Ecological divergence is marked by specific plastic gene expression and distinct rhizosphere among sibling allopolyploid marsh orchids (Dactylorhiza)

In addition to demographic and dispersal constraints, a complex interplay of biotic and abiotic factors shapes the distribution of taxa across landscapes, with each species adapting uniquely to the challenges and opportunities presented by its habitat. Plasticity, the capacity to dynamically adjust one’s phenotype in response to environmental shifts, is a pivotal trait enabling species to exploit a wider niche space and withstand suboptimal conditions. This adaptability is particularly crucial for newly formed allopolyploid lineages seeking to establish themselves in diverse environments. The amalgamation of different parental genomes in such hybrids can enhance their plastic responses to varying habitats, yet the long term impacts of plasticity on allopolyploid establishment success remain elusive. Here we use ecologically divergent sibling allopolyploid marsh orchids ( Dactylorhiza , Orchidaceae) in reciprocal transplantation experiments at two localities to gain insights into the drivers of ecological divergence and species persistence in the face of gene flow. We show that while consistent abiotic differences characterise the alternative environments, the great majority of gene expression differences between the sibling allopolyploids is plastic. Few genes exhibit fixed expression differences, suggesting that despite ecological divergence, plasticity may mediate gene flow between the sibling allopolyploids, thereby preventing the accumulation of genetically encoded expression differences. Extending our investigations to the rhizosphere, we uncover distinct fungal communities between the roots of the two sibling allopolyploids, consistent across the two localities. This finding suggests that both biotic and abiotic factors contribute to the distribution and delimitation of Dactylorhiza sibling allopolyploids. Altogether, our results suggest that plasticity can exert both diversifying and homogenising influences on the establishment of recurrently-formed allopolyploid species. ### Competing Interest Statement The authors have declared no competing interest.