Thesis Defence: Investigating the Formation of Dikaryotic Spores Between Closely Related Arbuscular Mycorrhizal Fungi in Soil

Selina Spence, supervised by Dr. Miranda Hart, will defend their thesis titled “Investigating the Formation of Dikaryotic Spores Between Closely Related Arbuscular Mycorrhizal Fungi in Soil” in partial fulfillment of the requirements for the degree of Master of Science in Biology.

An abstract for Selina Spence’s thesis is included below.

Defences are open to all members of the campus community as well as the general public. Registration is not required for in-person defences.

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Abstract

Arbuscular mycorrhizal fungi (AMF) form important symbiotic relationships with plants, where they provide nutrients to the plant in exchange for plant-derived carbon. Since this often results in enhanced plant growth, AMF are commercially produced as bioinoculants to reduce the need for chemical fertilizers. AMF contain thousands of nuclei that can flow throughout a continuous hyphal network. They can be homokaryotic, where all nuclei are genetically identical, or dikaryotic, where nuclei of two distinct genotypes can co-exist. It is believed that dikaryotic AMF are formed by the fusion (anastomosis) of two genetically compatible hyphae and nuclear exchange, though the formation of stable dikaryons to date has not been observed in vitro. To address whether dikaryons can form between homokaryotic strains in soil, a greenhouse study was conducted using two strains of Rhizophagus irregularis (DAOM 197198 and DAOM 229455). DAOM 197198, a strain used commercially as a bioinoculant, and DAOM 229455 were inoculated alone and together and grown in pots (n = 27) with a host for three to six months. Spores were individually analyzed to determine if they were homokaryotic, or dikaryotic. Out of the ~300 spores analyzed, 94% were classified as DAOM 197198, 5% were classified as dikaryotic, and only 1% were classified as DAOM 229455, showing a strong dominance of DAOM 197198. The abundance of fungi in the roots was also determined. When grown alone, both strains were equally abundant; however, when co-inoculated, DAOM 197198 was dominant and, in most cases, was the only strain detected in the roots. Despite this competition, there was no effect on the host, with consistent mycorrhizal growth and phosphorus responses across treatments. Nonetheless, the production of dikaryotic spores, even when one strain is in low abundance, suggests that anastomosis could be a gateway for foreign nuclei to “cheat” their way into pre-established mycelial networks and disrupt native mycelial dynamics; therefore, anastomosis and dikaryon formation should be further explored to better understand the invasive potential of these commercialized fungi.