector losses. MAX effectors displayed high levels of standing variation and high rates of non-synonymous substitutions, pointing to widespread positive selection shaping the molecular diversity of MAX effectors. The combination of these analyses with structural data revealed that positive selection acts mostly on residues located in particular structural elements and at specific positions. Our work provides unique insights into the evolutionary history of an extended fungal effector family and opens up new research avenues to deepen our understanding of the molecular coevolutionary interactions of fungi with plant hosts.

Polyenergid fungi and oomycetes are phylogenetically distant but structurally similar. To address whether they share similar nuclear dynamics we carried out time-lapse imaging of fluorescently labelled Phytophthora palmivora nuclei. Nuclei underwent coordinated bidirectional movements during plant infection. Within hyphal networks growing in planta or in axenic culture nuclei are either dragged passively with the cytoplasm or actively become rerouted toward nuclei-depleted hyphal sections and often display a very stretched shape. Benomyl-induced depolymerization of microtubules reduced active movements and the occurrence of stretched nuclei. A centrosome protein localized at the leading end of stretched nuclei, suggesting that like fungi astral microtubule-guided movements contribute to nuclear distribution within oomycete hyphae. The remarkable hydrodynamic shape adaptations of Phytophthora nuclei contrast those in fungi and likely enable them to migrate over longer distances. Therefore, our work summarises mechanisms which enable a near equal nuclear distribution in an oomycete. We provide a basis for computational modelling of hydrodynamic nuclear deformation within branched tubular networks.