Apomixis is a form of asexual reproduction through seeds in angiosperms. Apomictic plants bypass meiosis and fertilization, developing offspring that are genetically identical to their mother. In a genetic screen for maize (Zea mays) mutants mimicking aspects of apomixis, we identified a dominant mutation resulting in the formation of functional unreduced gametes. The mutant shows defects in chromatin condensation during meiosis and subsequent failure to segregate chromosomes. The mutated locus codes for AGO104, a member of the ARGONAUTE family of proteins. AGO104 accumulates specifically in somatic cells surrounding the female meiocyte, suggesting a mobile signal rather than cellautonomous control. AGO104 is necessary for non-CG methylation of centromeric and knob-repeat DNA. Digital gene expression tag profiling experiments using high-throughput sequencing show that AGO104 influences the transcription of many targets in the ovaries, with a strong effect on centromeric repeats. AGO104 is related to Arabidopsis thaliana AGO9, but while AGO9 acts to repress germ cell fate in somatic tissues, AGO104 acts to repress somatic fate in germ cells. Our findings show that female germ cell development in maize is dependent upon conserved small RNA pathways acting noncell-autonomously in the ovule. Interfering with this repression leads to apomixis-like phenotypes in maize.

Apomictic plants reproduce asexually through seeds by avoiding both meiosis and fertilization. Although apomixis is genetically regulated, its core genetic component(s) has not been determined yet. Using profiling experiments comparing sexual development in maize (Zea mays) to apomixis in maize-Tripsacum hybrids, we identified six loci that are specifically downregulated in ovules of apomictic plants. Four of them share strong homology with members of the RNA-directed DNA methylation pathway, which in Arabidopsis thaliana is involved in silencing via DNA methylation. Analyzing loss-of-function alleles for two maize DNA methyltransferase genes belonging to that subset, dmt102 and dmt103, which are downregulated in the ovules of apomictic plants and are homologous to the Arabidopsis CHROMOMETHYLASEs and DOMAINS REARRANGED METHYLTRANSFERASE families, revealed phenotypes reminiscent of apomictic development, including the production of unreduced gametes and formation of multiple embryo sacs in the ovule. Loss of DMT102 activity in ovules resulted in the establishment of a transcriptionally competent chromatin state in the archesporial tissue and in the egg cell that mimics the chromatin state found in apomicts. Interestingly, dmt102 and dmt103 expression in the ovule is found in a restricted domain in and around the germ cells, indicating that a DNA methylation pathway active during reproduction is essential for gametophyte development in maize and likely plays a critical role in the differentiation between apomictic and sexual reproduction.

Cytosine methylation is a key epigenetic mark in many organisms, important for both transcriptional control and genome integrity. While relatively stable during somatic growth, DNA methylation is reprogrammed genome-wide during mammalian reproduction. Reprogramming is essential for zygotic totipotency and to prevent transgenerational inheritance of epimutations. However, the extent of DNA methylation reprogramming in plants remains unclear. Here, we developed sensors reporting with single-cell resolution CG and non-CG methylation in Arabidopsis. Live imaging during reproduction revealed distinct and sex-specific dynamics for both contexts. We found that CHH methylation in the egg cell depends on DOMAINS REARRANGED METHYLASE 2 (DRM2) and RNA polymerase V (Pol V), two main actors of RNA-directed DNA methylation, but does not depend on Pol IV. Our sensors provide insight into global DNA methylation dynamics at the single-cell level with high temporal resolution and offer a powerful tool to track CG and non-CG methylation both during development and in response to environmental cues in all organisms with methylated DNA, as we illustrate in mouse embryonic stem cells.

Although mutualistic associations between animals and microbial symbionts are widespread in nature, the mechanisms that have promoted their evolutionary persistence remain poorly understood. A vertical mode of symbiont transmission (from parents to offspring) is thought to ensure partner fidelity and stabilization, although the efficiency of vertical transmission has rarely been investigated, especially in cases where hosts harbour a diverse microbial community. Here we evaluated vertical transmission rates of cellulolytic gut oxymonad and parabasalid protists in the wood‐feeding termite Reticulitermes grassei. We sequenced amplicons of the 18S rRNA gene of protists from 24 colonies of R. grassei collected in two populations. For each colony, the protist community was characterized from the gut of 14 swarming reproductives and from a pool of 10 worker guts. A total of 98 operational taxonomic units belonging to 13 species‐level taxa were found. The vertical transmission rate was estimated for each protist present in a colony based on its frequency among the reproductives. The results revealed that transmission rates were high, with an average of 0.897 (±0.164) per protist species. Overall, the protist community did not differ between reproductive sexes, suggesting that both the queen and the king could contribute to the gut microbiota of the offspring. A positive relationship between the transmission rate of protists and their prevalence within populations was also detected. However, transmission rates alone do not explain the prevalence of protists. In conclusion, these findings reveal key forces behind a conserved, multispecies mutualism, raising further questions on the roles of horizontal transfer and negative selection in shaping symbiont prevalence.

Bacillus cereus ATCC 14579 possesses five RNA helicase-encoding genes overexpressed under cold growth conditions. Out of the five corresponding mutants, only the ⌬cshA, ⌬cshB, and ⌬cshC strains were cold sensitive. Growth of the ⌬cshA strain was also reduced at 30°C but not at 37°C. The cold phenotype was restored with the cshA gene for the ⌬cshA strain and partially for the ⌬cshB strain but not for the ⌬cshC strain, suggesting different functions at low temperature.

Background: Most extracellular virulence factors produced by Bacillus cereus are regulated by the pleiotropic transcriptional activator PlcR. Among strains belonging to the B. cereus group, the plcR gene is always located in the vicinity of genes encoding the YvfTU two-component system. The putative role of YvfTU in the expression of the PlcR regulon was therefore investigated.

The mitochondrial calcium uniporter complex (MCUc) was recently characterized in details in metazoans and consists of pore-forming units (MCUs) and regulatory factors that channel calcium (Ca ) ion into the mitochondria. MCUs participate in many stress and developmentally related processes involving Ca . Although multiple homologues of MCUs and one regulatory subunit are usually present in plants, the first functional characterization and contribution to Ca related processes of these proteins have been reported recently. Here, we focused on two predicted Arabidopsis MCUs and studied their role in the germination and the growth of pollen tube, a tip-growing cell type highly dependent on Ca homeostasis. Heterologous expression of MCU1 or MCU2 in yeast is sufficient to generate a mitochondrial Ca influx. MCU1 and MCU2 fluorescent reporters are co-expressed in the vegetative cell mitochondria of the pollen grain but are undetectable in the embryo sac. We demonstrate that MCU1 and MCU2 can form a heterotypic complex. Phenotypic analyses revealed an impaired pollen tube germination and growth in vitro only for the mcu2 mutants suggesting a predominant role of MCU2. Our results show that mitochondrial Ca controlled by MCUs is an additional player in Arabidopsis pollen tube germination and growth.

13Evolutionary theory predicts that vertically transmitted symbionts are selected for low 14 virulence, as their fitness is directly correlated to that of their host. In contrast with this prediction, 15 the Wolbachia strain wMelPop drastically reduces its Drosophila melanogaster host lifespan at 16 high rearing temperatures. It is generally assumed that this feature is maintained because the 17 D. melanogaster-wMelPop symbiosis is usually not exposed to environmental conditions in which 18 the symbiont is virulent. To test this hypothesis, we submitted wMelPop-infected D. melanogaster 19 lines to 17 generations of experimental evolution at a high temperature, while enforcing late 20 reproduction. The fly survival was measured at different time points, as well as two traits that have 21 2 been proposed to be causally responsible for wMelPop virulence: its relative density and the mean 22 number of octomom copies present in its genome. We hypothesised that these conditions would 23 select for a reduced wMelPop virulence, a reduced wMelPop density, and a reduced octomom copy 24 number. Our results indicate that density, octomom copy number and virulence are correlated. 25However, contrary to our expectations, we could not detect any reduction in virulence during the 26 course of evolution. We discuss the significance of our results with respect to the evolutionary 27 causes of wMelPop virulence and propose that intra-host selection could explain this conundrum. 28 29 42correlated to that of their hosts, they are expected to be selected for low virulence (Ewald 1983). 43 3 This reasoning, just like the transmission/virulence trade-off hypothesis, rests solely on the 44inter-host level of selection, ignoring the potential effects of intra-host selection. Indeed, the most 45 competitive symbionts within a host may replicate faster and be more virulent than what would be 46 optimal from the standpoint of inter-host selection (Alizon et al. 2013). However, the causal link 47