Symbiosis: A duplicated host protein controlling a nascent mutualism
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Mutualism
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ABSTRACT Evolutionary theory suggests that the conditions required for the establishment of mutualistic symbioses through mutualism alone are highly restrictive, often requiring the evolution of complex stabilising mechanisms. Exploitation, whereby initially the host benefits at the expense of its symbiotic partner and mutual benefits evolve subsequently through trade-offs, offers an arguably simpler route to the establishment of mutualistic symbiosis. In this review, we discuss the theoretical and experimental evidence supporting a role for host exploitation in the establishment and evolution of mutualistic microbial symbioses, including data from both extant and experimentally evolved symbioses. We conclude that exploitation rather than mutualism may often explain the origin of mutualistic microbial symbioses.
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Confusion has afflicted the definition of symbiosis for over 130 years. Despite the lack of discussion in recent times, the usage of symbiosis has evolved and appears to be stabilizing to broader interpretations. Current usage of symbiosis and its associated terminology in 10 current general biology (GB) and 10 general ecology (GE) textbooks is presented. The restrictive definition (i.e. symbiosis = mutualism) has essentially disappeared. All GB textbooks (100%) surveyed used an explicit or implicit “de Bary” definition of symbiosis (i.e. mutualism, commensalism, and parasitism), while only 40% of GE textbooks did the same. General ecology textbooks also included 30% defining symbiosis to constitute all species interactions and 30% that completely avoided usage of the term. When combining GB and GE textbooks to analyze symbiotic usage, 85% defined mutualism, commensalism, and parasitism as symbiotic interactions. Also, 70% considered a symbiosis to be a species interaction that is “intimate,” with 45% of those both “intimate and constant.” Unfortunately, only 5% used the terms ecto-/endosymbiosis, which help discern intimacy and constancy in species interactions. Usage of symbiont (55%) was preferred over symbiote (0%). Predator and prey were defined as organisms (vs. animals) in 90% of GB and GE textbooks, while 55% and 75% described carnivores and herbivores as organisms, respectively. Only 25% discussed predation, parasitism, parasitoidism, and grazing/herbivory, with only one (5%) integrating these +/- agonistic interactions in relation to intimacy and lethality. Data reveals trend of biologists/ecologists using broader definitions.
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Mutualistic symbioses with mycorrhizal fungi are widespread in plants. The majority of plant species associate with arbuscular mycorrhizal (AM) fungi. By contrast, the minority associate with ectomycorrhizal (EM) fungi, have abandoned the symbiosis and are nonmycorrhizal (NM), or engage in an intermediate, weakly AM symbiosis (AMNM). To understand the processes that maintain the mycorrhizal symbiosis or cause its loss, we reconstructed its evolution using a ∼3,000-species seed plant phylogeny integrated with mycorrhizal state information. Reconstruction indicated that the common ancestor of seed plants most likely associated with AM fungi and that the EM, NM, and AMNM states descended from the AM state. Direct transitions from the AM state to the EM and NM states were infrequent and generally irreversible, implying that natural selection or genetic constraint could promote stasis once a particular state evolved. However, the evolution of the NM state was more frequent via an indirect pathway through the AMNM state, suggesting that weakening of the AM symbiosis is a necessary precursor to mutualism abandonment. Nevertheless, reversions from the AMNM state back to the AM state were an order of magnitude more likely than transitions to the NM state, suggesting that natural selection favors the AM symbiosis over mutualism abandonment.
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Endophyte
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Mutualism describes positive ecological interactions among species that benefit each other. There can be symbiosis when two different biological organisms interact closely for a long period of time. This symbiotic relationship can be mutualistic, communalistic, or parasitic. One such mutualistic relationship is shared between the Ants and Funguses. This relationship can be tri-way symbiosis also. In that case, ecosystems rely on interactions between fungi and plants, plants and ants and ants and fungi. But it is rare to find symbioses that involve all three partners. The cultivation of fungi thought various types of agricultural systems is considered as one of the prominent breakthroughs in ant evolution. The fungus-growing ants are commonly called as attines. The nests of these attines are made up of leaves and grasses, which they cut priory and then carry to their nests to grow fungus on which they feed on. An ant has been found to contain a bacterial symbiont that produces certain compounds that fight against infections caused by other group of fungi. These bacterial symbionts are from the genus Pseudonocardia.
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Symbioses, prolonged associations between organisms often widely separated phylogenetically, are more common in biology than we once thought and have been neglected as a phenomenon worthy of study on its own merits. Extending along a dynamic continuum from antagonistic to cooperative and often involving elements of both antagonism and mutualism, symbioses involve pathogens, commensals, and mutualists interacting in myriad ways over the evolutionary history of the involved "partners." In this first of 2 parts, some remarkable examples of symbiosis will be explored, from the coral-algal symbiosis and nitrogen fixation to the great diversity of dietary specializations enabled by the gastrointestinal microbiota of animals.
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Commensalism
Mutualism
Endophyte
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Mutualism
Glomeromycota
Endosymbiosis
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