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Arbuscular mycorrhizal (AM) symbiosis is one of the most prominent and beneficial plant-microbe interactions that facilitates mineral nutrition and confers tolerance to biotic and abiotic stresses. AM fungi colonize the root cortex and develop specialized structures called arbuscules where the nutrient exchange takes place. Arbuscule development is a highly controlled and coordinated process requiring the involvement of many plant proteins recruited at that interface. In contrast, much less is known about the fungal proteins involved in this process. Here, we have identified an AM fungal effector that participates in this developmental step of the symbiosis. RiCRN1 is a crinkler (CRN) effector that belongs to a subfamily of secreted CRN proteins from R. irregularis . CRNs have been so far only functionally characterized in pathogenic microbes and shown to participate in processes controlling plant cell death and immunity. RiCRN1 accumulates during symbiosis establishment parallel to MtPT4 , the gene coding for an arbuscule-specific phosphate transporter. Expression in Nicotiana benthamiana leaves and in Medicago truncatula roots suggest that RiCRN1 is not involved in cell death processes. RiCRN1 dimerizes and localizes to nuclear bodies, suggesting that, similar to other CRNs, it functions in the plant nucleus. Downregulation of RiCRN1 using host-induced gene silencing led to an impairment of the symbiosis in M. truncatula and to a reduction of MtPT4 , while ectopic expression of RiCRN1 , surprisingly, led to a drastic reduction in arbuscule size that correlated with a decrease not only in MtPT4 but also in MtBCP1 , a marker for initial stages of arbuscule development. Altogether, our results suggest that a tightly regulated expression in time and space of RiCRN1 is critical for symbiosis progression and for the proper initiation of arbuscule development.

Encephalitozoon cuniculi is a model microsporidian species with a mononucleate nucleus and a genome that has been extensively studied. To date, analyses of genome diversity have revealed the existence of four genotypes in E. cuniculi (EcI, II, III and IV). Genome sequences are available for EcI, II and III, and are all very divergent, possibly diploid and genetically homogeneous. The mechanisms that cause low genetic diversity in E. cuniculi (for example, selfing, inbreeding or a combination of both), as well as the degree of genetic variation in their natural populations, have been hard to assess because genome data have been so far gathered from laboratory-propagated strains. In this study, we aim to tackle this issue by analyzing the complete genome sequence of a natural strain of E. cuniculi isolated in 2013 from a steppe lemming. The strain belongs to the EcIII genotype and has been designated EcIII-L. The EcIII-L genome sequence harbors genomic features intermediate to known genomes of II and III lab strains, and we provide primers that differentiate the three E. cuniculi genotypes using a single PCR. Surprisingly, the EcIII-L genome is also highly homogeneous, harbors signatures of heterozygosity and also one strain-specific single-nucleotide polymorphism (SNP) that introduces a stop codon in a key meiosis gene, Spo11. Functional analyses using a heterologous system demonstrate that this SNP leads to a deficient meiosis in a model fungus. This indicates that EcIII-L meiotic machinery may be presently broken. Overall, our findings reveal previously unsuspected genome diversity in E. cuniculi, some of which appears to affect genes of primary importance for the biology of this pathogen.

Mikrocytos mackini is an intracellular protistan parasite of oysters whose position in the phylogenetic tree of eukaryotes has been a mystery for many years [1,2]. M. mackini is difficult to isolate, has not been cultured, and has no defining morphological feature. Furthermore, its only phylogenetic marker that has been successfully sequenced to date (the small subunit ribosomal RNA) is highly divergent and has failed to resolve its evolutionary position [2]. M. mackini is also one of the few eukaryotes that lacks mitochondria [1], making both its phylogenetic position and comparative analysis of mitochondrial function particularly important. Here, we have obtained transcriptomic data for M. mackini from enriched isolates and constructed a 119-gene phylogenomic data set. M. mackini proved to be among the fastest-evolving eukaryote lineages known to date, but, nevertheless, our analysis robustly placed it within Rhizaria. Searching the transcriptome for genetic evidence of a mitochondrion-related organelle (MRO) revealed only four mitochondrion-derived genes: IscS, IscU, mtHsp70, and FdxR. Interestingly, all four genes are involved in iron-sulfur cluster formation, a biochemical pathway common to other highly reduced "mitosomes" in unrelated MRO-containing lineages [7]. This is the first evidence of MRO in Rhizaria, and it suggests the parallel evolution of mitochondria to mitosomes in this supergroup., (Copyright © 2013 Elsevier Ltd. All rights reserved.)

About 20 new isolates of Carpediemonas-like organisms (CLOs) have been reported since 2006. Small subunit rRNA gene phylogenies divide CLOs into six major clades: four contain described exemplars (i.e. Carpediemonas, Dysnectes, Hicanonectes, and Kipferlia), but two include only undescribed organisms. Here we describe a representative of one of these latter clades as Ergobibamus cyprinoides n. g., n. sp., and catalogue its ultrastructure. Ergobibamus cyprinoides is a bean-shaped biflagellated cell, 7-11.5 μm long, with a conspicuous groove. Instead of classical mitochondria there are cristae-lacking rounded organelles 300-400 nm in diameter. The posterior flagellum has a broad ventral vane and small dorsal vane. There are normally four basal bodies, two non-flagellated. There is one anterior root (AR), containing six microtubules. The posterior flagellar apparatus follows the "typical excavate" pattern of a splitting right root supported by fibres "I,""B," and "A," a "composite" fibre, a singlet root, and a left root (LR) with a "C" fibre. The B fibre originates against the LR--a synapomorphy of the taxon Fornicata--supporting the assignation of Ergobibamus to Fornicata, along with diplomonads, retortamonads, and other CLOs. Distinctive features of E. cyprinoides include the complexity of the AR, which is intermediate between Hicanonectes, and Carpediemonas and Dysnectes, and a dorsal extension of the C fibre., (© 2010 The Author(s). Journal of Eukaryotic Microbiology © 2010 International Society of Protistologists.)