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Pace Kramer posted an update 6 months, 2 weeks ago
Entomopathogenic nematodes are parasitic organisms with an exceptional capacity to infect rapidly and efficiently a wide range of insect species. Their distinct pathogenic properties have established entomopathogenic nematodes as supreme biocontrol agents of insects as well as excellent models to simulate and dissect the molecular and physiological bases of conserved strategies employed by parasitic nematodes that cause infectious diseases in humans. The extreme infectivity of entomopathogenic nematodes is due in part to the presence of certain species of Gram-negative bacteria that live in mutualistic symbiosis during the infective juvenile stage, which forms the central part of the nematode life cycle. Both nematodes and their mutualistic bacteria are capable of interfering and undermining several aspects of the insect host innate immune system during the infection process. The mutualistic bacteria are also able to modulate other biological functions in their nematode host including growth, development, and reproduction. In this review, we will focus our attention on the mutualistic relationship between entomopathogenic nematodes and their associated bacteria to discuss the nature and distinct characteristics of the regulatory mechanisms, and their molecular as well as physiological components that control this specific biological partnership.Wolbachia symbionts, first observed in the 1920s, are now known to be present in about 30-70% of tested arthropod species, in about half of tested filarial nematodes (including the majority of human filarial nematodes), and some plant-parasitic nematodes. In arthropods, they are generally viewed as parasites while in nematodes they appear to be mutualists although this demarcation is not absolute. Their presence in arthropods generally leads to reproductive anomalies, while in nematodes, they are generally required for worm development and reproduction. In mosquitos, Wolbachia inhibit RNA viral infections, leading to populational reductions in human RNA virus pathogens, whereas in filarial nematodes, their requirement for worm fertility and survival has been channeled into their use as drug targets for filariasis control. While much more research on these ubiquitous symbionts is needed, they are viewed as playing significant roles in biological processes, ranging from arthropod speciation to human health.Legume-rhizobia symbiosis has a considerable ecological relevance because it replenishes the soil with fixed-nitrogen (e.g., ammonium) for other plants. Because of this benefit to the environment, the exploitation of the legume-rhizobia symbiosis can contribute to the development of the lower input, sustainable agriculture, thereby, reducing dependency on synthetic fertilizers. To achieve this goal, it is necessary to understand the different levels of regulation of this symbiosis to enhance its nitrogen-fixation efficiency. A different line of evidence attests to the relevance of early molecular events in the establishment of a successful symbiosis between legumes and rhizobia. In this chapter, we will review the early molecular signaling in the legume-rhizobia symbiosis. We will focus on the early molecular responses that are crucial for the recognition of the rhizobia as a potential symbiont.Nitrogen-fixing root nodule symbiosis can sustain the development of the host plants under nitrogen-limiting conditions. Such symbiosis occurs only in a clade of angiosperms known as the nitrogen-fixing clade (NFC). It has long been proposed that root nodule symbiosis evolved several times (in parallel) in the NFC. Two recent phylogenomic studies compared the genomes of nodulating and related non-nodulating species across the four orders of the NFC and found that genes essential for nodule formation are lost or pseudogenized in the non-nodulating species. As these symbiosis genes are specifically involved in the symbiotic interaction, it means that the presence of pseudogenes and the loss of symbiosis genes strongly suggest that their ancestor, which still had functional genes, most likely had a symbiosis with nitrogen-fixing bacteria. These findings agree with the hypothesis that nodulation evolved once at the common ancestor of the NFC, and challenge the hypothesis of parallel evolution. In this chapter, we will cover the current understandings on actinorhizal-type and legume nodule development, and discuss the evolution of the legume nodule type.Paulinella photosynthetic species are unicellular, silica shell-forming amoebas classified into the supergroup Rhizaria. click here They crawl at the bottom of freshwater and brackish environments with the help of filose pseudopodia. These protists have drawn the attention of the scientific community because of two photosynthetic bodies, called chromatophores, that fill up their cells permitting fully photoautotrophic existence. Paulinella chromatophores, similarly to primary plastids of the Archaeplastida supergroup (including glaucophytes, red algae as well as green algae and land plants), evolved from free-living cyanobacteria in the process of endosymbiosis. Interestingly, these both cyanobacterial acquisitions occurred independently, thereby undermining the paradigm of the rarity of endosymbiotic events. Chromatophores were derived from α-cyanobacteria relatively recently 60-140 million years ago, whereas primary plastids originated from β-cyanobacteria more than 1.5 billion years ago. Since their acquisition, chromatophore genomes have undergone substantial reduction but not to the extent of primary plastid genomes. Consequently, they have also developed mechanisms for transport of metabolites and nuclear-encoded proteins along with appropriate targeting signals. Therefore, chromatophores of Paulinella photosynthetic species, similarly to primary plastids, are true cellular organelles. They not only show that endosymbiotic events might not be so rare but also make a perfect model for studying the process of organellogenesis. In this chapter, we summarize the current knowledge and retrace the fascinating adventure of Paulinella species on their way to become photoautotrophic organisms.
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