Hyperaccumulation of Nickel but not Selenium drives floral microbiome differentiation: a study with six species of Brassicaceae

Intraspecific variation in flower microbiome composition can mediate pollination and reproduction, thus understanding the community assembly processes driving this variation is critical. Yet, the relative importance trait-based host filtering and dispersal in shaping among-species variation in floral microbiomes remains unknown. Within two clades of Brassicaceae, we compared diversity and composition of floral microbiomes in natural populations of focal Nickel and Selenium hyperaccumulator species and two of their non-accumulating relatives. We assessed relative strength of floral elemental composition, plant phylogenetic distance (host filtering) and geography (dispersal) in driving floral microbiome composition. Species in the Nickel hyperaccumulator clade had strongly divergent floral microbiomes, with the most variation driven by floral elemental composition, followed by geographic distance between plant populations, and phylogenetic distance. Conversely, within the Selenium hyperaccumulator clade, floral microbiome divergence was much lower among species, and elemental composition, geography, and plant phylogeny were far weaker determinates of microbiome variation. Our results show that the strength of elemental hyperaccumulation on floral microbiomes differ substantially among plant clades, possibly due to variation in elements as selective filters or in long-distance dispersal probability in different habitats.