Plants in all terrestrial biomes form root symbioses with arbuscular mycorrhizal (AM) fungi. These fungi (Phylum Glomeromycotina) reproduce primarily via asexual spores that differ substantially with respect to numerous quantifiable morphological traits. Previously, both interspecific and intraspecific variation in AM fungal spore size have been shown to be considerable, particularly for such a comparatively species-poor group. In addition to size, spores differ with respect to surface ornamentation and wall thickness, energetically costly traits that could aid in dispersal or persistence in inhospitable environments. Investigating variation in spore traits, both among and within species, as well as trade-offs between traits could help elucidate the dispersal ecology and life-history strategies of AM fungi. To this end, we built TraitAM, a database of quantitative AM fungal spore and life-history traits containing 2,330 measurements from 309 original taxonomic species descriptions. Relationships among traits were explored and trait divergence was tested using the most current SSU-ITS-LSU aligned consensus phylogenetic tree for AM fungi. We asked: 1) How much do AM fungal spore traits such as spore wall thickness and ornamentation vary both among and within species? 2) Do trade-offs exist between spore size and spore wall traits?
Results/Conclusions
Across all Glomeromycotinan fungi, interspecific variation in spore wall thickness differed by two orders of magnitude, ranging from 0.50µm to 55µm. Intraspecific variation in spore wall thickness was almost as large, with certain AM fungal species exhibiting no variation in wall thickness and others varying by nearly 50µm. Some species invest heavily in spore walls; up to 75% of the total spore diameter is comprised of spore wall indicating high investment in offspring protection. There is a positive relationship between spore size and wall thickness, suggesting that AM fungal species that form larger spores also invest in traits to protect them. Interspecific and intraspecific variation in spore ornamentation, as measured by the thickness of the ornamentation layer, also differed but to a lesser degree. These results suggest that other AM fungal spore traits in addition to size exhibit substantial interspecific and intraspecific variation, and that these traits are conserved across broad phylogenetic groups. Spore walls may protect larger spores from degradation in the soil environment or during animal or aerial dispersal. Finally, these results indicate that, instead of trading off energetically costly spore traits (i.e. spore size and wall thickness), AM fungi may “double down” to ensure offspring survival.