Temporal host–symbiont dynamics in community contexts: Impact of host fitness and vertical transmission efficiency on symbiosis prevalence

Symbiotic associations play a role in plant ecology and evolution, but the
outcome of the interaction depends on the life history traits of the
partners and the environmental context. Although symbiosis with vertically
transmitted microorganisms should result in mutualism, it is not clear how
the transmission process aligns with the outcome of the context-dependent
symbiosis. For three years, we sampled individuals of an annual
plant species that forms symbiosis with a vertically transmitted fungal
endophyte, in paired stands of two contrasting vegetation communities
[Humid Mesophytic Meadows (HMM): productive / low stress, and Humid
Prairies (HP): less productive / high stress]. We estimated the prevalence
of symbiosis at the population level, and the fitness of the plant, the
status of the symbiotic and the vertical transmission efficiency at the
individual level. Over three years, the prevalence of symbiosis
was ≈100% in HMM and ≈75% in HP. Plant fitness was very low and high in
years with precipitation below and above the yearly mean, respectively.
The higher fitness of endophyte-symbiotic plants was evident in the HMM
and high precipitation years. Vertical transmission of endophytes was
higher in HMM (≈96%) compared to HP (≈93%) and was not related to plant
fitness. Despite transmission inefficiencies in HP, changes in prevalence
within the growing season (from seeds to the final plant stand) suggest a
fitness advantage for symbiotic plants. Vertical transmission is
expected to promote mutualism as it aligns partners' fitness.
Although symbiotic plants showed higher fitness and the probability of
transmission failures was higher among low-fitness plants, the variation
in transmission efficiency between plants and vegetation communities was
not related to the fitness of the individual host. Our study provides
evidence that context-dependent vertical transmission efficiency and
endophyte-mediated fitness advantages interact complexly to determine the
prevalence of symbiosis in populations that occur in contrasting
vegetation communities.