Reassessing Banana Phylogeny and Organelle Inheritance Modes Using Genome Skimming Data

The CsPPR gene with RNA-editing function involved in leaf color asymmetry of the reciprocal hybrids derived from Cucumis sativus and C. hystrix

MAIN CONCLUSION

The leaf color asymmetry found in the reciprocal hybrids C. hystrix × C. sativus (HC) and C. sativus × C. hystrix (CH) could be influenced by the CsPPR gene (CsaV3_1G038250.1). Most angiosperm organelles are maternally inherited; thus, the reciprocal hybrids usually exhibit asymmetric phenotypes that are associated with the maternal parent. However, there are two sets of organelle genomes in the plant cytoplasm, and the mechanism of reciprocal differences are more complex and largely unknown, because the chloroplast genes are involved besides mitochondrial genes. Cucumis spp. contains the species, i.e., cucumber and melon, which chloroplasts and mitochondria are maternally inherited and paternally inherited, respectively, serving as good materials for the study of reciprocal differences. In this study, leaf color asymmetry was observed in the reciprocal hybrids (HC and CH) derived from C. sativus (2n = 14, CC) and C. hystrix (2n = 24, HH), where the leaves of HC were found to have reduced chlorophyll content, abnormal chloroplast structure and lower photosynthetic capacity. Transcriptomic analysis revealed that the chloroplast development-related genes were differentially expressed in leaf color asymmetry. Genetic analysis showed that leaf color asymmetry was caused by the maternal chloroplast genome. Comparative analysis of chloroplast genomes revealed that there was no mutation in the chloroplast genome during interspecific hybridization. Moreover, a PPR gene (CsaV3_1G038250.1) with RNA-editing function was found to be involved in the regulation of leaf color asymmetry. These findings provide new insights into the regulatory mechanisms of asymmetric phenotypes in plant reciprocal crosses.

Phylogenomics identifies parents of naturally occurring tetraploid bananas

BACKGROUND

Triploid bananas are almost sterile. However, we succeeded in harvesting seeds from two edible triploid banana individuals (Genotype: ABB) in our conservation repository where various wild diploid bananas were also grown. The resulting rare offspring survived to seedling stages. DNA content analyses reveal that they are tetraploid. Since bananas contain maternally inherited plastids and paternally inherited mitochondria, we sequenced and assembled plastomes and mitogenomes of these seedlings to trace their hybridization history.

RESULTS

The coding sequences of both organellar genomic scaffolds were extracted, aligned, and concatenated for constructing phylogenetic trees. Our results suggest that these tetraploid seedlings be derived from hybridization between edible triploid bananas and wild diploid Musa balbisiana (BB) individuals. We propose that generating female triploid gametes via apomeiosis may allow the triploid maternal bananas to produce viable seeds.

CONCLUSIONS

Our study suggests a practical avenue towards expanding genetic recombination and increasing genetic diversity of banana breeding programs. Further cellular studies are needed to understand the fusion and developmental processes that lead to formation of hybrid embryos in banana reproduction, polyploidization, and evolution.

Integration of large and diverse angiosperm DNA fragments into Asian Gnetum mitogenomes

BACKGROUND

Horizontal gene transfer (HGT) events have rarely been reported in gymnosperms. Gnetum is a gymnosperm genus comprising 25‒35 species sympatric with angiosperms in West African, South American, and Southeast Asian rainforests. Only a single acquisition of an angiosperm mitochondrial intron has been documented to date in Asian Gnetum mitogenomes. We wanted to develop a more comprehensive understanding of frequency and fragment length distribution of such events as well as their evolutionary history in this genus.

RESULTS

We sequenced and assembled mitogenomes from five Asian Gnetum species. These genomes vary remarkably in size and foreign DNA content. We identified 15 mitochondrion-derived and five plastid-derived (MTPT) foreign genes. Our phylogenetic analyses strongly indicate that these foreign genes were transferred from diverse eudicots-mostly from the Rubiaceae genus Coptosapelta and ten genera of Malpighiales. This indicates that Asian Gnetum has experienced multiple independent HGT events. Patterns of sequence evolution strongly suggest DNA-mediated transfer between mitochondria as the primary mechanism giving rise to these HGT events. Most Asian Gnetum species are lianas and often entwined with sympatric angiosperms. We therefore propose that close apposition of Gnetum and angiosperm stems presents opportunities for interspecific cell-to-cell contact through friction and wounding, leading to HGT.

CONCLUSIONS

Our study reveals that multiple HGT events have resulted in massive amounts of angiosperm mitochondrial DNA integrated into Asian Gnetum mitogenomes. Gnetum and its neighboring angiosperms are often entwined with each other, possibly accounting for frequent HGT between these two phylogenetically remote lineages.

Metapangenomics of wild and cultivated banana microbiome reveals a plethora of host-associated protective functions

BACKGROUND

Microbiomes are critical to plants, promoting growth, elevating stress tolerance, and expanding the plant's metabolic repertoire with novel defense pathways. However, generally microbiomes within plant tissues, which intimately interact with their hosts, remain poorly characterized. These endospheres have become a focus in banana (Musa spp.)-an important plant for study of microbiome-based disease protection. Banana is important to global food security, while also being critically threatened by pandemic diseases. Domestication and clonal propagation are thought to have depleted protective microbiomes, whereas wild relatives may hold promise for new microbiome-based biological controls. The goal was to compare metapangenomes enriched from 7 Musa genotypes, including wild and cultivated varieties grown in sympatry, to assess the host associations with root and leaf endosphere functional profiles.

RESULTS

Density gradients successfully generated culture-free microbial enrichment, dominated by bacteria, with all together 24,325 species or strains distinguished, and 1.7 million metagenomic scaffolds harboring 559,108 predicted gene clusters. About 20% of sequence reads did not match any taxon databases and ~ 62% of gene clusters could not be annotated to function. Most taxa and gene clusters were unshared between Musa genotypes. Root and corm tissues had significantly richer endosphere communities that were significantly different from leaf communities. Agrobacterium and Rhizobium were the most abundant in all samples while Chitinophagia and Actinomycetia were more abundant in roots and Flavobacteria in leaves. At the bacterial strain level, there were > 2000 taxa unique to each of M. acuminata (AAA genotype) and M. balbisiana (B-genotype), with the latter 'wild' relatives having richer taxa and functions. Gene ontology functional enrichment showed core beneficial functions aligned with those of other plants but also many specialized prospective beneficial functions not reported previously. Some gene clusters with plant-protective functions showed signatures of phylosymbiosis, suggesting long-standing associations or heritable microbiomes in Musa.

CONCLUSIONS

Metapangenomics revealed key taxa and protective functions that appeared to be driven by genotype, perhaps contributing to host resistance differences. The recovery of rich novel taxa and gene clusters provides a baseline dataset for future experiments in planta or in vivo bacterization or engineering of wild host endophytes.

The banana genome hub: a community database for genomics in the Musaceae

The Banana Genome Hub provides centralized access for genome assemblies, annotations, and the extensive related omics resources available for bananas and banana relatives. A series of tools and unique interfaces are implemented to harness the potential of genomics in bananas, leveraging the power of comparative analysis, while recognizing the differences between datasets. Besides effective genomic tools like BLAST and the JBrowse genome browser, additional interfaces enable advanced gene search and gene family analyses including multiple alignments and phylogenies. A synteny viewer enables the comparison of genome structures between chromosome-scale assemblies. Interfaces for differential expression analyses, metabolic pathways and GO enrichment were also added. A catalogue of variants spanning the banana diversity is made available for exploration, filtering, and export to a wide variety of software. Furthermore, we implemented new ways to graphically explore gene presence-absence in pangenomes as well as genome ancestry mosaics for cultivated bananas. Besides, to guide the community in future sequencing efforts, we provide recommendations for nomenclature of locus tags and a curated list of public genomic resources (assemblies, resequencing, high density genotyping) and upcoming resources-planned, ongoing or not yet public. The Banana Genome Hub aims at supporting the banana scientific community for basic, translational, and applied research and can be accessed at https://banana-genome-hub.southgreen.fr.

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.

Comparative plastome analysis of Musaceae and new insights into phylogenetic relationships

Background

Musaceae is an economically important family consisting of 70-80 species. Elucidation of the interspecific relationships of this family is essential for a more efficient conservation and utilization of genetic resources for banana improvement. However, the scarcity of herbarium specimens and quality molecular markers have limited our understanding of the phylogenetic relationships in wild species of Musaceae. Aiming at improving the phylogenetic resolution of Musaceae, we analyzed a comprehensive set of 49 plastomes for 48 species/subspecies representing all three genera of this family.

Results

Musaceae plastomes have a relatively well-conserved genomic size and gene content, with a full length ranging from 166,782 bp to 172,514 bp. Variations in the IR borders were found to show phylogenetic signals to a certain extent in Musa. Codon usage bias analysis showed different preferences for the same codon between species and three genera and a common preference for A/T-ending codons. Among the two genes detected under positive selection (dN/dS > 1), ycf2 was indicated under an intensive positive selection. The divergent hotspot analysis allowed the identification of four regions (ndhF-trnL, ndhF, matK-rps16, and accD) as specific DNA barcodes for Musaceae species.

Bayesian and maximum likelihood phylogenetic analyses using full plastome resulted in nearly identical tree topologies with highly supported relationships between species. The monospecies genus Musella is sister to Ensete, and the genus Musa was divided into two large clades, which corresponded well to the basic number of n = x = 11 and n = x =10/9/7, respectively. Four subclades were divided within the genus Musa. A dating analysis covering the whole Zingiberales indicated that the divergence of Musaceae family originated in the Palaeocene (59.19 Ma), and the genus Musa diverged into two clades in the Eocene (50.70 Ma) and then started to diversify from the late Oligocene (29.92 Ma) to the late Miocene. Two lineages (Rhodochlamys and Australimusa) radiated recently in the Pliocene /Pleistocene periods.

Conclusions

The plastome sequences performed well in resolving the phylogenetic relationships of Musaceae and generated new insights into its evolution. Plastome sequences provided valuable resources for population genetics and phylogenetics at lower taxon.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-022-08454-3.