The reproductive microbiome inhibits pollen germination in milkweed

We know very little about the reproductive microbiomes of plants. Microbes may play important roles in shaping pollination, fertilization, and seed production - processes which are important evolutionarily, ecologically, and agriculturally. Through a series of field and laboratory experiments, we show that the stigmatic microbiome in milkweeds influences the success of pollination. Isolation of individual bacterial and fungal taxa from stigmatic secretions allowed us to experimentally test their effects on pollen germination. These experiments demonstrate that individual taxa impact pollen differently, with many microbial taxa being neutral, but some being deleterious. Through isolation of microbes from the legs of pollinator insects we found that pollinators are a likely source for pollen-harmful bacterial taxa. Next, by utilizing a natural hybrid zone, we demonstrate species-specific responses to the stigmatic microbiome that be driving asymmetric patterns of gene-flow between species - with Asclepias exaltata being a better pollen host than A. syriaca. This study demonstrates that the reproductive microbiome is an underappreciated player in sexual reproduction of plants. Significance StatementThe results presented here demonstrate an important but previously unappreciated role of stigmatic microbes in plant sexual reproduction. This study demonstrates that the microbial taxa living in stigmatic secretions in milkweed impact pollen germination. We found that filtering out the microbes from stigmatic secretions of milkweed flowers dramatically increases pollen germination. Through isolating microbial taxa from both stigmatic secretions, and pollinator legs, we found that individual microbial taxa impact pollen differently, with many taxa being neutral, but some being deleterious. Finally, by utilizing a naturally occurring milkweed hybrid zone we demonstrated that microbial taxa in stigmatic secretions may be acting as asymmetric prezygotic barrier.