Is there a latitudinal diversity gradient for symbiotic microbes? A case study with sensitive partridge peas

Mutualism is thought to be more prevalent in the tropics than temperate
zones and may therefore play an important role in generating and
maintaining high species richness found at lower latitudes. However,
results on the impact of mutualism on latitudinal diversity gradients are
mixed, and few empirical studies sample both temperate and tropical
regions. We investigated whether a latitudinal diversity gradient exists
in the symbiotic microbial community associated with the legume
Chamaecrista nictitans. We sampled bacteria DNA from nodules and the
surrounding soil of plant roots across a latitudinal gradient (38.64 °N to
8.68 °N). Using 16S rRNA sequence data, we identified many non-rhizobial
species within C. nictitans nodules that cannot form nodules or fix
nitrogen. Species richness increased towards lower latitudes in the
non-rhizobial portion of the nodule community but not in the rhizobial
community. The microbe community in the soil did not effectively predict
the non-rhizobia community inside nodules, indicating that host selection
is important for structuring non-rhizobia communities in nodules. We next
factorially manipulated the presence of three non-rhizobia strains in
greenhouse experiments and found that co-inoculations of non-rhizobia
strains with rhizobia had a marginal effect on nodule number and no effect
on plant growth. Our results suggest that these non-rhizobia bacteria are
likely commensals – species that benefit from associating with a host but
are neutral for host fitness. Overall, our study suggests that temperate
C. nictitans plants are more selective in their associations with the
non-rhizobia community, potentially due to differences in soil nitrogen
across latitude.