PS 30-111 - Why are there so many kinds of arbuscular mycorrhizal fungal spores?

Background/Question/Methods

Arbuscular mycorrhizal (AM) fungi form belowground spores that differ substantially with respect to quantifiable morphological traits. Despite existing entirely belowground (in the dark), AM fungal spores represent a broad range of colors from translucent to white, red, green, brown, and black. The ecological function (if any) of spore color, along with other spore traits remains largely speculative; spore color could impact a fungus’s ability to be dispersed by small mammals or survive UV radiation during dispersal. AM fungal spore traits like color, size, or wall thickness could inform predictions regarding differential persistence, dispersal, or host C sink dynamics among AM fungal species. Relationships between traits could inform potential energetic tradeoffs and life history strategies of different AM fungal species. Here we report findings from the ongoing development of TraitAM, a database of quantitative AM fungal spore and life-history traits containing 27 variables for 311 original taxonomic species descriptions. We investigated relationships between traits and explored linkages between spore traits and mycorrhizal function. We asked: 1) Does a relationship exist between spore color and spore size that informs dispersal ecology and mycorrhizal symbiotic economies? 2) Can spore size predict symbiotic function to hosts in arbuscular mycorrhizas?

Results/Conclusions

We observed a positive relationship between spore size and color such that AM fungal species that produce larger spores also invest more resources in darker pigmentation. These data align with the physical expectations for larger spores that also form thicker protective walls and appear darker. AM fungal taxa that form larger spores (i.e. Gigasporaceae species) are more frequently found in coarse texture sandy soils which may possess temperature and moisture environments that require greater protective traits. Further, higher color contrast between large, darker spores and sandy soil may make the spores more attractive to animal dispersers. Using data from MycoDB, a global meta-analytic mycorrhizal function database, we also observed a positive relationship between spore traits and plant symbiotic function. We saw a negative correlation between spore size and plant growth response to mycorrhization. This indicates that AM fungal genera that form comparatively larger spores may confer less benefit to plant hosts, as spores represent a greater plant host resource drain. Future work will explore other relationships between traits, including wall thickness and ornamentation in relation to mycorrhizal function.