Roles of the Low-Affinity Calcium Uptake System in Nematode-Trapping Fungi

Calcium ions (Ca2+) serve as ubiquitous intracellular signaling molecules, regulating diverse cellular and developmental processes in eukaryotes. Among these eukaryotes, nematode-trapping fungi (NTF) have evolved diverse traps, including two-dimensional (2-D) adhesive traps such as knobs, columns, and non-constricting rings, three-dimensional (3-D) adhesive traps like networks, as well as mechanical traps like constricting rings (CRs), to ensnare living nematodes. Comparative genomic analyses of NTF have revealed the presence of two homologs to FIG_1, which is the sole member of the low-affinity calcium uptake system (LACS) within fungi. Specifically, one of two genes, FIG_1, is present in both NTF and non-NTF organisms. In contrast, the other gene, FIG_2, is exclusive to NTF. The functions of FIG_1 and FIG_2 were investigated through gene knockout or gene knockdown in the 3-D-trap forming Arthrobotrys oligospora, the 2-D-trap forming Dactylellina haptotyla and the mechanical-trap forming Drechslerella dactyloides. Results demonstrated that both FIG_1 and FIG_2 play a role in growth, conidiation, stress response, trap formation, and the predatory process in NTF. However, FIG_2 exhibited a more pronounced role in the predatory process. Notably, mutants lacking FIG_2 showed a loss of trap formation ability. Furthermore, suppressing FIG_2’s expression led to a substantial reduction in trap formation-34% in D. haptotyla and 41% in D. dactyloides-indicating the essential contribution of LACS in facilitating the predatory lifestyle unique to NTF. These findings shed light on the molecular underpinnings of NTF’s predation strategies and offer insights into the broader significance of calcium signaling in the evolution and adaptation of fungal species.