Conidial fusion in the asexual fungusVerticillium dahliae

Abstract

Cell-to-cell fusion is a fundamental biological process across the tree of life. In filamentous fungi, somatic fusion (or anastomosis) is required for the normal development of their syncytial hyphal networks, and it can initiate non-sexual genetic exchange processes, such as horizontal genetic transfer and the parasexual cycle. Although these could be important drivers of the evolution of asexual fungi, this remains a largely unexplored possibility due to the lack of suitable resources for their study in these puzzling organisms. In this study, we report that the spores of the important asexual plant-pathogenic fungus Verticillium dahliae often engage in cell fusion via Conidial Anastomosis Tubes (CATs). We optimized appropriate procedures for their highly reproducible quantification and live-cell imaging, which were used to characterize their physiology and cell biology, and to start elucidating their underlying genetic machinery. Formation of CATs was shown to depend on growth conditions and require functional Fus3 and Slt2 MAP kinases, as well as the NADPH oxidase NoxA, whereas the GPCR Ste2 and the mating-type protein MAT1-2-1 were dispensable. We show that nuclei and other organelles can migrate through CATs, which often leads to the formation of transient dikaryons. Their nuclei have possible windows of opportunity for genetic interaction before degradation of one by a presumably homeostatic mechanism. We establish here CAT-mediated fusion in V. dahliae as an experimentally convenient system for the cytological analysis of fungal non-sexual genetic interactions. We expect that it will facilitate the dissection of sexual alternatives in asexual fungi.

Sex Pheromone Receptor Ste2 Orchestrates Chemotropic Growth towards Pine Root Extracts in the Pitch Canker Pathogen Fusarium circinatum

“…Nevertheless, strains of F. oxysporum and V. dahliae harbor either the MAT1-1 or MAT1-2 versions of the MAT1 locus [57,58] and not both as is the case for homothallic species such as F. graminearum. The F. oxysporum strain in which chemotropism was studied carries the MAT1-1 version of the locus, while the V. dahliae strain studied carries the MAT1-2 version [15,59]. In the current study, we examined both MAT1-1 and MAT1-2 isolates in the sexual fungus F. circinatum and found that strains of either mating specificity responded chemotropically to synthetically produced α-peptide pheromone.…”