Genome skimming reveals novel plastid markers for the molecular identification of illegally logged African timber species

Reporting complete chloroplast genome of endangered red mulberry, useful for understanding hybridization and phylogenetic relationships

Introgressive hybridization of the North American native red mulberry (Morus rubra) with its invasive congener white mulberry (Morus alba) has severely threatened the genetic integrity of M. rubra, which is primarily found in pristine riparian forests. The major objectives of the present study were (1) to sequence, assemble, and annotate the complete chloroplast genome of M. rubra, and (2) to perform phylogenomic analyses of Morus species to assess their evolutionary history and hybridization events within the genus. We sampled 45 mulberry trees representing populations from eight US states for chloroplast genome sequencing. We found that the chloroplast genome of M. rubra ranged from 159,396 to 159,423 basepair (bp) and contained 128 genes coding for eight rRNAs, 37 tRNAs, and 83 proteins. The chloroplast genome of M. rubra was at least 103 bp larger than that of M. alba. The chloroplast DNA sequence polymorphism analysis of M. rubra showed the presence of 12 haplotypes. The chloroplast genome analysis across 12 Morus species identified trnK-UUU-rps16, psbI-trnG-UCC, psbC-psbZ, psbZ-trnM-CAU, rps4-trnT-UGU, trnT-UGU-trnL-UAA, ndhC-trnV-UAC, psbE-petL, clpP1, ndhF-rpl32, rpl32-trnL-UAG, and ccsA-ndhD as having higher DNA polymorphism (Pi > 0.01). Phylogenomic analysis of the 12 Morus species revealed distinct clades for Asian, North American, South American, and African Morus, supporting the monophyly of the genus. Divergence time analysis showed the most recent common ancestor of the genus Morus diverged 38.67 million years ago (MYA), separating the African clade of M. mesozygia and M. insignis from the others, while the American and Asian clades diverged 28.63 MYA. The results from this study provide insights into the chloroplast genome structure of M. rubra, offering a foundational data that bridges the knowledge gaps for addressing complex taxonomic complexities within the genus and for developing molecular markers to study population genetics, including introgressive hybridization.

Unlocking the geography of Azobé timber (Lophira alata): revealing spatial genetic structure beyond species boundaries

BACKGROUND

The illegal trade of tropical timber constitutes a major and persistent environmental problem. Since the detection of fraud in trade documents remains challenging, forensic tools that can independently trace timber origin are needed. In this study, we evaluated the potential of the chloroplast genome (plastome) as a genetic tool to verify the claimed species and geographic origin of timber from Azobé (Lophira alata), an intensively exploited and threatened tropical tree species.

RESULTS

We sampled 480 trees from Lophira alata and the congeneric species L. lanceolata across nine countries in Central and West Africa. Sampling included L. alata trees from 15 logging concessions in Cameroon, Gabon and the Republic of the Congo. DNA was isolated from the cambium or leaf tissue, and complete plastid genomes were assembled. A total of 228 SNPs from 436 trees were retained, which formed 35 pDNA haplotypes (with a length of 179 SNPs). The two Lophira species shared one plastid haplotype and contained several closely related plastid haplotypes. For the exploited L. alata, we detected a moderately strong correlation between genetic and spatial distances. Two haplotypes were widely spread across the core of Central Africa, while several others were more spatially constrained or endemic, for example, in West Gabon (potentially a L. alata cryptic species) and Northern Congo.

CONCLUSIONS

The distribution of haplotypes revealed a clear spatial structure. Some widely spread haplotypes potentially hamper site distinction of Azobé wood samples, but still reveal their wider region of origin. In regions where endemic haplotypes are present, differentiation may be successful at finer scales. Thus, the potential spatial resolution for timber tracing may vary across regions. We assembled the first reference database of plastome-wide SNP datasets for Azobé timber, with a focus on the major logging areas. Our work represents a step towards plastome-based timber tracing for this species, but also reveals limited potential of this method for species differentiation. To validate the potential of the plastid genome for timber tracing, further steps, including assignment and blind sample tests, will be needed.

Species delimitation and phylogenomics of economically important African Pterocarpus trees, with an implication for the development of DNA-based species identification tools

Reliable species delimitation is fundamental for establishing clear and equitable guidelines on the sustainable harvest of economically important organisms. Pterocarpus (Fabaceae) is a pantropical tree genus including several highly valuable timber-producing species, that face significant threats from intensive logging. However, a lack of taxonomic clarity has hindered the advance of logging regulations and has led to the inclusion of all African Pterocarpus populations under CITES regulations (CoP19). In our study, we addressed this issue by reassessing species delimitation of all twelve accepted African Pterocarpus species, including neotropical samples of the two trans-Atlantic species. Based on DNA sequences obtained by the Angiosperms353 probe set, we reconstructed a comprehensive phylogeny applying maximum likelihood and multispecies coalescent approaches. Additionally, we explored the potential of high-copy DNA for identifying African Pterocarpus species. This involved reconstructing chloroplast and ribosomal DNA phylogenies, using genome skimming and maximum likelihood approaches. Our results confirmed the monophyly of eleven Pterocarpus species and a paraphyletic P. rotundifolius, which presented geographically coherent subclades, suggesting the possibility of cryptic diversity within the complex. A similar situation arose in P. lucens, exhibiting two sister clades with disjunct distributions. Species delimitation based on high-copy DNA was congruent with the Angiosperms353 data for most species, indicating the reliability of chloroplast and ribosomal DNA markers for Pterocarpus species identification. Our findings give valuable insights into African Pterocarpus species delimitation, highlighting the need for further investigation of potential cryptic diversity within a clade including P. rotundifolius, P. brenanii and P. lucens. Finally, our study lays the groundwork for developing DNA-based tools aimed at improving logging regulations for African Pterocarpus species.

Characterization and phylogenetic analysis of the complete chloroplast genome of Curcuma comosa and C. latifolia

Members of the Curcuma genus, a crop in the Zingiberaceae, are widely utilized rhizomatous herbs globally. There are two distinct species, C. comosa Roxb. and C. latifolia Roscoe, referred to the same vernacular name "Wan Chak Motluk" in Thai. C. comosa holds economic importance and is extensively used as a Thai traditional medicine due to its phytoestrogenic properties. However, its morphology closely resembles that of C. latifolia, which contains zederone, a compound known for its hepatotoxic effects. They are often confused, which may affect the quality, efficacy and safety of the derived herbal materials. Thus, DNA markers were developed for discriminating C. comosa from C. latifolia. This study focused on analyzing core DNA barcode regions, including rbcL, matK, psbA-trnH spacer and ITS2, of the authentic C. comosa and C. latifolia species. As a result, no variable nucleotides in core DNA barcode regions were observed. The complete chloroplast (cp) genome was introduced to differentiate between the two species. The comparison revealed that the cp genomes of C. comosa and C. latifolia were 162,272 and 162,289 bp, respectively, with a total of 133 identified genes. The phylogenetic analysis revealed that C. comosa and C. latifolia exhibited a very close relationship with other Curcuma species. The cp genome of C. comosa and C. latifolia were identified for the first time, providing valuable insights for species identification and evolutionary research within the Zingiberaceae family.

New Insights Into The Evolution of Chloroplast Genomes in Ochna Species (Ochnaceae, Malpighiales)

Ochnaceae DC. includes more than 600 species that exhibit potential values for environmental ecology, ornamental, pharmaceutical, and timber industries. Although studies on phylogeny and phytochemicals have been intensively conducted, chloroplast genome data of Ochnaceae species have not been fully explored. In this study, the next-generation sequencing method was used to sequence the chloroplast genomes of Ochna integerrima and Ochna serrulata which were 157 329 and 157 835 bp in length, respectively. These chloroplast genomes had a quadripartite structure and contained 78 protein-coding genes, 30 tRNAs, and 4 rRNAs. Comparative analysis revealed 8 hypervariable regions, including trnK_UUU-trnQ_UUG, rpoB-psbM, trnS_GGA-rps4, accD-psaI, rpl33-rps18, rpl14-rpl16, ndhF-trnL_UAG, and rps15-ycf1 among 6 Ochnaceae taxa. Additionally, there were shared and unique repeats among 6 examined chloroplast genomes. The notable changes were the loss of rpl32 in Ochna species and the deletion of rps16 exon 2 in O. integerrima compared to other taxa. This study is the first comprehensive comparative genomic analysis of complete chloroplast genomes of Ochna species and related taxa in Ochnaceae. Consequently, the current study provides initial results for further research on genomic evolution, population genetics, and developing molecular markers in Ochnaceae and related taxa.

Developing the Protocol Infrastructure for DNA Sequencing Natural History Collections

Intentionally preserved biological material in natural history collections represents a vast repository of biodiversity. Advances in laboratory and sequencing technologies have made these specimens increasingly accessible for genomic analyses, offering a window into the genetic past of species and often permitting access to information that can no longer be sampled in the wild. Due to their age, preparation and storage conditions, DNA retrieved from museum and herbarium specimens is often poor in yield, heavily fragmented and biochemically modified. This not only poses methodological challenges in recovering nucleotide sequences, but also makes such investigations susceptible to environmental and laboratory contamination. In this paper, we review the practical challenges associated with making the recovery of DNA sequence data from museum collections more routine. We first review key operational principles and issues to address, to guide the decision-making process and dialogue between researchers and curators about when and how to sample museum specimens for genomic analyses. We then outline the range of steps that can be taken to reduce the likelihood of contamination including laboratory set-ups, workflows and working practices. We finish by presenting a series of case studies, each focusing on protocol practicalities for the application of different mainstream methodologies to museum specimens including: (i) shotgun sequencing of insect mitogenomes, (ii) whole genome sequencing of insects, (iii) genome skimming to recover plant plastid genomes from herbarium specimens, (iv) target capture of multi-locus nuclear sequences from herbarium specimens, (v) RAD-sequencing of bird specimens and (vi) shotgun sequencing of ancient bovid bone samples.

First comparative analysis of complete chloroplast genomes among six Hedysarum (Fabaceae) species

Hedysarum is one of the largest genera in the Fabaceae family, mainly distributed in the Northern Hemisphere. Despite numerous molecular studies on the genus Hedysarum, there is still a lack of research aimed at defining the specific characteristics of the chloroplast genome (cp genome) of the genus. Furthermore, the interrelationships between sections in the genus based on the cp genome have not yet been studied. In this study, comprehensive analyses of the complete cp genomes of six Hedysarum species, corresponding to sections Multicaulia, Hedysarum, and Stracheya were conducted. The complete cp genomes of H. drobovii, H. flavescens, and H. lehmannianum were sequenced for this study. The cp genomes of six Hedysarum species showed high similarity with regard to genome size (except for H. taipeicum), gene sequences, and gene classes, as well as the lacking IR region. The whole cp genomes of the six species were found to contain 110 genes ranging from 121,176 bp to 126,738 bp in length, including 76 protein-coding genes, 4 rRNA genes, and 30 tRNA genes. In addition, chloroplast SSRs and repetitive sequence regions were reported for each species. The six Hedysarum species shared 7 common SSRs and exhibited 14 unique SSRs. As well, three highly variable genes (clpP, accD, and atpF) with high Pi values were detected among protein-coding genes. Furthermore, we conducted phylogenetic analyses using the complete cp genomes and 76 protein-coding genes of 14 legume species, including the seven Hedysarum species. The results showed that the Hedysarum species form a monophyletic clade closely related to the genera Onobrychis and Alhagi. Furthermore, both of our phylogenetic reconstructions showed that section Stracheya is more closely related to section Hedysarum than to section Multicaulia. This study is the first comprehensive work to investigate the genome characteristics of the genus Hedysarum, which provides useful genetic information for further research on the genus, including evolutionary studies, phylogenetic relationships, population genetics, and species identification.

Target capture and genome skimming for plant diversity studies

Recent technological advances in long-read high-throughput sequencing and assembly methods have facilitated the generation of annotated chromosome-scale whole-genome sequence data for evolutionary studies; however, generating such data can still be difficult for many plant species. For example, obtaining high-molecular-weight DNA is typically impossible for samples in historical herbarium collections, which often have degraded DNA. The need to fast-freeze newly collected living samples to conserve high-quality DNA can be complicated when plants are only found in remote areas. Therefore, short-read reduced-genome representations, such as target capture and genome skimming, remain important for evolutionary studies. Here, we review the pros and cons of each technique for non-model plant taxa. We provide guidance related to logistics, budget, the genomic resources previously available for the target clade, and the nature of the study. Furthermore, we assess the available bioinformatic analyses, detailing best practices and pitfalls, and suggest pathways to combine newly generated data with legacy data. Finally, we explore the possible downstream analyses allowed by the type of data generated using each technique. We provide a practical guide to help researchers make the best-informed choice regarding reduced genome representation for evolutionary studies of non-model plants in cases where whole-genome sequencing remains impractical.

The complete chloroplast genome of Calophyllum soulattri Burm. f. (Calophyllaceae)

Calophyllum soulattri Burm. f. (1768) is an evergreen tree native to Southeast Asia, Australia, and the Solomon Islands. It is known for its medicinal uses and has been utilized in traditional folk medicine. However, genomic resources for this species are still unavailable. In this study, we sequenced and assembled the first complete chloroplast genome of C. soulattri using next-generation sequencing data. The chloroplast genome of C. soulattri is 161,381 bp in length with a total GC content of 36.36%. The chloroplast genome contains a large single copy (LSC) region of 88,680 bp, a small single copy (SSC) region of 17,453 bp, and two inverted repeat (IR) regions of 27,624 bp each. Furthermore, the chloroplast genome has 131 genes, which include 86 protein-coding genes, 37 tRNA genes, and 8 rRNA genes. Phylogenetic analysis indicated that C. soulattri is clustered in the same branch with C. inophyllum and is closely related to Mesua ferrea.

The complete chloroplast genome sequence of Sedum bulbiferum (Crassulaceae)

Sedum bulbiferum is a traditional medicinal plant in China, with few reports on its chloroplast genome. In this study, the chloroplast genome of Sedum bulbiferum was characterized, and its phylogenetic position among other closely related species was studied. The results showed that the full length of the chloroplast genome was 150,074 bp, containing a large single-copy (LSC) region and a small single-copy (SSC) region of 81,730 and 16,726 bp, respectively, as well as two inverted repeat regions (IRs) of 25,809 bp like other plants. A total of 128 genes were found, including 83 protein-coding genes, 37 tRNA genes, and eight rRNA genes. Phylogenetic analysis showed that Sedum bulbiferum is closely related to Sedum emarginatum, Sedum alfredii, Sedum tricarpum, Sedum plumbizincicola, and Sedum sarmentosum.

Species discrimination in Schima (Theaceae): Next-generation super-barcodes meet evolutionary complexity

Plastid genome and nrDNA arrays, proposed recently as "super barcodes", might provide additional discriminatory power and overcome the limitations of traditional barcoding loci, yet super barcodes need to be tested for their effectiveness in more plant groups. Morphological homoplasy among Schima species makes the genus a model for testing the efficacy of super barcodes. In this study, we generated multiple datasets comprising standard DNA barcodes (matK, rbcL, trnH-psbA, nrITS) and super-barcodes (plastid genome, nrDNA arrays) across 58 individuals from 12 out of 13 species of Schima from China. No samples were correctly assigned to species using standard DNA barcodes and nrDNA arrays, while only 27.27% of species with multiple accessions were distinguished using the plastid genome and its partitioned datasets-the lowest estimated rate of super barcode success in the literature so far. For Schima and other taxa with similarly recently divergence and low levels of genetic variation, incomplete lineage sorting, hybridization, or taxonomic oversplitting are all possible causes of the failure. Taken together, our study suggests that by no means are super barcodes immune to the challenges imposed by evolutionary complexity. We therefore call for developing multi-locus nuclear markers for species discrimination in plant groups.