During embryonic development, maintenance of cell identity and lineage commitment requires the Polycomb-group PRC2 complex, which catalyzes histone H3 lysine 27 trimethylation (H3K27me3). However, the developmental origins of this regulation are unknown. Here we show that H3K27me3 enrichment increases from blastula stages onward in embryos of the Western clawed frog (Xenopus tropicalis) within constrained domains strictly defined by sequence. Strikingly, although PRC2 also binds widely to active enhancers, H3K27me3 is only deposited at a small subset of these sites. Using a Support Vector Machine algorithm, these sequences can be predicted accurately on the basis of DNA sequence alone, with a sequence signature conserved between humans, frogs, and fish. These regions correspond to the subset of blastula-stage DNA methylation-free domains that are depleted for activating promoter motifs, and enriched for motifs of developmental factors. These results imply a genetic-default model in which a preexisting absence of DNA methylation is the major determinant of H3K27 methylation when not opposed by transcriptional activation. The sequence and motif signatures reveal the hierarchical and genetically inheritable features of epigenetic cross-talk that impose constraints on Polycomb regulation and guide H3K27 methylation during the exit of pluripotency.

Comparison of the molecular diversity in all plankton populations present in geographically distant water columns may allow for a holistic view of the connectivity, isolation and adaptation of organisms in the marine environment. In this context, a large‐scale detection and analysis of genomic variants directly in metagenomic data appeared as a powerful strategy for the identification of genetic structures and genes under natural selection in plankton. Here, we used discosnp++, a reference‐free variant caller, to produce genetic variants from large‐scale metagenomic data and assessed its accuracy on the copepod Oithona nana in terms of variant calling, allele frequency estimation and population genomic statistics by comparing it to the state‐of‐the‐art method. discosnp ++ produces variants leading to similar conclusions regarding the genetic structure and identification of loci under natural selection. discosnp++ was then applied to 120 metagenomic samples from four size fractions, including prokaryotes, protists and zooplankton sampled from 39 tara Oceans sampling stations located in the Atlantic Ocean and the Mediterranean Sea to produce a new set of marine genomic markers containing more than 19 million of variants. This new genomic resource can be used by the community to relocate these markers on their plankton genomes or transcriptomes of interest. This resource will be updated with new marine expeditions and the increase of metagenomic data (availability: http://bioinformatique.rennes.inria.fr/taravariants/).

14Copepods are the most numerous animals and play an essential role in the marine trophic web 15 and biogeochemical cycles. The genus Oithona is described as having the highest numerical 16 density, as the most cosmopolite copepod and iteroparous. The Oithona male paradox obliges 17 it to alternate feeding (immobile) and mating (mobile) phases. As the molecular basis of this 18 trade-off is unknown, we investigated this sexual dimorphism at the molecular level by 19 integrating genomic, transcriptomic and protein-protein interaction analyses. 20While a ZW sex-determination system was predicted in O. nana, a fifteen-year time-series in 21 the Toulon Little Bay showed a biased sex ratio toward females (male / female ratio < 0.15± 22 neurotransmitter glutamate and permit new axons and dendrites formation suggesting a sexual 33 nervous system dimorphism. This supports the hypothesis of a sacrificial behaviour in males 34 at the metabolic level and constitutes the first case of sacrificial behaviour in an iteroparous 35 animal. 36 37 Copepods are small planktonic crustacean forming the most abundant metazoan subclass on 38Earth and occupy all ecological aquatic niches (Huys and Boxshall 1991; Kiørboe 2011). 39 Among them, the genus Oithona is described as having the highest numerical density 40 (Gallienne and Robins 2001), cosmopolite (Nishida 1985) and playing a key role of secondary 41 producer in the marine food web and biogeochemical cycles (Steinberg and Landry 2017). 42Because of its importance, Oithona phylogeography, ecology, behaviour, life cycle, anatomy 43 and genomics are studied (Cornils,