Complex evolutionary history of coffees revealed by full plastid genomes and 28,800 nuclear SNP analyses, with particular emphasis on Coffea canephora (Robusta coffee)

Crop-to-wild gene flow in wild coffee species: the case of Coffea canephora in the Democratic Republic of the Congo

BACKGROUND AND AIMS

Plant breeders are increasingly turning to crop wild relatives (CWRs) to ensure food security in a rapidly changing environment. However, CWR populations are confronted with various human-induced threats, including hybridization with their nearby cultivated crops. This might be a particular problem for wild coffee species, which often occur near coffee cultivation areas. Here, we briefly review the evidence for wild Coffea arabica (cultivated as Arabica coffee) and Coffea canephora (cultivated as Robusta coffee) and then focused on C. canephora in the Yangambi region in the Democratic Republic of the Congo. There, we examined the geographical distribution of cultivated C. canephora and the incidence of hybridization between cultivated and wild individuals within the rainforest.

METHODS

We collected 71 C. canephora individuals from home gardens and 12 C. canephora individuals from the tropical rainforest in the Yangambi region and genotyped them using genotyping-by-sequencing (GBS). We compared the fingerprints with existing GBS data from 388 C. canephora individuals from natural tropical rainforests and the INERA Coffee Collection, a Robusta coffee field gene bank and the most probable source of cultivated genotypes in the area. We then established robust diagnostic fingerprints that genetically differentiate cultivated from wild coffee, identified cultivated-wild hybrids and mapped their geographical position in the rainforest.

KEY RESULTS

We identified cultivated genotypes and cultivated-wild hybrids in zones with clear anthropogenic activity, and where cultivated C. canephora in home gardens may serve as a source for crop-to-wild gene flow. We found relatively few hybrids and backcrosses in the rainforests.

CONCLUSIONS

The cultivation of C. canephora in close proximity to its wild gene pool has led to cultivated genotypes and cultivated-wild hybrids appearing within the natural habitats of C. canephora. Yet, given the high genetic similarity between the cultivated and wild gene pool, together with the relatively low incidence of hybridization, our results indicate that the overall impact in terms of risk of introgression remains limited so far.

The evolutionary history of three Baracoffea species from western Madagascar revealed by chloroplast and nuclear genomes

The wild species of the Coffea genus present a very wide morphological, genetic, and biochemical diversity. Wild species are recognized more resistant to diseases, pests, and environmental variations than the two species currently cultivated worldwide: C. arabica (Arabica) and C. canephora (Robusta). Consequently, wild species are now considered as a crucial resource for adapting cultivated coffee trees to climate change. Within the Coffea genus, 79 wild species are native to the Indian Ocean islands of Comoros, Mayotte, Mauritius, Réunion and Madagascar, out of a total of 141 taxa worldwide. Among them, a group of 9 species called "Baracoffea" are particularly atypical in their morphology and adaptation to the sandy soils of the dry deciduous forests of western Madagascar. Here, we have attempted to shed light on the evolutionary history of three Baracoffea species: C. ambongensis, C. boinensis and C. bissetiae by analyzing their chloroplast and nuclear genomes. We assembled the complete chloroplast genomes de novo and extracted 28,800 SNP (Single Nucleotide Polymorphism) markers from the nuclear genomes. These data were used for phylogenetic analysis of Baracoffea with Coffea species from Madagascar and Africa. Our new data support the monophyletic origin of Baracoffea within the Coffea of Madagascar, but also reveal a divergence with a sister clade of four species: C. augagneurii, C. ratsimamangae, C. pervilleana and C. Mcphersonii (also called C. vohemarensis), belonging to the Subterminal botanical series and living in dry or humid forests of northern Madagascar. Based on a bioclimatic analysis, our work suggests that Baracoffea may have diverged from a group of Malagasy Coffea from northern Madagascar and adapted to the specific dry climate and low rainfall of western Madagascar. The genomic data generated in the course of this work will contribute to the understanding of the adaptation mechanisms of these particularly singular species.

The chloroplast protein HCF164 is predicted to be associated with Coffea SH9 resistance factor against Hemileia vastatrix

To explore the connection between chloroplast and coffee resistance factors, designated as SH1 to SH9, whole genomic DNA of 42 coffee genotypes was sequenced, and entire chloroplast genomes were de novo assembled. The chloroplast phylogenetic haplotype network clustered individuals per species instead of SH factors. However, for the first time, it allowed the molecular validation of Coffea arabica as the maternal parent of the spontaneous hybrid "Híbrido de Timor". Individual reads were also aligned on the C. arabica reference genome to relate SH factors with chloroplast metabolism, and an in-silico analysis of selected nuclear-encoded chloroplast proteins (132 proteins) was performed. The nuclear-encoded thioredoxin-like membrane protein HCF164 enabled the discrimination of individuals with and without the SH9 factor, due to specific DNA variants linked to chromosome 7c (from C. canephora-derived sub-genome). The absence of both the thioredoxin domain and redox-active disulphide center in the HCF164 protein, observed in SH9 individuals, raises the possibility of potential implications on redox regulation. For the first time, the identification of specific DNA variants of chloroplast proteins allows discriminating individuals according to the SH profile. This study introduces an unexplored strategy for identifying protein/genes associated with SH factors and candidate targets of H. vastatrix effectors, thereby creating new perspectives for coffee breeding programs.

Hedyotislongiramulis (Rubiaceae), a new species from south China

Hedyotislongiramulissp. nov. (Rubiaceae) is described from Guangdong Province, China. It is similar to H.caudatifolia but differs in having puberulent, more or less tetragonal and decussately sulcate juvenile stems, waxy leaf surface, short inflorescence peduncles, high length ratio of corolla lobe to tube, and subglobose capsules. The phylogenetic analysis reveals that H.longiramulis is sister to H.pubirachis. Dimorphism concerning pollen size was observed in the heterostylous flowers. The complete chloroplast genome of the new species comprises a typical quadripartite structure of 153,616 bp in length, with two inverted repeats of 25,457 bp, a large single-copy of 85,050 bp and a small single-copy of 17,652 bp. It contains 112 unique genes, including 79 protein-coding genes, 29 tRNA genes, and four rRNA genes, the GC content of the chloroplast genome is 32.4%. The new species is provisionally evaluated as "Least Concern" because it is common and well-protected in two Provincial Nature Reserves.

Metagenomics of African Empogona and Tricalysia (Rubiaceae) reveals the presence of leaf endophytes

BACKGROUND

Leaf symbiosis is a phenomenon in which host plants of Rubiaceae interact with bacterial endophytes within their leaves. To date, it has been found in around 650 species belonging to eight genera in four tribes; however, the true extent in Rubiaceae remains unknown. Our aim is to investigate the possible occurrence of leaf endophytes in the African plant genera Empogona and Tricalysia and, if present, to establish their identity.

METHODS

Total DNA was extracted from the leaves of four species of the Coffeeae tribe (Empogona congesta, Tricalysia hensii, T. lasiodelphys, and T. semidecidua) and sequenced. Bacterial reads were filtered out and assembled. Phylogenetic analysis of the endophytes was used to reveal their identity and their relationship with known symbionts.

RESULTS

All four species have non-nodulated leaf endophytes, which are identified as Caballeronia. The endophytes are distinct from each other but related to other nodulated and non-nodulated endophytes. An apparent phylogenetic or geographic pattern appears to be absent in endophytes or host plants. Caballeronia endophytes are present in the leaves of Empogona and Tricalysia, two genera not previously implicated in leaf symbiosis. This interaction is likely to be more widespread, and future discoveries are inevitable.

A review of the indigenous coffee resources of Uganda and their potential for coffee sector sustainability and development

Uganda is a major global coffee exporter and home to key indigenous (wild) coffee resources. A comprehensive survey of Uganda's wild coffee species was undertaken more than 80 years ago (in 1938) and thus a contemporary evaluation is required, which is provided here. We enumerate four indigenous coffee species for Uganda: Coffea canephora, C. eugenioides, C. liberica (var. dewevrei) and C. neoleroyi. Based on ground point data from various sources, survey of natural forests, and literature reviews we summarise taxonomy, geographical distribution, ecology, conservation, and basic climate characteristics, for each species. Using literature review and farm survey we also provide information on the prior and exiting uses of Uganda's wild coffee resources for coffee production. Three of the indigenous species (excluding C. neoleroyi) represent useful genetic resources for coffee crop development (e.g. via breeding, or selection), including: adaptation to a changing climate, pest and disease resistance, improved agronomic performance, and market differentiation. Indigenous C. canephora has already been pivotal in the establishment and sustainability of the robusta coffee sector in Uganda and worldwide, and has further potential for the development of this crop species. Coffea liberica var. dewevrei (excelsa coffee) is emerging as a commercially viable coffee crop plant in its own right, and may offer substantial potential for lowland coffee farmers, i.e. in robusta coffee growing areas. It may also provide useful stock material for the grafting of robusta and Arabica coffee, and possibly other species. Preliminary conservation assessments indicate that C. liberica var. dewevrei and C. neoleroyi are at risk of extinction at the country-level (Uganda). Adequate protection of Uganda's humid forests, and thus its coffee natural capital, is identified as a conservation priority for Uganda and the coffee sector in general.

Ecological and genomic vulnerability to climate change across native populations of Robusta coffee (Coffea canephora)

The assessment of population vulnerability under climate change is crucial for planning conservation as well as for ensuring food security. Coffea canephora is, in its native habitat, an understorey tree that is mainly distributed in the lowland rainforests of tropical Africa. Also known as Robusta, its commercial value constitutes a significant revenue for many human populations in tropical countries. Comparing ecological and genomic vulnerabilities within the species' native range can provide valuable insights about habitat loss and the species' adaptive potential, allowing to identify genotypes that may act as a resource for varietal improvement. By applying species distribution models, we assessed ecological vulnerability as the decrease in climatic suitability under future climatic conditions from 492 occurrences. We then quantified genomic vulnerability (or risk of maladaptation) as the allelic composition change required to keep pace with predicted climate change. Genomic vulnerability was estimated from genomic environmental correlations throughout the native range. Suitable habitat was predicted to diminish to half its size by 2050, with populations near coastlines and around the Congo River being the most vulnerable. Whole-genome sequencing revealed 165 candidate SNPs associated with climatic adaptation in C. canephora, which were located in genes involved in plant response to biotic and abiotic stressors. Genomic vulnerability was higher for populations in West Africa and in the region at the border between DRC and Uganda. Despite an overall low correlation between genomic and ecological vulnerability at broad scale, these two components of vulnerability overlap spatially in ways that may become damaging. Genomic vulnerability was estimated to be 23% higher in populations where habitat will be lost in 2050 compared to regions where habitat will remain suitable. These results highlight how ecological and genomic vulnerabilities are relevant when planning on how to cope with climate change regarding an economically important species.

An 82 bp tandem repeat family typical of 3' non-coding end of Gypsy/TAT LTR retrotransposons is conserved in Coffea spp. pericentromeres

Coffea spp. chromosomes are very small and accumulate a variety of repetitive DNA families around the centromeres. However, the proximal regions of Coffea chromosomes remain poorly understood, especially regarding the nature and organisation of the sequences. Taking advantage of the genome sequences of C. arabica (2n = 44), C. canephora, and C. eugenioides (C. arabica progenitors with 2n = 22) and good coverage genome sequencing of dozens of other wild Coffea spp., repetitive DNA sequences were identified, and the genomes were compared to decipher particularities of pericentromeric structures. The searches revealed a short tandem repeat (82 bp length) typical of Gypsy/TAT LTR retrotransposons, named Coffea_sat11. This repeat organises clusters with fragments of other transposable elements, comprising regions of non-coding RNA production. Cytogenomic analyses showed that Coffea_sat11 extends from the pericentromeres towards the middle of the chromosomal arms. This arrangement was observed in the allotetraploid C. arabica chromosomes, as well as in its progenitors. This study improves our understanding of the role of the Gypsy/TAT LTR retrotransposon lineage in the organisation of Coffea pericentromeres, as well as the conservation of Coffea_sat11 within the genus. The relationships between fragments of other transposable elements and the functional aspects of these sequences on the pericentromere chromatin were also evaluated. Highlights: A scattered short tandem repeat, typical of Gypsy/TAT LTR retrotransposons, associated with several fragments of other transposable elements, accumulates in the pericentromeres of Coffea chromosomes. This arrangement is preserved in all clades of the genus and appears to have a strong regulatory role in the organisation of chromatin around centromeres.