Genetic diversity of native and cultivated Ugandan Robusta coffee (Coffea canephora Pierre ex A. Froehner): Climate influences, breeding potential and diversity conservation

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.

Convention on Biological Diversity (CBD) and the Nagoya Protocol: Implications and Compliance Strategies for the Global Coffee Community

The Nagoya Protocol on Access and Benefit-sharing (ABS) of the Convention on Biological Diversity (CBD) is a fundamental international agreement that plays a crucial role in the protection and equitable utilization of plant genetic resources. While this agreement is essential for conservation and sustainable use, it presents specific challenges to coffee research and industry. One major issue is the requirement to obtain prior informed consent (PIC) from the source country or community, which can be a complex and time-consuming process, especially in regions with limited governance capacity. Additionally, the mandates of this agreement necessitate benefit-sharing with the source community, a requirement that poses implementation challenges, particularly for small businesses or individual researchers. Despite these challenges, the importance of the Nagoya Protocol in the coffee sector cannot be overstated. It contributes significantly to the conservation of coffee genetic resources and the sustainable utilization of these resources, ensuring fair distribution of benefits. To address the complexities presented by this international framework, coffee researchers and industry need to engage proactively with source countries and communities. This includes developing clear and equitable benefit-sharing and implementing strategies for compliance. This article explores the impact of the Nagoya Protocol on the coffee industry, particularly emphasizing the need for balancing scientific investigation with the ethical considerations of resource sharing. It also discusses practical strategies for navigating the complexities of this agreement, including research focused on authenticity control and the challenges in conducting large-scale coffee studies. The conclusion underscores the potential for international collaboration, particularly through platforms like the International Coffee Organization (ICO), to harmonize research activities with the ethical imperatives of the Nagoya Protocol.

Genetic diversity and structure in wild Robusta coffee (Coffea canephora A. Froehner) populations in Yangambi (DR Congo) and their relation to forest disturbance

Degradation and regeneration of tropical forests can strongly affect gene flow in understorey species, resulting in genetic erosion and changes in genetic structure. Yet, these processes remain poorly studied in tropical Africa. Coffea canephora is an economically important species, found in the understorey of tropical rainforests of Central and West Africa, and the genetic diversity harboured in its wild populations is vital for sustainable coffee production worldwide. Here, we aimed to quantify genetic diversity, genetic structure, and pedigree relations in wild C. canephora populations, and we investigated associations between these descriptors and forest disturbance and regeneration. Therefore, we sampled 256 C. canephora individuals within 24 plots across three forest categories in Yangambi (DR Congo), and used genotyping-by-sequencing to identify 18,894 SNPs. Overall, we found high genetic diversity, and no evidence of genetic erosion in C. canephora in disturbed old-growth forest, as compared to undisturbed old-growth forest. In addition, an overall heterozygosity excess was found in all populations, which was expected for a self-incompatible species. Genetic structure was mainly a result of isolation-by-distance, reflecting geographical location, with low to moderate relatedness at finer scales. Populations in regrowth forest had lower allelic richness than populations in old-growth forest and were characterised by a lower inter-individual relatedness and a lack of isolation-by-distance, suggesting that they originated from different neighbouring populations and were subject to founder effects. Wild Robusta coffee populations in the study area still harbour high levels of genetic diversity, yet careful monitoring of their response to ongoing forest degradation remains required.

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.

Intraspecific variation in growth response to drought stress across geographic locations and genetic groups in Coffea canephora

Uganda lies within the drier end of the natural distribution range of Coffea canephora and contains unexplored genetic material that could be drought-adapted and useful for developing climate-resilient varieties. Using water treatment: (i) ample and (ii) restricted-water, the response of 148 genotypes were studied comprising wild, feral and cultivated C. canephora. Biomass allocation, standing leaf area and leaf area growth data were collected. Linear mixed effect models and PCA were used to the analyze effect of water treatment on genotypes from different: (i) cultivation status, (ii) genetic groups and (iii) locations. We also assessed the relationship between drought tolerance for relative growth rate in leaf area (RGRA), total number of leaves (TNL), total leaf area (TLA) and total leaf dry weight (TLDW) of genotypes at final harvest. Restricted-water reduced RGRA across genetic groups (3.2-32.5%) and locations (7.1-36.7%) but not cultivation status. For TNL, TLA and TLDW, genotypes that performed well in ample-water performed worse under restricted-water, indicating growth-tolerance trade-off. Drought tolerance in RGRA and TNL were negatively correlated with wetness index suggesting some degree of adaptation to local climate. Findings indicate a growth-tolerance trade-off within this tropical tree species and drought tolerance of Uganda's C. canephora is somewhat associated with local climate.

Coffea canephora: Heterotic Crosses Indicated by Molecular Approach

The genus Coffea comprises more than 100 species, of which the most commercially important are Coffea arabica and Coffea canephora. The latter is a self-incompatible plant with high natural genetic variability. The detection of polymorphism at the DNA level by molecular markers allowed significant progress with the selection of superior plants. The objective of this study was the molecular characterization of C. canephora using microsatellite markers. To this end, a population of forty-four C. canephora genotypes and one C. arabica genotype, was evaluated with 21 primers. These primers identified 61 alleles in the population and between 2 and 5 alleles per locus. The information index indicated a high level of polymorphism of the analyzed markers. According to the observed and expected heterozygosity, the genetic diversity in the population is high. The overall inbreeding coefficient of the population detected high heterozygosity and zero inbreeding within this population. Genetic diversity among the accessions was also evaluated by the unweighted pair-group method based on arithmetic averages (UPGMA). Six groups were formed based on Mojena's cutting rule and three using the Bayesian approach. These results confirmed the existence of genetic diversity, genetic variability and a potential for selection in future breeding efforts involving the 45 genotypes studied.

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.

Whole-genome resequencing of Coffea arabica L. (Rubiaceae) genotypes identify SNP and unravels distinct groups showing a strong geographical pattern

BACKGROUND

Coffea arabica L. is an economically important agricultural crop and the most popular beverage worldwide. As a perennial crop with recalcitrant seed, conservation of the genetic resources of coffee can be achieved through the complementary approach of in-situ and ex-situ field genebank. In Ethiopia, a large collection of C. arabica L. germplasm is preserved in field gene banks. Here, we report the whole-genome resequencing of 90 accessions from Choche germplasm bank representing garden and forest-based coffee production systems using Illumina sequencing technology.

RESULTS

The genome sequencing generated 6.41 billion paired-end reads, with a mean of 71.19 million reads per sample. More than 93% of the clean reads were mapped onto the C. arabica L. reference genome. A total of 11.08 million variants were identified, among which 9.74 million (87.9%) were SNPs (Single nucleotide polymorphisms) and 1.34 million (12.1%) were InDels. In all accessions, genomic variants were unevenly distributed across the coffee genome. The phylogenetic analysis using the SNP markers displayed distinct groups.

CONCLUSIONS

Resequencing of the coffee accessions has allowed identification of genetic markers, such as SNPs and InDels. The SNPs discovered in this study might contribute to the variation in important pathways of genes for important agronomic traits such as caffeine content, yield, disease, and pest in coffee. Moreover, the genome resequencing data and the genetic markers identified from 90 accessions provide insight into the genetic variation of the coffee germplasm and facilitate a broad range of genetic studies.

Genetic diversity of wild and cultivated Coffea canephora in northeastern DR Congo and the implications for conservation

PREMISE

Many cultivated coffee varieties descend from Coffea canephora, commonly known as Robusta coffee. The Congo Basin has a century-long history of Robusta coffee cultivation and breeding, and is hypothesized to be the region of origin of many of the cultivated Robusta varieties. Since little is known about the genetic composition of C. canephora in this region, we assessed the genetic diversity of wild and cultivated C. canephora shrubs in the Democratic Republic of the Congo.

METHODS

Using 18 microsatellite markers, we studied the genetic composition of wild and backyard-grown C. canephora shrubs in the Tshopo and Ituri provinces and multiple accessions from the INERA Yangambi Coffee Collection. We assessed genetic clustering patterns, genetic diversity, and genetic differentiation between populations.

RESULTS

Genetic differentiation was relatively strong between wild and cultivated C. canephora shrubs, and both gene pools harbored multiple unique alleles. Strong genetic differentiation was also observed between wild populations. The level of genetic diversity in wild populations was similar to that of the INERA Yangambi Coffee Collection, but local wild genotypes were mostly missing from that collection. Shrubs grown in the backyards were genetically similar to the breeding material from INERA Yangambi.

CONCLUSIONS

Most C. canephora that is grown in local backyards originated from INERA breeding programs, while a few shrubs were obtained directly from surrounding forests. The INERA Yangambi Coffee Collection could benefit from an enrichment with local wild genotypes to increase the genetic resources available for breeding purposes and to support ex situ conservation.