Vegetative compatibility in arbuscular mycorrhizal fungi.

摘要

The process of anastomosis promotes interconnectivity between hyphal branches during formation of a mycelial network and between fungal colonies. Self anastomosis involves fusion within a mycelium from a germinating spore, and this process is common in all species of fungi. Non-self anastomosis involves fusion between hyphae from different spores or colonies and allows lateral gene transfer, but this process is much rarer. Anastomosis in Ascomycota is well studied, but little is known for asexual arbuscular mycorrhizal fungi (AMF) in Glomeromycota. Because AMF are obligate symbionts in plant roots, studies have focused on anastomosis in asymbiotic hyphae of germinating spores. To study behavior of symbiotic hyphae growing from a mycorrhizal plant, a “rhizohyphatron” was designed which allowed symbiotic hyphae to grow from single or paired containerized mycorrhizal plants onto an agar-coated surface that could be removed and examined microscopically. In Chapter 1, anastomosis was studied in genetically uncharacterized populations of AMF species representing major genera. Hyphal fusions occurred at very low frequencies in Glomus clarum and Glomus intraradices (<15%) but were absent in Ambispora, Gigaspora, Paraglomus and Scutellospora. These results suggest that anastomosis in symbiotic mycelia may not be biologically important for most clades of AMF. In Chapter 2, self and non-self anastomosis were studied in both asymbiotic and symbiotic hyphae of G. clarum from single spore cultures genotyped using two anonymous microsatellite-flanking markers. Frequency of self anastomosis was similar in asymbiotic hyphae from germinating spores and symbiotic mycelia from plants, ranging from 8 to 38%. Non-self anastomosis was observed only in asymbiotic hyphae of isolates that were genetically identical as well as different isolates. However, fusions occurred in less than 6% of hyphal contacts. Such low levels of hyphal fusion may still permit gene flow in AMF populations, but anastomosis is restricted to asymbiotic hyphae of isolates from proximate locations. Results from this study show that vegetative compatibility between individuals occurs in AMF, but it is limited to only a few lineages and is constrained to only a small phase of the AMF life cycle.