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According to Wikipedia, galls (cecidia) are formed by parasitic insects and mites like gall wasps (Neuroterus albipes). At some metamorphic stage, these organisms alter cell division processes in meristematic tissues of their host plants, which creates a tumour on (typically) the surface of leaves, branches or roots. These organisms use the galls as their habitat and/or food source (exploiting the sugars present).

However, I also know that bullhorn acacia and ants form an interspecific mutualistic relationship (with galls), so apparently galls don't need to be parasitic. Are there any other specific plant-insect pairs that have the same type of mutualistic gall relationship as the acacia-ant system?

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    $\begingroup$ Pardon the initial link and thank you for clarifying the question $\endgroup$ – Chimango Chisuwo Dec 22 '15 at 9:30
  • $\begingroup$ Thanks for fixing it. I'll remove the now obsolete comment. $\endgroup$ – fileunderwater Dec 22 '15 at 9:36
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From what I know of ant-Acacia mutualisms, I've never heard of an ant gall in an Acacia. The link in your question also never mentions a gall. You've likely confused either domatia or Beltian bodies as a gall. Both differ greatly from galls in that domatia or Beltian bodies are created by the plant and not by the insect.

Domatia are small plant-made chambers that house mutualistic insects (though are often invaded by non-mutualists).

Beltian bodies are a type of food body (or plant-made structures containing nutritional substances (often high in protein) that attract a mutualist species of ants in Acacia).

See here or here for more info about galls.

To answer your overarching question of whether there are mutualistic galls, I would have to say no. I do not know of any instances of which this is the case. Further, galls are inherently parasitic (not mutualistic). Even if the swollen cells do not interfere with normal plant function (e.g., as is common in Solidago spp.)1, the parasitic insect (or other arthropod) is still required to chew its way out of the gall. Last I checked, chewing on your host's tissues is never very mutualistic.


1: Crutsinger, G.M., Habenicht, M.N., Classen, A.T., Schweitzer, J.A. and Sanders, N.J., 2008. Galling by Rhopalomyia solidaginis alters Solidago altissima architecture and litter nutrient dynamics in an old-field ecosystem. Plant and soil, 303(1-2), pp.95-103.

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Yes, pollinating fig wasps are gall inducing and mutualistic at the same time, and actually essential for the pollination of figs (see e.g Martinson et al., 2015). During the very intricate mutualism, fig wasps deposit eggs in some of the flowers and leave others. The flowers with eggs and later larvae will develop into galls that will produce new wasps but no seeds, while the flowers that lack wasp larvae will produce seeds. However, the hatched females that leave the figs will pick up pollen before leaving, and are essentially the only vector for cross pollinations between fig plants.

(with the aim of expanding the answer later...)

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I do not believe that galls are categorically non-mutualistic. However, as noted above, ant domatia (of acacia at least) are not galls, the trees produce them in the absence of ants. However, the size and number of domatia seems to be mildly inducible by the presence of their symbiotic ants.

But back to galls as mutualistic, my argument is that seemingly purely parasitic/herbivorous galls may also provide indirect fitness benefits to their host through their affect on the endophytic community. There is a vastly underappreciated universe of endyphytes, many are commensal and benign, but there undoubtedly parasites among them, and perhaps their predators as well, qualifying those endophytes as 'mutualists', and thus there are nested ecological interactions in many wild plants which are not apparent when we zoom in on two characters in the whole scene. Insects introduce certain hormonal factors, antimicrobials, and other chemicals which directly affect the dynamics of these endophyte communities. Basically the parasite benefits from keeping its host alive, and may provide immune function to its host in order to keep its food source alive longer, increasing the fitness of both the gall-maker and the host plant. Thus, from a somewhat game theory, lesser-of-two-evils kind of argument, this would qualify an apparent herbivore (a gall-making insect) as a mutualist under certain conditions.

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