While all known natural isolates of C. elegans contain multiple copies of the Tc1 transposon, which are active in the soma, Tc1 transposition is fully silenced in the germline of many strains. We mutagenized one such silenced strain and isolated mutants in which Tc1 had been activated in the germline ("mutators"). Interestingly, many other transposons of unrelated sequence had also become active. Most of these mutants are resistant to RNA interference (RNAi). We found one of the mutated genes, mut-7, to encode a protein with homology to RNaseD. This provides support for the notion that RNAi works by dsRNA-directed, enzymatic RNA degradation. We propose a model in which MUT-7, guided by transposon-derived dsRNA, represses transposition by degrading transposon-specific messengers, thus preventing transposase production and transposition.

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Picocyanobacteria of the genus Synechococcus span a range of different colours, from red strains rich in phycoerythrin (PE) to green strains rich in phycocyanin (PC). Here, we show that coexistence of red and green picocyanobacteria in the Baltic Sea is widespread. The diversity and phylogeny of red and green picocyanobacteria was analysed using three different genes: 16S rRNA-ITS, the cpeBA operon of the red PE pigment and the cpcBA operon of the green PC pigment. Sequencing of 209 clones showed that Baltic Sea picocyanobacteria exhibit high levels of microdiversity. The partial nucleotide sequences of the cpcBA and cpeBA operons from the clone libraries of the Baltic Sea revealed two distinct phylogenetic clades: one clade containing mainly sequences from cultured PC-rich picocyanobacteria, while the other contains only sequences from cultivated PE-rich strains. A third clade of phycourobilin (PUB) containing strains of PE-rich Synechococcus spp. did not contain sequences from the Baltic Sea clone libraries. These findings differ from previously published phylogenies based on 16S rRNA gene analysis. Our data suggest that, in terms of their pigmentation, Synechococcus spp. represent three different lineages occupying different ecological niches in the underwater light spectrum. Strains from different lineages can coexist in light environments that overlap with their light absorption spectra.

Summary The mosaic distribution of interbreeding taxa with contrasting ecology and morphology offers an opportunity to study microevolutionary dynamics during ecological divergence. We investigate here the evolutionary history of an alpine and a montane ecotype of Heliosperma pusillum (Caryophyllaceae) in the south‐eastern Alps.From six pairs of geographically close populations of the two ecotypes (120 individuals) we obtained a high‐coverage restriction site associated DNA sequencing (RADseq) dataset that was used for demographic inference to test the hypothesis of parallel evolution of the two ecotypes.The data are consistent with repeated ecological divergence in H. pusillum, uncovering up to five polytopic origins of one ecotype from the other. A complex evolutionary history is evidenced, with local isolation‐with‐migration in two population pairs and intra‐ecotype migration in two others. In all cases, the time of divergence or secondary contact was inferred as postglacial. A metagenomic analysis on exogenous contaminant RAD sequences suggests divergent microbial communities between the ecotypes.The lack of shared genomic regions of high divergence across population pairs illustrates the action of drift and/or local selection in shaping genetic divergence across repeated cases of ecological divergence.

Synechococcus is a cosmopolitan genus of picocyanobacteria living in the photic zone of freshwater and marine ecosystems. Here, we describe the isolation of 46 closely related picocyanobacterial strains from the Baltic Sea. The isolates showed considerable variation in their cell size and pigmentation phenotypes, yielding a colorful variety of red, pink and blue-green strains. These pigmentation phenotypes could not be differentiated on the basis of their 16S rRNA-internal transcribed spacer (ITS) sequences. Thirty-nine strains, designated BSea, possessed 16S rRNA-ITS sequences almost identical with Synechococcus strain WH5701. Despite their similar 16S rRNA-ITS sequences, the BSea strains separated into several different clusters when comparing the phycocyanin (cpcBA) operon. This separation was largely consistent with the phycobiliprotein composition of the different BSea strains. The majority of phycocyanin (PC)-rich Bsea strains clustered with WH5701. Remarkably, the phycoerythrin (PE)-rich strains of BSea formed an as yet unidentified cluster within the cpcBA phylogeny, distantly related to other PE-rich groups. Detailed analysis of the cpcBA operon using neighbour-net analysis indicated that the PE-rich BSea strains are probably endemic for the Baltic Sea. Comparison of the phylogenies obtained by the 16S rRNA-ITS, the cpcBA, and the concatenated 16S rRNA-ITS and cpcBA operon sequences revealed possible events of horizontal gene transfer among different Synechococcus lineages. Our results show that microdiversity is important in Synechococcus populations and that it can reflect extensive diversification of different pigmentation phenotypes into different ecological niches.