Background Deep-branching phylogenetic relationships are often difficult to resolve because phylogenetic signals are obscured by the long history and complexity of evolutionary processes, such as ancient introgression/hybridization, polyploidization, and incomplete lineage sorting (ILS). Phylogenomics has been effective in providing information for resolving both deep- and shallow-scale relationships across all branches of the tree of life. The olive family (Oleaceae) is composed of 25 genera classified into five tribes with tribe Oleeae consisting of four subtribes. Previous phylogenetic analyses showed that ILS and/or hybridization led to phylogenetic incongruence in the family. It was essential to distinguish phylogenetic signal conflicts, and explore mechanisms for the uncertainties concerning relationships of the olive family, especially at the deep-branching nodes. Results We used the whole plastid genome and nuclear single nucleotide polymorphism (SNP) data to infer the phylogenetic relationships and to assess the variation and rates among the main clades of the olive family. We also used 2608 and 1865 orthologous nuclear genes to infer the deep-branching relationships among tribes of Oleaceae and subtribes of tribe Oleeae, respectively. Concatenated and coalescence trees based on the plastid genome, nuclear SNPs and multiple nuclear genes suggest events of ILS and/or ancient introgression during the diversification of Oleaceae. Additionally, there was extreme heterogeneity in the substitution rates across the tribes. Furthermore, our results supported that introgression/hybridization, rather than ILS, is the main factor for phylogenetic discordance among the five tribes of Oleaceae. The tribe Oleeae is supported to have originated via ancient hybridization and polyploidy, and its most likely parentages are the ancestral lineage of Jasmineae or its sister group, which is a “ghost lineage,” and Forsythieae. However, ILS and ancient introgression are mainly responsible for the phylogenetic discordance among the four subtribes of tribe Oleeae. Conclusions This study showcases that using multiple sequence datasets (plastid genomes, nuclear SNPs and thousands of nuclear genes) and diverse phylogenomic methods such as data partition, heterogeneous models, quantifying introgression via branch lengths (QuIBL) analysis, and species network analysis can facilitate untangling long and complex evolutionary processes of ancient introgression, paleopolyploidization, and ILS.

Populus not only has significant economic and ecological values, but also serves as a model tree that is widely used in the basic research of tree growth, physiology, and genetics. However, high levels of morphological variation and extensive interspecific hybridization of Populus pose an obstacle for taxonomy, and also to the understanding of phylogenetic interspecific relationships and biogeographical history. In this study, a total of 103 accessions representing almost all the wild species of Populus were collected and whole-genome re-sequenced to examine the phylogenetic relationships and biogeography history. On the basis of 12,916,788 nuclear single nucleotide polymorphisms (SNPs), we reconstructed backbone phylogenies using concatenate and coalescent methods, we highly disentangled the species relationships of Populus, and several problematic taxa were treated as species complexes. Furthermore, the phylogeny of the chloroplast genome showed extensive discordance with the trees from the nuclear genome data, and due to extensive chloroplast capture and hybridization of Populus species, plastomes could not accurately evaluate interspecies relationships. Ancient gene flow between clades and some hybridization events were also identified by ABBA–BABA analysis. The reconstruction of chronogram and ancestral distributions suggested that North America was the original region of this genus, and subsequent long dispersal and migration across land bridges were contributed to the modern range of Populus. The diversification of Populus mainly occurred in East Asia in recent 15 Ma, possibly promoted by the uplift of the Tibetan Plateau. This study provided comprehensive evidence on the phylogeny of Populus and proposed a four-subgeneric classification and a new status, subgenus Abaso. Meanwhile, ancestral distribution reconstruction with nuclear data advanced the understanding of the biogeographic history of Populus.

Arnebiae Radix is a traditional medicine with pleiotropic properties that has been used for several 100 years. There are five species of Arnebia in China, and the two species Arnebia euchroma and Arnebia guttata are the source plants of Arnebiae Radix according to the Chinese Pharmacopoeia. Molecular markers that permit species identification and facilitate studies of the genetic diversity and divergence of the wild populations of these two source plants have not yet been developed. Here, we sequenced the chloroplast genomes of 56 samples of five Arnebia species using genome skimming methods. The Arnebia chloroplast genomes exhibited quadripartite structures with lengths from 149,539 and 152,040 bp. Three variable markers (rps16-trnQ, ndhF-rpl32, and ycf1b) were identified, and these markers exhibited more variable sites than universal chloroplast markers. The phylogenetic relationships among the five Arnebia species were completely resolved using the whole chloroplast genome sequences. Arnebia arose during the Oligocene and diversified in the middle Miocene; this coincided with two geological events during the late Oligocene and early Miocene: warming and the progressive uplift of Tianshan and the Himalayas. Our analyses revealed that A. euchroma and A. guttata have high levels of genetic diversity and comprise two and three subclades, respectively. The two clades of A. euchroma exhibited significant genetic differences and diverged at 10.18 Ma in the middle Miocene. Three clades of A. guttata diverged in the Pleistocene. The results provided new insight into evolutionary history of Arnebia species and promoted the conservation and exploitation of A. euchroma and A. guttata.

Bretschneidera sinensis is an endangered woody species found in East and South China. Comprehensive intraspecies chloroplast genome studies have demonstrated novel genetic resources to assess the genetic variation and diversity of this species. Using genome skimming method, we assembled the whole chloroplast genome of 12 genotypes of B. sinensis from different geographical locations, covering most wild populations. The B. sinensis chloroplast genome size ranged from 158,959 to 159,045 base pairs (bp) and displayed a typical circular quadripartite structure. Comparative analyses of 12 B. sinensis chloroplast genome revealed 33 polymorphic simple sequence repeats (SSRs), 105 polymorphic single nucleotide polymorphisms (SNPs), and 55 indels. Phylogenetic analysis showed that the 12 genotypes were grouped into 2 branches, which is consistent with the geographical distribution (Eastern clade and Western clade). Divergence time estimates showed that the two clades were divergent from 0.6 Ma in the late Pleistocene. Ex situ conservation is essential for this species. In this study, we identified SNPs, indels, and microsatellites of B. sinensis by comparative analyses of chloroplast genomes and determined genetic variation between populations using these genomic markers. Chloroplast genomic resources are also important for further domestication, population genetic, and phylogenetic analysis, possibly in combination with molecular markers of mitochondrial and/or nuclear genomes.

Background Eriocaulon is a wetland plant genus with important ecological value, and one of the famous taxonomically challenging groups among angiosperms, mainly due to the high intraspecific diversity and low interspecific variation in the morphological characters of species within this genus. In this study, 22 samples representing 15 Eriocaulon species from China, were sequenced and combined with published samples of Eriocaulon to test the phylogenetic resolution using the complete chloroplast genome. Furthermore, comparative analyses of the chloroplast genomes were performed to investigate the chloroplast genome evolution of Eriocaulon. Results The 22 Eriocaulon chloroplast genomes and the nine published samples were proved highly similar in genome size, gene content, and order. The Eriocaulon chloroplast genomes exhibited typical quadripartite structures with lengths from 150,222 bp to 151,584 bp. Comparative analyses revealed that four mutation hotspot regions (psbK-trnS, trnE-trnT, ndhF-rpl32, and ycf1) could serve as effective molecular markers for further phylogenetic analyses and species identification of Eriocaulon species. Phylogenetic results supported Eriocaulon as a monophyletic group. The identified relationships supported the taxonomic treatment of section Heterochiton and Leucantherae, and the section Heterochiton was the first divergent group. Phylogenetic tree supported Eriocaulon was divided into five clades. The divergence times indicated that all the sections diverged in the later Miocene and most of the extant Eriocaulon species diverged in the Quaternary. The phylogeny and divergence times supported rapid radiation occurred in the evolution history of Eriocaulon. Conclusion Our study mostly supported the taxonomic treatment at the section level for Eriocaulon species in China and demonstrated the power of phylogenetic resolution using whole chloroplast genome sequences. Comparative analyses of the Eriocaulon chloroplast genome developed molecular markers that can help us better identify and understand the evolutionary history of Eriocaulon species in the future.