Investigation of mitochondrial-derived plastome sequences in the Paspalum lineage (Panicoideae; Poaceae)

Invited Review Beyond parasitic convergence: unravelling the evolution of the organellar genomes in holoparasites

BACKGROUND

The molecular evolution of organellar genomes in angiosperms has been studied extensively, with some lineages, such as parasitic ones, displaying unique characteristics. Parasitism has emerged 12 times independently in angiosperm evolution. Holoparasitism is the most severe form of parasitism, and is found in ~10 % of parasitic angiosperms. Although a few holoparasitic species have been examined at the molecular level, most reports involve plastomes instead of mitogenomes. Parasitic plants establish vascular connections with their hosts through haustoria to obtain water and nutrients, which facilitates the exchange of genetic information, making them more susceptible to horizontal gene transfer (HGT). HGT is more prevalent in the mitochondria than in the chloroplast or nuclear compartments.

SCOPE

This review summarizes current knowledge on the plastid and mitochondrial genomes of holoparasitic angiosperms, compares the genomic features across the different lineages, and discusses their convergent evolutionary trajectories and distinctive features. We focused on Balanophoraceae (Santalales), which exhibits extraordinary traits in both their organelles.

CONCLUSIONS

Apart from morphological similarities, plastid genomes of holoparasitic plants also display other convergent features, such as rampant gene loss, biased nucleotide composition and accelerated evolutionary rates. In addition, the plastomes of Balanophoraceae have extremely low GC and gene content, and two unexpected changes in the genetic code. Limited data on the mitochondrial genomes of holoparasitic plants preclude thorough comparisons. Nonetheless, no obvious genomic features distinguish them from the mitochondria of free-living angiosperms, except for a higher incidence of HGT. HGT appears to be predominant in holoparasitic angiosperms with a long-lasting endophytic stage. Among the Balanophoraceae, mitochondrial genomes exhibit disparate evolutionary paths with notable levels of heteroplasmy in Rhopalocnemis and unprecedented levels of HGT in Lophophytum. Despite their differences, these Balanophoraceae share a multichromosomal mitogenome, a feature also found in a few free-living angiosperms.

Poaceae Chloroplast Genome Sequencing: Great Leap Forward in Recent Ten Years

The first complete chloroplast genome of rice (Oryza sativa) was published in 1989, ushering in a new era of studies of chloroplast genomics in Poaceae. Progresses in Next-Generation Sequencing (NGS) and Third-Generation Sequencing (TGS) technologiesand in the development of genome assembly software, have significantly advanced chloroplast genomics research. Poaceae is one of the most targeted families in chloroplast genome research because of its agricultural, ecological, and economic importance. Over the last 30 years, 2,050 complete chloroplast genome sequences from 40 tribes and 282 genera have been generated, most (97%) of them in the recent ten years. The wealth of data provides the groundwork for studies on species evolution, phylogeny, genetic transformation, and other aspects of Poaceae chloroplast genomes. As a result, we have gained a deeper understanding of the properties of Poaceae chloroplast genomes. Here, we summarize the achievements of the studies of the Poaceae chloroplast genomes and envision the challenges for moving the area ahead.

Chloroplast Genome Evolution and Species Identification of Styrax (Styracaceae)

The genus Styrax L. consists of approximately 130 species distributed in the Americas, eastern Asia, and the Mediterranean region. The phylogeny and evolutionary history of this genus are not clear. Knowledge of the phylogenetic relationships and the method for species identification will be critical for the evolution of this genus. In this study, we sequenced the chloroplast genome of 17 Styrax samples and added 17 additional chloroplast genome sequences from GenBank. The data were used to investigate chloroplast genome evolution, infer phylogenetic relationships, and access the species identification rate within Styrax. The Styrax chloroplast genome contains typical quadripartite structures, ranging from 157,641 bp to 159,333 bp. The chloroplast genome contains 114 unique genes. The P distance among the Styrax species ranged from 0.0003 to 0.00611. Seventeen small inversions and SSR sites were discovered in the Styrax chloroplast genome. By comparing with the chloroplast genome sequences, six mutation hotspots were identified, and the markers ycf1b and trnT-trnL were identified as the best Styrax-specific DNA barcodes. The specific barcodes and superbarcode exhibited higher discriminatory power than universal barcodes. Chloroplast phylogenomic results improved the resolution of the phylogenetic relationships of Styrax compared to previous analyses.

Unprecedented organelle genomic variations in morning glories reveal independent evolutionary scenarios of parasitic plants and the diversification of plant mitochondrial complexes

Background

The morning glories (Convolvulaceae) are distributed worldwide and produce economically important crops, medicinal herbs, and ornamentals. Members of this family are diverse in morphological characteristics and trophic modes, including the leafless parasitic Cuscuta (dodders). Organelle genomes were generally used for studying plant phylogeny and genomic variations. Notably, plastomes in parasitic plants always show non-canonical features, such as reduced size and accelerated rates. However, few organelle genomes of this group have been sequenced, hindering our understanding of their evolution, and dodder mitogenome in particular.

Results

We assembled 22 new mitogenomes and 12 new plastomes in Convolvulaceae. Alongside previously known ones, we totally analyzed organelle genomes of 23 species in the family. Our sampling includes 16 leafy autotrophic species and 7 leafless parasitic dodders, covering 8 of the 12 tribes. Both the plastid and mitochondrial genomes of these plants have encountered variations that were rarely observed in other angiosperms. All of the plastomes possessed atypical IR boundaries. Besides the gene and IR losses in dodders, some leafy species also showed gene and intron losses, duplications, structural variations, and insertions of foreign DNAs. The phylogeny reconstructed by plastid protein coding sequences confirmed the previous relationship of the tribes. However, the monophyly of ‘Merremieae’ and the sister group of Cuscuta remained uncertain. The mitogenome was significantly inflated in Cuscuta japonica, which has exceeded over 800 kb and integrated massive DNAs from other species. In other dodders, mitogenomes were maintained in small size, revealing divergent evolutionary strategies. Mutations unique to plants were detected in the mitochondrial gene ccmFc, which has broken into three fragments through gene fission and splicing shift. The unusual changes likely initially happened to the common ancestor of the family and were caused by a foreign insertion from rosids followed by double-strand breaks and imprecise DNA repairs. The coding regions of ccmFc expanded at both sides after the fission, which may have altered the protein structure.

Conclusions

Our family-scale analyses uncovered unusual scenarios for both organelle genomes in Convolvulaceae, especially in parasitic plants. The data provided valuable genetic resources for studying the evolution of Convolvulaceae and plant parasitism.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12915-022-01250-1.

The chicken or the egg? Plastome evolution and an independent loss of the inverted repeat in papilionoid legumes

The plastid genome (plastome), while surprisingly constant in gene order and content across most photosynthetic angiosperms, exhibits variability in several unrelated lineages. During the diversification history of the legume family Fabaceae, plastomes have undergone many rearrangements, including inversions, expansion, contraction and loss of the typical inverted repeat (IR), gene loss and repeat accumulation in both shared and independent events. While legume plastomes have been the subject of study for some time, most work has focused on agricultural species in the IR-lacking clade (IRLC) and the plant model Medicago truncatula. The subfamily Papilionoideae, which contains virtually all of the agricultural legume species, also comprises most of the plastome variation detected thus far in the family. In this study three non-papilioniods were included among 34 newly sequenced legume plastomes, along with 33 publicly available sequences, to assess plastome structural evolution in the subfamily. In an effort to examine plastome variation across the subfamily, approximately 20% of the sampling represents the IRLC with the remainder selected to represent the early-branching papilionoid clades. A number of IR-related and repeat-mediated changes were identified and examined in a phylogenetic context. Recombination between direct repeats associated with ycf2 resulted in intraindividual plastome heteroplasmy. Although loss of the IR has not been reported in legumes outside of the IRLC, one genistoid taxon was found to completely lack the typical plastome IR. The role of the IR and non-IR repeats in the progression of plastome change is discussed.

Lineage-specific plastid degradation in subtribe Gentianinae (Gentianaceae)

The structure and sequence of plastid genomes is highly conserved across most land plants, except for a minority of lineages that show gene loss and genome degradation. Understanding the early stages of plastome degradation may provide crucial insights into the repeatability and predictability of genomic evolutionary trends. We investigated these trends in subtribe Gentianinae of the Gentianaceae, which encompasses ca. 450 species distributed around the world, particularly in alpine and subalpine environments. We sequenced, assembled, and annotated the plastomes of 41 species, representing all six genera in subtribe Gentianinae and all main sections of the species-rich genus Gentiana L. We reconstructed the phylogeny, estimated divergence times, investigated the phylogenetic distribution of putative gene losses, and related these to substitution rate shifts and species' habitats. We obtained a strongly supported topology consistent with earlier studies, with all six genera in Gentianinae recovered as monophyletic and all main sections of Gentiana having full support. While closely related species have very similar plastomes in terms of size and structure, independent gene losses, particularly of the ndh complex, have occurred in multiple clades across the phylogeny. Gene loss was usually associated with a shift in the boundaries of the small single-copy and inverted repeat regions. Substitution rates were variable between clades, with evidence for both elevated and decelerated rate shifts. Independent lineage-specific loss of ndh genes occurred at a wide range of times, from Eocene to Pliocene. Our study illustrates that diverse degradation patterns shape the evolution of the plastid in this species-rich plant group.

Plastid Genomes of Flowering Plants: Essential Principles

The plastid genome (plastome ) has proved a valuable source of data for evaluating evolutionary relationships among angiosperms. Through basic and applied approaches, plastid transformation technology offers the potential to understand and improve plant productivity, providing food, fiber, energy, and medicines to meet the needs of a burgeoning global population. The growing genomic resources available to both phylogenetic and biotechnological investigations is allowing novel insights and expanding the scope of plastome research to encompass new species. In this chapter, we present an overview of some of the seminal and contemporary research that has contributed to our current understanding of plastome evolution and attempt to highlight the relationship between evolutionary mechanisms and the tools of plastid genetic engineering.

Evolutionary and phylogenetic aspects of the chloroplast genome of Chaenomeles species

Chaenomeles (family Rosaceae) is a genus of five diploid species of deciduous spiny shrubs that are native to Central Asia and Japan. It is an important horticultural crop (commonly known as flowering quinces) in Europe and Asia for its high yield in fruits that are rich in juice, aroma, and dietary fiber. Therefore, the development of effective genetic markers of Chaenomeles species is advantageous for crop improvement through breeding and selection. In this study, we successfully assembled and analyzed the chloroplast genome of five Chaenomeles species. The chloroplast genomes of the five Chaenomeles species were very similar with no structural or content rearrangements among them. The chloroplast genomes ranged from 159,436 to 160,040 bp in length and contained a total of 112 unique genes, including 78 protein-coding genes, 30 tRNAs, and 4 rRNAs. Three highly variable regions, including trnR-atpA, trnL-F, and rpl32-ccsA, were identified. Phylogenetic analysis based on the complete chloroplast genome showed that Chaenomeles forms a monophyletic clade and had a close relationship with the genera Docynia and Malus. Analyses for phylogenetic relationships and the development of available genetic markers in future could provide valuable information regarding genetics and breeding mechanisms of the Chaenomeles species.

Plastome phylogenomics and phylogenetic diversity of endangered and threatened grassland species (Poaceae) in a North American tallgrass prairie

Native grasslands are one of the most endangered ecosystems in North America. In this study, we examined the ecological and evolutionary roles of endangered and threatened (e/t) grasses by establishing robust evolutionary relationships with other nonthreatened native and introduced grass species of the community. We hypothesized that the phylogenomic distribution of e/t species of grasses in Illinois would be phylogenetically clustered because closely related species would be vulnerable to the same threats and have similar requirements for survival. This study presents the first time a phylogeny based on complete plastome DNA of Poaceae was analyzed by phylogenetic diversity analysis. To avoid the disturbance of e/t populations, DNA was extracted from herbarium specimens. Next-generation sequencing (NGS) techniques were used to sequence DNA of plastid genomes (plastomes). The resulting phylogenomic tree was analyzed by phylogenetic diversity metrics. The extracted DNA successfully produced complete plastomes demonstrating that herbarium material is a practical source of DNA for genomic studies. The phylogenomic tree was strongly supported and defined Dichanthelium as a separate clade from Panicum. The phylogenetic metrics revealed phylogenetic clustering of e/t species, confirming our hypothesis.

Zygnema circumcarinatum UTEX 1559 chloroplast and mitochondrial genomes provide insight into land plant evolution

The complete chloroplast and mitochondrial genomes of Charophyta have shed new light on land plant terrestrialization. Here, we report the organellar genomes of the Zygnema circumcarinatum strain UTEX 1559, and a comparative genomics investigation of 33 plastomes and 18 mitogenomes of Chlorophyta, Charophyta (including UTEX 1559 and its conspecific relative SAG 698-1a), and Embryophyta. Gene presence/absence was determined across these plastomes and mitogenomes. A comparison between the plastomes of UTEX 1559 (157 548 bp) and SAG 698-1a (165 372 bp) revealed very similar gene contents, but substantial genome rearrangements. Surprisingly, the two plastomes share only 85.69% nucleotide sequence identity. The UTEX 1559 mitogenome size is 215 954 bp, the largest among all sequenced Charophyta. Interestingly, this large mitogenome contains a 50 kb region without homology to any other organellar genomes, which is flanked by two 86 bp direct repeats and contains 15 ORFs. These ORFs have significant homology to proteins from bacteria and plants with functions such as primase, RNA polymerase, and DNA polymerase. We conclude that (i) the previously published SAG 698-1a plastome is probably from a different Zygnema species, and (ii) the 50 kb region in the UTEX 1559 mitogenome might be recently acquired as a mobile element.