Making sure that you get out what you put in can sometimes be difficult, and understanding how species cooperate, without being exploited by opportunistic free-loaders, is tricky. In the paper behind the cover of this month’s issue of New Phytologist, Guillaume Chomicki and co-workers describe a trick, used by a plant to conceal a sugary reward, meant only for its mutualistic ant partner.
While on fieldwork in Fiji, the researchers observed a plant that had adopted a special strategy to make sure that the extra-floral nectar it uses to reward its ant partner isn’t taken by opportunists. Dolichoderine ants live in symbiosis with Rubiacieae species: the plants provide the ants with somewhere to live, and nectar, while the ants provide nutrients for the plants through defecation and protect the plants from herbivory by other species.
It is in the plant’s interest to keep its mutualistic ant partner, so ensuring a supply of nectar to keep the ants sweet is important. By continuing to function for 10 days after the end of flowering, the nectar glands maintain a hidden source of nectar. The nectar is hidden by a thick epidermis, and can only be accessed by the true mutualist, Philidris nagasau, which is able to bite through the thick outer skin to feed on the nectar within.
As symbiotic interactions between species shift towards a more focused interaction between two or fewer insect species and one plant species (towards an obligate mutualism), the level of reciprocation between the species in the relationship ought to increase, so the partners can get more out of each other. However, this makes the rewards more attractive to other species that aren’t part of the relationship. Hiding the nectar rewards in this way reinforces the mutualistic relationship between the Squamellaria plants pictured and the ants that live with them, by ensuring that the reward for the ants’ services to the plant remain exclusive to them.
Read the paper: Chomicki, G., Staedler, Y. M., Schönenberger, J. and Renner, S. S. (2016) Partner choice through concealed floral sugar rewards evolved with the specialization of ant–plant mutualisms. New Phytologist 211: 1358–1370. doi: 10.1111/nph.13990
Mike Whitfield (@mgwhitfield)