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Composition of Extrafloral Nectar Influences Interactions between the Myrmecophyte Humboldtia brunonis and its Ant Associates

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Abstract

Ant–plant interactions often are mediated by extrafloral nectar (EFN) composition that may influence plant visitation by ants. Over a 300 km range in the Indian Western Ghats, we investigated the correlation between the EFN composition of the myrmecophytic ant-plant Humboldtia brunonis (Fabaceae) and the number and species of ants visiting EFN. EFN composition varied among H. brunonis populations and between plant organs (floral bud vs. young leaf EFN). In general, EFN was rich in sugars with small quantities of amino acids, especially essential amino acids, and had moderate invertase activity. In experiments at the study sites with sugar and amino acid solutions and with leaf or floral bud EFN mimics, dominant EFN-feeding ants differentiated between solutions as well as between mimics. The castration parasite Crematogaster dohrni (northern study site) was the least selective and did not exhibit any clear feeding preferences, while the largely trophobiont-tending non-protective Myrmicaria brunnea (middle study site) preferred higher sucrose concentrations and certain essential/non-essential amino acid mixtures. The mutualistic Technomyrmex albipes (southern study site) preferred sucrose over glucose or fructose solutions and consumed the leaf EFN mimic to a greater extent than the floral bud EFN mimic. This young leaf EFN mimic had low sugar concentrations, the lowest viscosity and sugar:amino acid ratio, was rich in essential amino acids, and appeared ideally suited to the digestive physiology of T. albipes. This preference for young leaf EFN may explain the greater protection afforded to young leaves than to floral buds by T. albipes, and may also help to resolve ant–pollinator conflicts. The differential response of dominant ants to sugar, amino acids, or solution viscosity suggests that plants can fine-tune their interactions with local ants via EFN composition. Thus, EFN can mediate local partner-choice mechanisms in ant–plant interactions.

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Acknowledgements

This work was funded by the Ministry of Environment and Forests, Government of India. We are grateful to T. M. Musthak Ali for ant identification. We thank Anupama, Karthik, Megha, Priya, Shaji, Sivan, Sudheesh, and G. Yathiraj for assistance, P. Balaram and S. Chandrasekaran for help with the GC-MS analysis, and the Karnataka Forest Department for research permissions. We are especially grateful to Yuvaraj Ranganathan for doing the multivariate analyses and for help with the final figures. We thank Judie Bronstein, Doyle McKey, Martin Heil, and an anonymous reviewer for helpful comments on the manuscript.

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  1. Center for Industrial Ecology, School of Forestry and Environment, Yale University, New Haven, CT, USA

    Megha Shenoy

  2. Department of Bioorganic Chemistry, Max Planck Institute for Chemical Ecology, Jena, Germany

    Venkatesan Radhika

  3. Centre for Ecological Sciences, Indian Institute of Science, Bangalore, 560012, India

    Megha Shenoy, Venkatesan Radhika, Suma Satish & Renee M. Borges

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Correspondence to Renee M. Borges.

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Shenoy, M., Radhika, V., Satish, S. et al. Composition of Extrafloral Nectar Influences Interactions between the Myrmecophyte Humboldtia brunonis and its Ant Associates. J Chem Ecol 38, 88–99 (2012). https://doi.org/10.1007/s10886-011-0052-z

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