Project description:Climatic fluctuations in the Pleistocene caused glacial expansion-contraction cycles in Eurasia and other parts of the world. Consequences of these cycles, such as population expansion and subsequent subdivision, have been studied in many taxa at intraspecific population level across much of the Northern Hemisphere. However, the consequences for the potential of hybridization and polyploidization are poorly understood. Here, we investigated the phylogeographic structure of two widespread, closely related species, Veronica spicata and Veronica longifolia, across their European distribution ranges. We assessed the extent and the geographic pattern of polyploidization in both species and hybridization between them. We used genome-scale SNP data to clarify phylogenetic relationships and detect possible hybridization/introgression events. In addition, crossing experiments were performed in different combination between V. spicata and V. longifolia individuals of two ploidy levels and of different geographic origins. Finally, we employed ecological niche modeling to infer macroclimatic differences between both species and both ploidy levels. We found a clear genetic structure reflecting the geographical distribution patterns in both species, with V. spicata showing higher genetic differentiation than V. longifolia. We retrieved significant signals of hybridization and introgression in natural populations from the genetic data and corroborated this with crossing experiments. However, there were no clear phylogeographic patterns and unequivocal macroclimatic niche differences between diploid and tetraploid lineages. This favors the hypothesis, that autopolyploidization has happened frequently and in different regions. The crossing experiments produced viable hybrids when the crosses were made between plants of the same ploidy levels but not in the interploidy crosses. The results suggest that hybridization occurs across the overlapping areas of natural distribution ranges of both species, with apparently directional introgression from V. spicata to V. longifolia. Nevertheless, the two species maintain their species-level separation due to their adaptation to different habitats and spatial isolation rather than reproductive isolation.

Project description:Background and aimsPolyploidy is considered one of the main mechanisms of plant evolution and speciation. In the Mediterranean Basin, polyploidy has contributed to making this region a biodiversity hotspot, along with its geological and climatic history and other ecological and biogeographical factors. The Mediterranean genus Centaurium (Gentianaceae) comprises ~25 species, of which 60 % are polyploids, including tetraploids and hexaploids. To date, the evolutionary history of centauries has been studied using Sanger sequencing phylogenies, which have been insufficient to fully understand the phylogenetic relationships in this lineage. The goal of this study is to gain a better understanding of the evolutionary history of Centaurium by exploring the mechanisms that have driven its diversification, specifically hybridization and polyploidy. We aim to identify the parentage of hybrid species, at the species or clade level, as well as assessing whether morphological traits are associated with particular ploidy levels.MethodsWe sequenced RADseq markers from 42 samples of 28 Centaurium taxa, and performed phylogenomic analyses using maximum likelihood, summary coalescent SVDquartets and Neighbor-Net approaches. To identify hybrid taxa, we used PhyloNetworks and the fastSTRUCTURE algorithm. To infer the putative parental species of the allopolyploids, we employed genomic analyses (SNIPloid). The association between different traits and particular ploidy levels was explored with non-metric multidimensional scaling.Key resultsOur phylogenetic analyses confirmed the long-suspected occurrence of recurrent hybridization. The allopolyploid origin of the tetraploid C. serpentinicola and the hexaploids C. mairei, C. malzacianum and C. centaurioides was also confirmed, unlike that of C. discolor. We inferred additional signatures of hybridization events within the genus and identified morphological traits differentially distributed in different ploidy levels.ConclusionsThis study highlights the important role that hybridization has played in the evolution of a Mediterranean genus such as Centaurium, leading to a polyploid complex, which facilitated its diversification and may exemplify that of other Mediterranean groups.

Project description:The Lemnaceae (duckweeds) are the world’s smallest but fastest growing flowering plants, with a drastically reduced morphology and predominant clonal reproductive habit capable of continuous exponential growth. Here, we present assemblies of 10 Lemna chromosome sets by single molecule nanopore sequencing and chromosome conformation capture. Dynamics of genome evolution in the family are revealed by syntenic comparisons with Wolffia and Spirodela, and diversification of these genera was found to coincide with the “Azolla event”, in which blooms of aquatic macrophytes reduced atmospheric CO2 from greenhouse levels found in the Eocene to those of the current ice age. Orthologous gene comparisons with other aquatic and terrestrial plants uncovered candidate genes for the unique metabolic and developmental features of the family, such as frequent hybrid polyploidy, lack of stomatal closure in high CO2, and accumulation of calcium oxalate, a promising candidate for carbon sequestration. Loss of a spermine-triggered gene network accounts for drastic reduction in stature and preferentially adaxial stomata, a feature of floating aquatic plants. Strikingly, Lemnaceae genomes have selectively lost some of the genes required for RNA interference, including Argonaute genes required for post-zygotic reproductive isolation (the triploid block) and reduced gamete formation. Triploid hybrids arise commonly among Lemna, presumably by hybridization with unreduced gametes, and we have found mutations in highly-conserved ZMM crossover pathway genes that could support polyploid meiosis. Rapid but stable clonal propagation makes Lemna an ideal platform for protein and starch micro-cropping, and for sequestration of dissolved nutrients and atmospheric CO2. Facile regeneration of transgenic fronds from tissue culture, aided by reduced epigenetic silencing, makes Lemna a powerful biotechnological platform, as exemplified by our recent engineering of high-oil Lemna lines that out-perform with oil seed crops.

Dataset Information

Genome size as a key to evolutionary complex aquatic plants: polyploidy and hybridization in Callitriche (Plantaginaceae).

Publications

Genome size as a key to evolutionary complex aquatic plants: polyploidy and hybridization in Callitriche (Plantaginaceae).

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