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Worldwide arbuscular mycorrhizal (AM) plants in grassland ecosystems associate with multiple AM symbionts; the literature suggests that the typical range varies from 7 to 11 AM symbionts. Using a model based on group selection, we examine the evolutionary traits that could be responsible for maintaining this situation. We assumed that from a pool of 160 potential symbionts some of them would deliver a profit for the plant while the others (“defectors”) would deliver comparable damage. The plant could control the number of simultaneous fungi it associated with but not their identities. Plants admitting higher numbers of symbionts can gain greater benefits but run a greater risk of associating with defectors. Based on simulation results we can demonstrate convergence toward an optimal number of fungal symbionts which occurs faster for plants that experience low survivorship and higher selection pressure. Additionally, for seedling survivorship rates close to those experienced in grassland biomes (i.e. around 50%) there appears to be a steady plateau with an optimum of 7 symbionts over a wide range of defection probabilities. This does not hold for other biomes with higher plant mortality such as woodlands in which there is an unsteady situation with highly likely modification of the number of simultaneous symbionts. This may hold important clues about to the origin of the ectomycorrhizal associations where only one symbiont is optimum. ⺠A model was developed to explain diversity in arbuscular mycorrhizas. ⺠The model predicts for grasslands 7 simultaneous symbionts per plant. ⺠A richness close to 7 symbionts has been confirmed in the literature. ⺠The model may hold important clues about the origin of ectomycorrhizal associations.
"We define “root” as the underground part of the plant which is colonized by several simultaneous symbionts – a definition consistent with most AM studies. Singh et al. (2008) were able to estimate the numbers of simultaneous AM symbionts in 1 cm root sections of different plants in a grazed pasture, in terms of operational taxonomic units (OTUs), at values ranging from 6.88 to 9.29. Using 20 cm root sections, on the other hand, Öpik et al. (2009) following harvesting of plant species in a boreonemoral forest plot were able to demonstrate that the roots of typical forest species exhibited a higher (mean OTUs ± standard deviation (S.D.)) arbuscular mycorrhizal taxa diversity (28.8 ± 7.6) compared to generalist species that occur in various ecosystems that only contained 13.0 ± 3.0 taxa per plant species."
Fitness in AMF is difficult:
"we assume that the fitness outcome of the group selection on the AM fungi is negligible because of (i) their asexual nature that signifies that any fitness gains may only be invested in propagule formation; (ii) their ubiquitous occurrence in nature at high propagule densities (e.g. Öpik et al., 2010) – higher propagule abundance of an AM fungal species does not necessary increase chances of colonization; and (iii) their big investment differences in reproductive propagules versus hyphal network construction (e.g. Hart and Reader, 2002 and Klironomos and Hart, 2002) that render any comparative claims on AM fitness difficult."
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