Urban ecology and evolution of an arbuscular mycorrhizal interaction with the plant Ruellia nudiflora in a tropical city

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Diego S. Méndez, José Ramos-Zapata and Diego Carmona, Ecología Tropical, Universidad Autónoma de Yucatán, Merida, YC, Mexico, René Garruña-Hernández, Instituto Tecnológico de Conkal, Merida, YC, Mexico, Hector Estrada-Medina, Departamento de Manejo y Conservación de Recursos Naturales Tropicales, Universidad Autonoma de Yucatan, Merida, YC, Mexico

Background/Question/Methods
Cities provide new and extreme environments that can affect the ecology and evolution of organisms, as well as their ecological interactions; however, very few studies have analyzed the effects of urbanization on the evolution of ecological interactions such as the one between plants and arbuscular mycorrhizal fungi (AMF). Based on one field survey, we explore the impact of downtown urban sidewalks on the plant-AMF interaction by contrasting with plants growing under rural conditions. A reciprocal transplant experiment allowed us to test for genetic differentiation, local adaptation to soil conditions, and to explore the relative importance of the presence of the soil biotic factor, including AMF, on the arising patterns during seedling establishment. For this, we focus our research on Ruellia nudiflora (Acanthaceae), a perennial weed that benefits from the AMF association by increasing its survival and reproduction, in the tropical city of Merida, Mexico, in which the sidewalks can reach 47 °C.
Results/Conclusions
Our preliminary results, based on the field survey, showed that the density, diversity, and colonization rate of AMF were higher in rural than urban soil. Similar contrasts were detected in soil attributes such as pH, N, and K, while P was higher in urban soils. A structural equation modelling approach suggested that the observed P and N effects on AMF colonization rates on rural environment were lost in the city, while the K effect became positive. These results suggest that the urban environment changes the effects of abiotic soil factors on plant-AMF interactions. Our experiment did not detect genetic differentiation in AMF colonization rates nor local adaptation; however, we found that the biotic component can buffer and mask genetic differentiation between urban and rural phenotypes and possible local adaptation patterns. For instance, in the absence of the biotic component a soil origin × seed origin interaction was detected on emergence and first true leaf production rates in seedlings (both proxies of survival). On the other hand, we detected that urban plants are genetically more capable to respond to physiological stress (Fv/Fm), regardless of presence of the biotic factor. Thus, the biotic component of the soil in this system compensates for genetic differences that affect plant fitness via survival in the process of establishment. Interestingly, this compensatory effect is not enough to help rural plants match the physiological profile that urban plants might have evolved to cope with the stress suffered in the sidewalks.