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To expand on prior evolutionary studies conducted on the genus Azorella (Apiaceae), we here present analyses based on more intensive sampling from Australasian representatives of the genus, which had been less thoroughly represented in the prior studies, especially species from section Schizeilema , a group largely native to New Zealand. In this study, 144 specimens representing 56 of the 57 recognized species and putatively new species were sampled for nine DNA sequence regions from the plastid genome and two regions from the nuclear genome. The study also provides deeper analyses of biogeography (DEC model selected by BioGeoBears + ModelTest in RASP 4), divergence (BEAST), and diversification (RevBayes) compared to prior work. Our phylogenetic analyses corroborated earlier results regarding the relationships among the ten sections of Azorella and provided stronger evidence for other relationships that had been less well established. Incongruencies between phylogenetic trees based on plastid and nuclear data suggest reticulation events within the group, especially regarding the origin of section Schizeilema and among the subclades within that section, where speciation by hybridization and polyploidy appears common. The two nuclear datasets provide evidence of nonuniform concerted evolution and incongruence among sections Ranunculus , Schizeilema , and Stilbocarpa . These analyses also suggest that the origin and diversification of Azorella has been affected by events related to the cooling and break up of Antarctica during the Oligocene. Since that time, parallel dispersals to and radiations in Australasia (especially New Zealand) and the Andes have occurred, and that diversification rates accelerated during the Pliocene and Pleistocene concurrent with the opening of new high-altitude ecological niches in New Zealand’s Southern Alps and the Andes of South American.

Premise of the Study Polyploidy has been long recognized as an important force in plant evolution. Previous studies had suggested widespread occurrence of polyploidy and the allopolyploid origin of several species in the diverse neotropical genus Lachemilla (Rosaceae). Nonetheless, this evidence has relied mostly on patterns of cytonuclear discordance, and direct evidence from nuclear allelic markers is still needed. Methods Here we used PCR target enrichment in combination with high throughput sequencing to obtain multiple copies of the nuclear ribosomal (nr) DNA cistron and 45 regions of the plastid genome (cpDNA) from 219 accessions representing 48 species of Lachemilla and to explore the allopolyploid origin of species in this group. Key Results We were able to identify multiple nrDNA ribotypes and establish clear evidence of allopolyploidy in 33 species of Lachemilla, showing that this condition is common and widespread in the genus. Additionally, we found evidence for three autopolyploid species. We also established multiple, independent origins of several allopolyploid species. Finally, based solely on the cpDNA phylogeny, we identified that the monotypic genus Farinopsis is the sister group of Lachemilla and allied genera within subtribe Fragariinae. Conclusions Our study demonstrates the utility of the nuclear ribosomal DNA cistron to detect allopolyploidy when concerted evolution of this region is not complete. Additionally, with a robust chloroplast phylogeny in place, the direction of hybridization events can be established, and multiple, independent origins of allopolyploid species can be identified.

• Background and Aims Isolation and drift are the main causes for geographic structure of molecular variation. In contrast, the one found in a previous survey in Armeria (Plumbaginaceae) for nuclear ribosomal ITS multicopy regions was species‐independent and has been hypothesized to be due to extensive gene‐flow and biased concerted evolution. Since this was inferred from a genus‐level phylogenetic analysis, the aim of this study was to check for the occurrence of such structure and the validity of the proposed model at a local scale, in a southern Spanish massif (Sierra Nevada), as well as to examine the evolutionary implications at the organism level. • Methods In addition to 117 sequences of direct PCR products from genomic DNA, 50 sequences of PCR products from cloned DNA were obtained to analyse cases of intragenomic polymorphisms for the ITS regions. • Key Results Sequence data confirm the occurrence of a species‐independent structure at a local scale and reveal insights through the analysis of contact areas between different ITS copies (ribotypes). A comparison between cloned and direct sequences (a) confirms that, within these contact areas, ITS copies co‐occur both in different individuals and within single genomes; and (b) reveals recombination between different copies. • Conclusions This study supports the utility of direct sequences for detecting intra‐individual polymorphism and for partially inferring the ITS copies involved, given previous knowledge of the variability. The main evolutionary implication at the organism level is that gene‐flow and concerted evolution shape the geographic structure of ITS variation.

The cnidarian Nematostella vectensis has become an established lab model, providing unique opportunities for venom evolution research. The Nematostella venom system is multimodal: involving both nematocytes and ectodermal gland cells, which produce a toxin mixture whose composition changes throughout the life cycle. Additionally, their modes of interaction with predators and prey vary between eggs, larvae, and adults, which is likely shaped by the dynamics of the venom system. Nv1 is a major component of adult venom, with activity against arthropods (through specific inhibition of sodium channel inactivation) and fish. Nv1 is encoded by a cluster of at least 12 nearly identical genes that were proposed to be undergoing concerted evolution. Surprisingly, we found that Nematostella venom includes several Nv1 paralogs escaping a pattern of general concerted evolution, despite belonging to the Nv1-like family. Here, we show two of these new toxins, Nv4 and Nv5, are lethal for zebrafish larvae but harmless to arthropods, unlike Nv1. Furthermore, unlike Nv1, the newly identified toxins are expressed in early life stages. Using transgenesis and immunostaining, we demonstrate that Nv4 and Nv5 are localized to ectodermal gland cells in larvae. The evolution of Nv4 and Nv5 can be described either as neofunctionalization or as subfunctionalization. Additionally, the Nv1-like family includes several pseudogenes being an example of nonfunctionalization and venom evolution through birth-and-death mechanism. Our findings reveal the evolutionary history for a toxin radiation and point toward the ecological function of the novel toxins constituting a complex cnidarian venom.

Abstract Although both are salient features of genomes, at first glance ribosomal DNAs and transposable elements are genetic elements with not much in common: whereas ribosomal DNAs are mainly viewed as housekeeping genes that uphold all prime genome functions, transposable elements are generally portrayed as selfish and disruptive. These opposing characteristics are also mirrored in other attributes: organization in tandem (ribosomal DNAs) versus organization in a dispersed manner (transposable elements); evolution in a concerted manner (ribosomal DNAs) versus evolution by diversification (transposable elements); and activity that prolongs genomic stability (ribosomal DNAs) versus activity that shortens it (transposable elements). Re-visiting relevant instances in which ribosomal DNA–transposable element interactions have been reported, we note that both repeat types share at least four structural and functional hallmarks: (1) they are repetitive DNAs that shape genomes in evolutionary timescales, (2) they exchange structural motifs and can enter co-evolution processes, (3) they are tightly controlled genomic stress sensors playing key roles in senescence/aging, and (4) they share common epigenetic marks such as DNA methylation and histone modification. Here, we give an overview of the structural, functional, and evolutionary characteristics of both ribosomal DNAs and transposable elements, discuss their roles and interactions, and highlight trends and future directions as we move forward in understanding ribosomal DNA–transposable element associations.

Background Despite GC variation constitutes a fundamental element of genome and species diversity, the precise mechanisms driving it remain unclear. The abundant sequence data available for the ITS2, a commonly employed phylogenetic marker in plants, offers an exceptional resource for exploring the GC variation across angiosperms. Results A comprehensive selection of 8666 species, comprising 165 genera, 63 families, and 30 orders were used for the analyses. The alignment of ITS2 sequence-structures and partitioning of secondary structures into paired and unpaired regions were performed using 4SALE. Substitution rates and frequencies among GC base-pairs in the paired regions of ITS2 were calculated using RNA-specific models in the PHASE package. The results showed that the distribution of ITS2 GC contents on the angiosperm phylogeny was heterogeneous, but their increase was generally associated with ITS2 sequence homogenization, thereby supporting the occurrence of GC-biased gene conversion (gBGC) during the concerted evolution of ITS2. Additionally, the GC content in the paired regions of the ITS2 secondary structure was significantly higher than that of the unpaired regions, indicating the selection of GC for thermodynamic stability. Furthermore, the RNA substitution models demonstrated that base-pair transformations favored both the elevation and fixation of GC in the paired regions, providing further support for gBGC. Conclusions Our findings highlight the significance of secondary structure in GC investigation, which demonstrate that both gBGC and structure-based selection are influential factors driving angiosperm ITS2 GC content.

Tandemly repeated ribosomal DNA (rDNA) arrays are among the most evolutionary dynamic loci of eukaryotic genomes. The loci code for essential cellular components, yet exhibit extensive copy number (CN) variation within and between species. CN might be partly determined by the requirement of dosage balance between the 5S and 45S rDNA arrays. The arrays are nonhomologous, physically unlinked in mammals, and encode functionally interdependent RNA components of the ribosome. Here we show that the 5S and 45S rDNA arrays exhibit concerted CN variation (cCNV). Despite 5S and 45S rDNA elements residing on different chromosomes and lacking sequence similarity, cCNV between these loci is strong, evolutionarily conserved in humans and mice, and manifested across individual genotypes in natural populations and pedigrees. Finally, we observe that bisphenol A induces rapid and parallel modulation of 5S and 45S rDNA CN. Our observations reveal a novel mode of genome variation, indicate that natural selection contributed to the evolution and conservation of cCNV, and support the hypothesis that 5S CN is partly determined by the requirement of dosage balance with the 45S rDNA array. We suggest that human disease variation might be traced to disrupted rDNA dosage balance in the genome. Significance Ribosomes are essential intracellular machines composed of proteins and RNA molecules. The DNA sequences [i.e., ribosomal DNA (rDNA)] encoding rRNAs are tandemly repeated and give rise to the nucleolus. The rRNAs are transcribed from two array kinds (the 5S and the 45S arrays). Here we show that variation in the 5S and 45S rDNA arrays is tightly coupled, despite their location on different chromosomes. Our observations suggest that natural selection contributes to maintain balanced rDNA dosage across unlinked rDNA arrays. Finally, we show that bisphenol A can induce parallel loss of rDNA units in 5S and 45S arrays. These observations raise the prospect that human diseases might be traced to disrupted rDNA dosage balance in the genome.

Until around 1990, most multigene families were thought to be subject to concerted evolution, in which all member genes of a family evolve as a unit in concert. However, phylogenetic analysis of MHC and other immune system genes showed a quite different evolutionary pattern, and a new model called birth-and-death evolution was proposed. In this model, new genes are created by gene duplication and some duplicate genes stay in the genome for a long time, whereas others are inactivated or deleted from the genome. Later investigations have shown that most non-rRNA genes including highly conserved histone or ubiquitin genes are subject to this type of evolution. However, the controversy over the two models is still continuing because the distinction between the two models becomes difficult when sequence differences are small. Unlike concerted evolution, the model of birth-and-death evolution can give some insights into the origins of new genetic systems or new phenotypic characters.

Abstract Spliced leader (SL) trans-splicing is a key step in the processing of many mRNAs in different eukaryotic lineages, including in parasitic flatworms. Despite its importance, efforts for its characterization in this phylum have remained a collection of single-species studies with little exploration at a wider phylogenetic context. In this work, we present a comprehensive analysis of this process, based on the available genomic and transcriptomic data of 24 cestode and trematode species, including the identification of the SL-RNA sequences and their splicing acceptor transcripts and sites. We identified a main pattern of concerted evolution of SL-RNA loci in most flatworm species, as well as divergence of SL-RNA loci in selected species. However, even in species with several divergent SL-RNAs, there was no specialization in their targets. This, along with low SL trans-splicing levels, is in stark contrast with the global patterns of SL trans-splicing usage in nematodes. SL trans-splicing could be detected for a limited number of mRNAs in all species (<31%), and we found extensive use of the same splice acceptor sites for cis-splicing, especially for monocistronic transcripts. Ancestral SL trans-splicing sites are found in many conserved genes, including in putative ancestral operons that are shared between cestodes and trematodes. Graphical Abstract Graphical Abstract

The 5S rRNA genes are among the most conserved nucleotide sequences across all species. Similar to the 5S preservation we observe the occurrence of 5S-related nonautonomous retrotransposons, so-called Cassandras. Cassandras harbor highly conserved 5S rDNA-related sequences within their long terminal repeats, advantageously providing them with the 5S internal promoter. However, the dynamics of Cassandra retrotransposon evolution in the context of 5S rRNA gene sequence information and structural arrangement are still unclear, especially: (1) do we observe repeated or gradual domestication of the highly conserved 5S promoter by Cassandras and (2) do changes in 5S organization such as in the linked 35S-5S rDNA arrangements impact Cassandra evolution? Here, we show evidence for gradual co-evolution of Cassandra sequences with their corresponding 5S rDNAs. To follow the impact of 5S rDNA variability on Cassandra TEs, we investigate the Asteraceae family where highly variable 5S rDNAs, including 5S promoter shifts and both linked and separated 35S-5S rDNA arrangements have been reported. Cassandras within the Asteraceae mirror 5S rDNA promoter mutations of their host genome, likely as an adaptation to the host's specific 5S transcription factors and hence compensating for evolutionary changes in the 5S rDNA sequence. Changes in the 5S rDNA sequence and in Cassandras seem uncorrelated with linked/separated rDNA arrangements. We place all these observations into the context of angiosperm 5S rDNA-Cassandra evolution, discuss Cassandra's origin hypotheses (single or multiple) and Cassandra's possible impact on rDNA and plant genome organization, giving new insights into the interplay of ribosomal genes and transposable elements.