Mutualism between co-introduced species facilitates invasion and alters plant community structure

Generalized mutualisms are often predicted to be resilient to changes in partner identity. Variation in mutualism-related traits between native and invasive species however, can exacerbate the spread of invasive species (‘invasional meltdown’) if invasive partners strongly interact. Here we show how invasion by a seed-dispersing ant (Myrmica rubra) promotes recruitment of a co-introduced invasive over native ant-dispersed (myrmecochorous) plants. We created experimental communities of invasive (M. rubra) or native ants (Aphaenogaster rudis) and invasive and native plants and measured seed dispersal and plant recruitment. In our mesocosms, and in laboratory and field trials, M. rubra acted as a superior seed disperser relative to the native ant. By contrast, previous studies have found that invasive ants are often poor seed dispersers compared with native ants. Despite belonging to the same behavioural guild, seed-dispersing ants were not functionally redundant. Instead, native and invasive ants had strongly divergent effects on plant communities: the invasive plant dominated in the presence of the invasive ant and the native plants dominated in the presence of the native ant. Community changes were not due to preferences for coevolved partners: variation in functional traits of linked partners drove differences. Here, we show that strongly interacting introduced mutualists can be major drivers of ecological change.     

Economic game theory for mutualism and cooperation

We review recent work at the interface of economic game theory and evolutionary biology that provides new insights into the evolution of partner choice, host sanctions, partner fidelity feedback and public goods. (1) The theory of games with asymmetrical information shows that the right incentives allow hosts to screen-out parasites and screen-in mutualists, explaining successful partner choice in the absence of signalling. Applications range from ant-plants to microbiomes. (2) Contract theory distinguishes two longstanding but weakly differentiated explanations of host response to defectors: host sanctions and partner fidelity feedback. Host traits that selectively punish misbehaving symbionts are parsimoniously interpreted as pre-adaptations. Yucca-moth and legume-rhizobia mutualisms are argued to be examples of partner fidelity feedback. (3) The theory of public goods shows that cooperation in multi-player interactions can evolve in the absence of assortment, in one-shot social dilemmas among non-kin. Applications include alarm calls in vertebrates and exoenzymes in microbes.     

The devil to pay: a cost of mutualism with Myrmelachista schumanni ants in 'devil's gardens' is increased herbivory on Duroia hirsuta trees

'Devil's gardens' are nearly pure stands of the myrmecophyte, Duroia hirsuta, that occur in Amazonian rainforests. Devil's gardens are created by Myrmelachista schumanni ants, which nest in D. hirsuta trees and kill other plants using formic acid as an herbicide. Here, we show that this ant-plant mutualism has an associated cost; by making devil's gardens, M. schumanni increases herbivory on D. hirsuta. We measured standing leaf herbivory on D. hirsuta trees and found that they sustain higher herbivory inside than outside devil's gardens. We also measured the rate of herbivory on nursery-grown D. hirsuta saplings planted inside and outside devil's gardens in ant-exclusion and control treatments. We found that when we excluded ants, herbivory on D. hirsuta was higher inside than outside devil's gardens. These results suggest that devil's gardens are a concentrated resource for herbivores. Myrmelachista schumanni workers defend D. hirsuta against herbivores, but do not fully counterbalance the high herbivore pressure in devil's gardens. We suggest that high herbivory may limit the spread of devil's gardens, possibly explaining why devil's gardens do not overrun Amazonian rainforests.     

The direct and ecological costs of an ant-plant symbiosis

How strong is selection for cheating in mutualisms? The answer depends on the type and magnitude of the costs of the mutualism. Here we investigated the direct and ecological costs of plant defense by ants in the association between Cordia nodosa, a myrmecophytic plant, and Allomerus octoarticulatus, a phytoecious ant. Cordia nodosa trees produce food and housing to reward ants that protect them against herbivores. For nearly 1 year, we manipulated the presence of A. octoarticulatus ants and most insect herbivores on C. nodosa in a full-factorial experiment. Ants increased plant growth when herbivores were present but decreased plant growth when herbivores were absent, indicating that hosting ants can be costly to plants. However, we did not detect a cost to ant colonies of defending host plants against herbivores. Although this asymmetry in costs suggests that the plants may be under stronger selection than the ants to cheat by withholding investment in their partner, the costs to C. nodosa are probably at least partly ecological, arising because ants tend scale insects on their host plants. We argue that ecological costs should favor resistance or traits other than cheating and thus that neither partner may face much temptation to cheat.     

Thallous ion activation of p-nitrophenylphosphatase of rat myometrium plasma membrane

Addition of thallous ion (Tl+) inhibited the spontaneous mechanical activity of rat myometrium in K-free Krebs solutions with an I50 value of 30 microM. The corresponding value for I50 for similar inhibition by addition of K+ was 150 microM. Tl+ as well as K+-activated p-nitrophenylphosphatase (PNPPase) of isolated rat myometrium plasma membrane vesicles. Half maximal activation was caused by 0.47 mM Tl+ or 1.6 mM K+. Maximal enzyme activities obtained using Tl+ and K+ were comparable. The Km values for the substance p-nitrophenylphosphatase using Tl+ (1.24 mM) and K+ (1.46 mM) were also similar. Activation by either ion was inhibited by ouabain, Na+, inorganic phosphate, and vanadate (V +5). The results suggest that Tl+ can substitute for K+ for activation of the Na-K pump of rat myometrium plasma membrane.     

Allosteric regulation of glycerol kinase by enzyme IIIglc of the phosphotransferase system in Escherichia coli and Salmonella typhimurium.

The mechanism by which enzyme IIIglc of the bacterial phosphotransferase system regulates the activity of crystalline glycerol kinase from Escherichia coli has been studied, and the inhibitory effects have been compared with those produced by fructose-1,6-diphosphate. It was shown that the free, but not the phosphorylated, form of enzyme IIIglc inhibits the kinase. Mutants of Salmonella typhimurium were isolated which were resistant to inhibition by either enzyme IIIglc (glpKr mutants) or fructose-1,6-diphosphate (glpKi mutants), and each mutant type was shown to retain full sensitivity to inhibition by the other regulatory agent. Other mutants were fully or partially resistant to regulation by both agents. The two regulatory sites on the kinase are evidently distinct but must overlap or interact functionally. Kinetic analyses have revealed several mechanistic features of the regulatory interactions. (i) Inhibition by both allosteric regulatory agents is strongly pH dependent, with maximal inhibition occurring at ca. pH 6.5 under the assay conditions employed. (ii) Binding of enzyme IIIglc to glycerol kinase is also pH dependent, the Ki being near 4 microM at pH 6.0 but near 10 microM at pH 7.0. (iii) Whereas fructose-1,6-diphosphate inhibition apparently requires that the enzyme exist in a tetrameric state, both the dimer and the tetramer appear to be fully sensitive to enzyme IIIglc inhibition. (iv) Inhibition by enzyme IIIglc (like that by fructose-1,6-diphosphate) is noncompetitive with respect to both substrates. (v) The inhibitory responses of glycerol kinase to fructose-1, 6-diphosphate and enzyme IIIglc show features characteristic of positive cooperativity at low inhibitor concentration. (vi) Neither agent inhibits completely at high inhibitor concentration. (vii) Apparent negative cooperativity with respect to ATP binding is observed with purified E. coli glycerol kinase, with glycerol kinase in crude extracts of wild-type S. typhimurium cells, and with glpKr and glpKi mutant forms of glycerol kinase from S. typhimurium. These results serve to characterize the regulatory interactions which control the activity of glycerol kinase by fructose-1,6-diphosphate and by enzyme IIIglc of the phosphotransferase system.     

Synaptically released zinc: Physiological functions and pathological effects

In addition to its familiar role as a component of metalloproteins, zinc is also sequestered in the presynaptic vesicles of a specialized type of neurons called `zinc-containing' neurons. Here we review the physiological and pathological effects of the release of zinc from these zinc-containing synaptic terminals. The best-established physiological role of synaptically released zinc is the tonic modulation of brain excitability through modulation of amino acid receptors; prominent pathological effects include acceleration of plaque deposition in Alzheimer's disease and exacerbation of excitotoxic neuron injury. Synaptically released zinc functions as a conventional synaptic neurotransmitter or neuromodulator, being released into the cleft, then recycled into the presynaptic terminal. Beyond this, zinc also has the highly unconventional property that it passes into postsynaptic neurons during synaptic events, functioning analogously to calcium in this regard, as a transmembrane neural signal. To stimulate comparisons of zinc signals with calcium signals, we have compiled a list of the important parameters of calcium signals and zinc signals. More speculatively, we hypothesize that zinc signals may loosely mimic phosphate `signals' in the sense that signal zinc ions may commonly bind to proteins in a lasting manner (i.e., `zincylating' the proteins) with consequential changes in protein structure and function.