In diverse mutualisms, it is common for potential partners to vary in the quality of benefits they provide. When weakly beneficial mutualists and parasites have a competitive advantage over strongly beneficial mutualists, it is not clear how strongly beneficial mutualists persist. If mutualism is destabilized by competitive superiority of weakly beneficial mutualists or cheaters, then mechanisms providing for stable coexistence among competing species may also provide for the persistence of mutualism. We analyze coexistence of species within a mutualist guild using a simple spatial model of patch occupancy to suggest hypotheses about the ecological persistence of mutualism in the interaction between plants and ectomycorrhizal fungi. We suggest that plants could facilitate the persistence of mutualistic mycorrhizal fungi by enhancing the mortality of root tips colonized by competitively superior and less mutualistic fungi. We also discuss previous empirical studies and present original data from field observations in plant-ectomycorrhizal systems to address our predictions and to suggest profitable avenues for further work.
Can choice of mutualistic partners and the degree of their utilization determine (1) mutualistic partner coexistence, (2) relative abundance of mutualistic partners, and (3) environment-dependent changes in relative abundance? We investigate these questions in the context of the plant-mycorrhizal fungal mutualism by building a biological market model potentially applicable to other mutualisms as well. We examine the situation where a single plant selectively utilizes member(s) of a group of ectomycorrhizal potential trading partners. Under biologically realistic circumstances, the plant may simultaneously utilize multiple partners, its degree of utilization determining the community structure of the fungi. If utilization of multiple partners is optimal, the marginal cost of acquiring additional nitrogen from every trading partner must be equal while the marginal cost of acquiring it from any unutilized partner must be larger. Because the plant's nitrogen demand is light dependent, the composition of the fungal species among its trading partners changes along light-availability gradients. We discuss the design of an experiment to test the key prediction of our model, the equalization of marginal cost.
Using a 30 day time series of aphid Aphis helianthi and coccinellid counts on 107 mapped racemes of Yucca glauca, we demonstrate progressive, predation-induced self-organization of aphid colonies on individual racemes into extremely low and extremely high population sizes. This was driven by a two-attractor structure of density dependence that developed only in the presence of coccinellid predators. Foraging movements of the coccinellids among plants produced a power law relationship (average power 0.142) between aphid and coccinellid numbers. This resulted in increased predation pressure on mid-size colonies and decreased predation pressure on small and large populations. A field-parameterized mathematical model predicts a two-attractor structure in broad agreement with our observations. The overall system was integrated by the influence of the largest aphid populations, which determined the total number of coccinellids present, and thus the predation pressure throughout the system. Our study provides clear evidence of predator-driven self-organization of prey populations in a patchy environment.
We report two cases of prolapse of the fallopian tube associated with an exuberant angiomyofibroblastic stroma response, which occurred after hysterectomy and which is a hitherto unreported feature of this lesion. The tumors were composed of richly vascularized stroma arranged in a retiform pattern and mildly atypical glandular inclusions, which had the morphology of tubal epithelium. The stroma of the lesion was composed of either thin bipolar cells with tapered nuclei and stellate-shaped cells with minimal amount of cytoplasm or small epithelioid-looking cells with eosinophilic cytoplasm. The tubal glandular inclusions displayed mildly atypical nuclear features. If the tubal glandular component was overlooked, these tumors might be erroneously diagnosed as mesenchymal lesions of the vagina, such as vaginal fibroepithelial polyp, angiomyofibroblastoma, aggressive angiomyxoma, or superficial myofibroblastoma.
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