Genome skimming provides new insight into the relationships in Ludwigia section Macrocarpon, a polyploid complex

New insights into polyploid evolution and dynamic nature of Ludwigia section Isnardia (Onagraceae)

BACKGROUND

While polyploids are common in plants, the evolutionary history and natural dynamics of most polyploid groups are still unclear. Owing to plentiful earlier systematic studies, Ludwigia sect. Isnardia (comprising 22 wetland taxa) is an ideal allopolyploid complex to investigate polyploid evolution and natural dynamics within and among taxa. With a considerable sampling, we concentrated on revisiting earlier phylogenies of Isnardia, reevaluating the earlier estimated age of the most recent common ancestor (TMRCA), exploring the correlation between infraspecific genetic diversity and ploidy levels, and inspecting interspecific gene flows among taxa.

RESULTS

Phylogenetic trees and network concurred with earlier phylogenies and hypothesized genomes by incorporating 192 atpB-rbcL and ITS sequences representing 91% of Isnardia taxa. Moreover, we detected three multi-origin taxa. Our findings on L. repens and L. sphaerocarpa were consistent with earlier studies; L. arcuata was reported as a multi-origin taxon here, and an additional evolutionary scenario of L. sphaerocarpa was uncovered, both for the first time. Furthermore, estimated Isnardia TMRCA ages based on our data (5.9 or 8.9 million years ago) are in accordance with earlier estimates, although younger than fossil dates (Middle Miocene). Surprisingly, infraspecific genetic variations of Isnardia taxa did not increase with ploidy levels as anticipated from many other polyploid groups. In addition, the exuberant, low, and asymmetrical gene flows among Isnardia taxa indicated that the reproductive barriers may be weakened owing to allopolyploidization, which has rarely been reported.

CONCLUSIONS

The present research gives new perceptions of the reticulate evolution and dynamic nature of Isnardia and points to gaps in current knowledge about allopolyploid evolution.

Blumea chishangensis sp. nov. (Asteraceae: Inuleae) from Taiwan and new insights into the phylogeny of Blumea

BACKGROUND

Blumea plants are widely distributed in the tropical areas of Asia, Africa, and Australia, especially tropical Asia. Limited studies left the taxonomy and infrageneric phylogeny of Blumea insubstantial. Here, a new species, Blumea chishangensis S. W. Chung, Z. H. Chen, S. H. Liu & W. J. Huang, from Taiwan is described, and an extended phylogeny is reconstructed to provide new perceptions of Blumea evolution.

RESULTS

The new species is distinguished from B. hieraciifolia by the following features: leaf blade sparsely pilose or glabrescent (vs. silky villous), the leaves margins regularly remote mucronulate (vs. double serrate or dentate), capitula pedicelled (vs. capitula sessile or subsessile), and leaves basal rosette or sub-basal rosette and a few cauline (vs. mostly cauline). Phylogenetic analysis based on the ITS, trnL-trnF, and trnH-psbA regions places the new species in the subclade II in B. lacera clade and shows a close relationship with B. axillaris and B. oxyodonta. A key to Blumea species in Taiwan and the studied species in the subclade II is provided. Moreover, the evolutionary inferences of B. conspicua, B. linearis, and B. sinuata are first reported here. The paraphyly of B. formosana and B. sinuata are also revealed for the first time.

CONCLUSIONS

Both morphological and molecular data support that B. chishangensis is a new species. Our phylogeny highlights the need for further taxonomic and evolutionary studies on Blumea.

ORTHOSKIM: in silico sequence capture from genomic and transcriptomic libraries for phylogenomic and barcoding applications

Low-coverage whole genome shotgun sequencing (or genome skimming) has emerged as a cost-effective method for acquiring genomic data in non-model organisms. This method provides sequence information on chloroplast genome (cpDNA), mitochondrial genome (mtDNA) and nuclear ribosomal regions (rDNA), which are over-represented within cells. However, numerous bioinformatic challenges remain to accurately and rapidly obtain such data in organisms with complex genomic structures and rearrangements, in particular for mtDNA in plants or for cpDNA in some plant families. Here we introduce the pipeline ORTHOSKIM, which performs in silico capture of targeted sequences from genomic and transcriptomic libraries without assembling whole organelle genomes. ORTHOSKIM proceeds in three steps: 1) global sequence assembly, 2) mapping against reference sequences, and 3) target sequence extraction; importantly it also includes a range of quality control tests. Different modes are implemented to capture both coding and non-coding regions of cpDNA, mtDNA and rDNA sequences, along with predefined nuclear sequences (e.g. ultra-conserved elements) or collections of single-copy ortholog genes. Moreover, aligned DNA matrices are produced for phylogenetic reconstructions, by performing multiple alignments of the captured sequences. While ORTHOSKIM is suitable for any eukaryote, a case study is presented here, using 114 genome-skimming libraries and 4 RNAseq libraries obtained for two plant families, Primulaceae and Ericaceae, the latter being a well-known problematic family for cpDNA assemblies. ORTHOSKIM recovered with high success rates cpDNA, mtDNA and rDNA sequences, well suited to accurately infer evolutionary relationships within these families. ORTHOSKIM is released under a GPL-3 license and is available at: https://github.com/cpouchon/ORTHOSKIM.

Resolving the Phylogeny of the Olive Family (Oleaceae): Confronting Information from Organellar and Nuclear Genomes

The olive family, Oleaceae, is a group of woody plants comprising 28 genera and ca. 700 species, distributed on all continents (except Antarctica) in both temperate and tropical environments. It includes several genera of major economic and ecological importance such as olives, ash trees, jasmines, forsythias, osmanthuses, privets and lilacs. The natural history of the group is not completely understood yet, but its diversification seems to be associated with polyploidisation events and the evolution of various reproductive and dispersal strategies. In addition, some taxonomical issues still need to be resolved, particularly in the paleopolyploid tribe Oleeae. Reconstructing a robust phylogenetic hypothesis is thus an important step toward a better comprehension of Oleaceae's diversity. Here, we reconstructed phylogenies of the olive family using 80 plastid coding sequences, 37 mitochondrial genes, the complete nuclear ribosomal cluster and a small multigene family encoding phytochromes (phyB and phyE) of 61 representative species. Tribes and subtribes were strongly supported by all phylogenetic reconstructions, while a few Oleeae genera are still polyphyletic (Chionanthus, Olea, Osmanthus, Nestegis) or paraphyletic (Schrebera, Syringa). Some phylogenetic relationships among tribes remain poorly resolved with conflicts between topologies reconstructed from different genomic regions. The use of nuclear data remains an important challenge especially in a group with ploidy changes (both paleo- and neo-polyploids). This work provides new genomic datasets that will assist the study of the biogeography and taxonomy of the whole Oleaceae.

Plastome phylogenomics, biogeography, and clade diversification of Paris (Melanthiaceae)

BACKGROUND

Paris (Melanthiaceae) is an economically important but taxonomically difficult genus, which is unique in angiosperms because some species have extremely large nuclear genomes. Phylogenetic relationships within Paris have long been controversial. Based on complete plastomes and nuclear ribosomal DNA (nrDNA) sequences, this study aims to reconstruct a robust phylogenetic tree and explore historical biogeography and clade diversification in the genus.

RESULTS

All 29 species currently recognized in Paris were sampled. Whole plastomes and nrDNA sequences were generated by the genome skimming approach. Phylogenetic relationships were reconstructed using the maximum likelihood and Bayesian inference methods. Based on the phylogenetic framework and molecular dating, biogeographic scenarios and historical diversification of Paris were explored. Significant conflicts between plastid and nuclear datasets were identified, and the plastome tree is highly congruent with past interpretations of the morphology. Ancestral area reconstruction indicated that Paris may have originated in northeastern Asia and northern China, and has experienced multiple dispersal and vicariance events during its diversification. The rate of clade diversification has sharply accelerated since the Miocene/Pliocene boundary.

CONCLUSIONS

Our results provide important insights for clarifying some of the long-standing taxonomic debates in Paris. Cytonuclear discordance may have been caused by ancient and recent hybridizations in the genus. The climatic and geological changes since the late Miocene, such as the intensification of Asian monsoon and the rapid uplift of Qinghai-Tibet Plateau, as well as the climatic fluctuations during the Pleistocene, played essential roles in driving range expansion and radiative diversification in Paris. Our findings challenge the theoretical prediction that large genome sizes may limit speciation.