Heterospecific Mating Triggers Disruptive Postmating Gene Expression Response in Mimulus

Mating is both a driver of biodiversity and a battleground for competing evolutionary interests, particularly when postmating interactions impact fertilization success. One such suite of interactions is the postmating prezygotic (PMPZ) transcriptional response, a highly coordinated and dynamic interplay of molecular responses that trigger a cascade of signaling events after mating but before the formation of the zygote. PMPZ transcriptional responses can either facilitate or inhibit successful fertilization, thereby playing a pivotal role in reinforcing species boundaries. While this phenomenon is well documented in animal systems, the transcriptional response to heterospecific mating remains largely unexplored in plants. We used RNA-seq to characterize gene expression in styles of sister species pair Mimulus guttatus and Mimulus nasutus following conspecific and heterospecific crosses. Heterospecific matings elicited a significantly greater number and magnitude of differentially expressed genes, with strong enrichment of stress, immunity, and defense pathways. Some gene families showed concordant expression across treatments, while others, such as NAC transcription factors and heat shock proteins, showed divergent or amplified responses in the heterospecific condition. These results suggest that the pollinated style tissue mounts a heightened transcriptional response to heterospecific pollen, consistent with a shift toward biotic stress signaling. Our findings demonstrate that plants, like animals, exhibit PMPZ transcriptional disruption in response to heterospecific mating. This early molecular response may serve as a barrier to gene flow and contribute to the maintenance of species boundaries.