ACKNOWLEDGMENTS
We thank Alan Kergunteuil for comments on previous versions of the
manuscript. This work was supported by Swiss National Science Foundation
grant 31003A_179481 to SR.
Author contributions: SR and MB initiated the project and
collected the data, GG performed chemical analyses, ED, SR and MB
analysed the data, all authors wrote the manuscript.
Data accessibility statement : Data underlying this article can
be accessed on Dryad Digital Repository athttp://dx.doi.org/XXX/doi.goes.here , and used under the Creative
Commons Attribution licence.