West African Crop Wild Relative Checklist, Prioritization and Inventory

Crop wild relatives (CWR) are wild plant taxa genetically related to domesticated crops with trait diversity that can be used in plant breeding to sustain food security. Prioritization is a prerequisite for the cost–effective conservation of CWR as it allows CWR in a checklist to be reduced to a manageable number for active conservation action. In this study, a partial CWR checklist comprising 1651 taxa was compiled for West Africa. Prioritization of the annotated CWR checklist was based on three criteria: (i) economic value of the related crop in West Africa, (ii) CWR genetic closeness to its related crop and (iii) threat status. After applying the three criteria using the parallel method of prioritization, 102 priority CWR were selected for active conservation action. The priority CWR are related to food crops that are nationally, regionally and globally important, such as white guinea yam (Dioscorea cayenensis subsp. rotundata (Poir) J. Miège), cassava (Manihot esculenta Crantz), rice (Oryza sativa L.), wheat (Triticum aestivum L.), cowpea (Vigna unguiculata (L.) Walp.), sweet potato (Ipomea batatas (L.) Lam.), common bean (Phaseolus vulgaris L.) and sorghum (Sorghum bicolor (L.) Moench). This CWR checklist and prioritization will help in the development of a regional conservation action plan for West Africa.


Introduction
The significant effects of climate change on agriculture and livelihood in West Africa recently show the need to develop varieties of crops that can adapt to the rising temperatures, desertification, unpredictable rainfalls, floods and droughts and new diseases and pests, as well as meet the yield quality and quantity requirements of producers and consumers Mousavi-Derazmahalleh et al, 2018;Allen et al, 2019). Climate change has led to yield losses in different crops and will continue to adversely affect agriculture with considerable yield decline predicted in West Africa (Sultan et al, 2019). IPCC reported that the duration of the growing season in West Africa may be reduced by 20% in 2050, resulting in about 40% yield reduction in cereals (Zougmoré et al, 2016). The increasing population in the Western Africa region may further limit the ability of the region to meet the food and nutrient security needs of its growing population. CWR are wild plant taxa genetically related to domesticated crops and are widely recognised as a major reservoir of valuable diversity that can be used in plant breeding to sustain food and nutrient security in the future (Maxted et al, 2006;Magos Brehm et al, 2017;Herden et al, 2020;Kioukis et al, 2020). Many CWR thrive in marginal environments (Jarvis et al, 2015;Phillips et al, 2017;Vincent et al, 2019), making them better suited to withstand changing climate conditions. The extensive genetic diversity in CWR has been used globally in plant breeding programmes to produce crop cultivars with traits for high yield, drought tolerance, disease resistance, good handling quality, seed weight, early flowering time, cooking quality and better storage quality (Maxted and Kell, 2009;USDA, 2011).
CWR conservation and use contributes to the Sustainable Development Goal (SDG) of the United Nations (United Nations, 2015). Also, the United Nations' Intergovernmental Science -Policy Platform on Biodiversity and Ecosystem Service (IPBES), described CWR as vital for future food and nutrient security, ameliorating ecosystems and adapting crops to marginal environments (IPBES, 2019).
There are about 45,000 plant species in Sub-Saharan Africa (Linder, 2014). In Nigeria alone, there are thought to be 7,895 different plant species (Federal Republic of Nigeria, 2010). However, the diversity of CWR is widely threatened by unsustainable use of natural resources, urbanization, deteriorating environmental conditions, the introduction of exotic species and climate change (Maxted and Kell, 2009;Magos Brehm et al, 2017). Several CWR thrive on farmlands and are therefore threatened by agrochemical inputs and intensive agricultural systems (Jarvis et al, 2015;Capistrano-Gossmann et al, 2017;Vincent et al, 2019). Also, increasing population, poverty, habitat destruction, overgrazing, lack of land use planning and deforestation causes biodiversity loss in West Africa (Adejuwon, 2000). There is therefore the need for active in situ and ex situ conservation of CWR in West Africa, to ensure they continue to provide profitable genes to produce plant cultivars to meet the growing demand for ample food supply for the people of West Africa and beyond. Developing a regional and national conservation plan is essential if poverty alleviation and food provision is to be maximised. This starts with making an inventory of CWR. Several countries already have CWR inventory, such as UK (Fielder et al, 2012), USA (Khoury et al, 2013), China  and Indonesia (Rahman et al, 2019). A CWR checklist is a list of CWR taxa found in a defined geographical area. A CWR checklist may contain additional information on the priority CWR which are important for conservation planning including taxon distribution, reproduction and conservation status, turning the checklist into a CWR inventory. As reported by Magos Brehm et al (2017) the steps involved in the generation of a CWR inventory are: (i) compilation of a national flora, (ii) matching the national flora against an existing digitized list of crop genera to obtain a list of taxa of the same genera as the list in the national flora, thereby producing the CWR checklist, (iii) prioritization of the CWR checklist to generate a realistic and manageable number of priority CWR, and (iv) annotation of the priority list of CWR with additional information for active conservation action to produce a CWR inventory (Maxted et al, 2007;Magos Brehm et al, 2017). Prioritization involves reducing the number of taxa in the CWR checklist into a number manageable for active conservation actions due to resource constraints and funding limitations. The prioritization criteria may include crop socio-economic value, CWR genetic closeness and ability to donate genes to the related crop, endemicity, occurrence, threat status and other related parameters Thormann et al, 2017). There is presently no complete CWR checklist or inventory for West Africa.
This paper aims at the generation of a regional CWR checklist for West Africa, prioritization of this CWR checklist and the compilation of a CWR inventory, using the method described by Maxted et al (2007).

Creation of a CWR checklist for West Africa
A monographic approach (for selected crop genera) was carried out in order to produce a digitized CWR checklist  for West Africa, including the following countries: Benin, Burkina Faso, Cape Verde, Cote d' Ivoire, Gambia, Ghana, Guinea, Guinea-Bissau, Liberia, Mali, Mauritania, Niger, Nigeria, Senegal, Sierra Leone and Togo. A digitized flora for families known to contain CWR taxa was compiled for 12 selected plant families. WCSP (2020) was used for the Araceae, Arecaceae, Convolvulaceae, Dioscoraceae, Euphorbiaceae, Musaceae, Poaceae and Zingiberaceae, while the regional printed flora (Huchinson and Dalziel, 1958) was used for the Malvaceae, Papilionaceae, Sterculiaceae and Caricaceae.
The following steps were involved in generating the CWR checklist: (i) Produce a digitized list of regional flora: All taxa (i.e. species, subspecies, and varieties) belonging to the selected plant families were included in the floristic checklist. Information related to the different taxa of the regional flora was entered in the CWR checklist and inventory data template v.1 , including: family, genus, species and authorities, various sub-ranks, taxon, subtaxon, taxon common name, synonyms, related crop(s) and common name of the related crop Rahman et al, 2019).
(ii) Produce a digitized list of crops: A digitized list of crop genera was produced from the following sources: (i) all crops cultivated in the world (FAO, 2021), (ii) major and minor food crops from the World Atlas of Biodiversity (Groombridge et al, 2002), and (iii) Annex 1 of the International Treaty on Plant Genetic Resources for Food and Agriculture (ITPGRFA) for both forage and food crops (FAO, 2009). The digitized list of crop genera was obtained from a published crop and crop genus list for CWR checklist and prioritization (Kell, 2016).
(iii) Match the crop genera against the floristic checklist to produce the CWR checklist: The digitized list of crops was matched against the floristic list to produce the CWR checklist . Taxa cultivated but with no wild relatives in West Africa such as cocoyam (Colocasia esculenta (L.) Schott), coconut (Cocos nucifera L.), oil palm (Elaeis guineensis Jacq) or maize (Zea mays L.) were removed. The draft CWR checklist was sent to experts and agricultural stakeholders for validation. The draft CWR checklist was approved by the experts and agricultural stakeholders for prioritization.

CWR Prioritization
Different criteria and methods have been used to prioritize CWR checklists in the past for several countries and regions of the world, depending on the country and who will fund the CWR conservation action . In this work, three criteria were applied in the prioritization of the CWR checklist for West Africa : (i) crop value in West Africa from FAOSTAT, (ii) CWR closeness to the crop from the Harlan and de Wet CWR diversity (https://www.cwrdiversity.org/che cklist/) and (https://npgsweb.ars-grin.gov/gringlobal/ taxon/taxonomysearchcwr.aspx), with CWR closeness restricted to gene pool or proven use in breeding within tertiary gene pool (GP3) (Maxted and Kell (2009), and (iii) global threat status according to IUCN (https://ww w.iucn.org/).
A parallel method was used through a point scoring process in which taxa were scored for all criteria, ranked according to their total score, and selected based on a 'cut off' score. For all criteria, taxa with a score of ≥3 were selected for prioritization. In assigning scores to criterion one (value), human food crops were scored (7 points), crops used as food additive (5), material (3), animal feed (1) and environmental use (1). Food crops (important for nutrition and food security), food additives and materials were selected for prioritization, excluding animal feed and environmental use crops. In assigning scores to the second criterion (genetic closeness), GP1 was scored (9 points), GP2 (7), GP3 (3), and CWR that lack this information [i.e. those belonging to Taxon Group 4 (TG4)] were scored (1 point). TG4 are CWR that belong to the same genus with their related crop. Applying the second criterion, CWR belonging to the primary gene pool (GP1B), secondary gene pool (GP2), and tertiary gene pool (GP3) with proven use in crop improvement were selected for prioritization . Based on the third criterion (threat status), all evaluated CWR were selected for prioritization, excluding CWR that have not been evaluated .

Results
The monographic CWR checklist for West Africa contains 1651 taxa from 379 genera. After the digitized list of crop genera was matched with the floristic list, a total of 392 CWR (and crops) resulted, belonging to 46 genera. Cultivated taxa without wild relatives in West Africa were removed, bringing the number to 379 taxa belonging to 33 genera. After applying the three criteria of the parallel method for prioritization Ng'uni et al, 2019), the CWR checklist was reduced to a total of 102 priority CWR from 18 genera with 24 sub-taxon (subspecies/varieties). The priority CWR are related to 15 crops or crop groups important for  (13), Oryza (6) and Cola (5) ( Table 1).

CWR Closeness to Related Crops
Forty-five 45 (44 %) of the taxa were selected for prioritization using the criterion of taxa belonging to gene pools GP1B, GP2 or proven use of GP3 in crop improvement. Among the CWR selected, 21 (20.58 %) belong to GP1B, 28 (27.4%) are GP2 while 53 (51.9%) belong to GP3 or Taxon Group 4 (Supplemental Table  S1). Among the 53 CWR belonging to GP3/Taxon Group 4, three (2.9%) have potential and confirmed use in crop improvement. Twenty-two (21.7%) of the CWR have confirmed use in crop improvement for crops such as wheat, rice, yam, sorghum, cassava, cowpea, millet and cotton, contributing to yield improvement, drought tolerance and resistance to several diseases in different crops as well as fibre quality in cotton ( Table 2). Out of the 14 CWR taxa in the genus Dioscorea, five have confirmed use in crop improvement against various diseases such as anthracnose, yam mosaic virus (YMV) and yam nematode. Four CWR out of six   (Figure 2). Rice (Oryza sativa L.) was the only socio-economically valuable crop that had all its CWR assessed for threat status (Huchinson and Dalziel, 1958).
Potential for aphid resistance (Badiane et al, 2014) Wheat Triticum turgidum L. Stripe rust resistance, powdery mildew resistance (Chaudhary et al, 2014) Discussion Adejuwon (2000) reported that 20 species of plants in Nigeria were extinct, 431 were endangered species while 20 were vulnerable. Urbanization, soil degradation, natural calamities, deforestation, forest fires, overgrazing and other anthropogenic activities, particularly climate change, are reducing the availability of CWR for sustainable agricultural productivity (Maxted et al, 1997;Moore et al, 2008;Mounce et al, 2017). For an effective and sustainable regional conservation strategy and its subsequent implementation, a priority CWR inventory is essential. A CWR inventory serves as a guide for a sustainable conservation action plan. The outcome of this research will form the blueprint for a systematic conservation and use strategy for West Africa. It will provide a starting point for a coordinated policy in the conservation and sustainable utilization of CWR diversity in the West African region. In this study, 379 taxa were identified as priority plant taxa, of which 122 were subsequently prioritized for urgent active conservation action. The remaining 257 plant species and their CWR could be considered for active conservation in the future as and when resources become available.
CWR of socio-economic valuable crops in West Africa have been reportedly used in crop improvement. For instance, Sood et al (2015) reported the use of E. crusgalli (P. Beauv) and E. frumentacea (Link) to increase yield quality in Barnyard millet. D. abysinica (Hochst ex Kunth) and D. praehensilis have been reported to show resistance to yam mosaic virus and anthracnose , while D. bulbifera showed resistance to yam nematode and tolerance to drought  (Table 2). Traits for drought tolerance (Hajjar and Hodgkin, 2007), yield improvement (Brar, 2004) and bacterial blight resistance (Brar, 2004;Jena, 2010) (IRRI, 2006), iron tolerance (Brar and Khush, 2002), rapid leaf canopy establishment (Jones et al, 1997) and potential for tolerance to acid soil (Brar and Khush, 2002) and heat (Atwell et al, 2014). Also, S. bicolor subsp. verticillifolia (L.) Moench has reportedly shown resistance to leaf rust (Park et al, 2015), spot blotch (Yun et al, 2006), stem rust (Fetch Jr et al, 2009) and tolerance to drought (Nevo and Chen, 2010). Resistance to white mold (Mkwaila et al, 2011), bruchids (Osborn et al, 2003), common bacterial blight and web blight (Beaver et al, 2012), and tolerance to drought have been documented in Phaseolus vulgaris var, aborigineus L. Chaudhary et al (2014) reported stripe rust and powdery mildew resistance in T. turgidum L. (Table 2). Maxted et al (2015) and Kell et al (2017) have opined that regional conservation is supplemental to national efforts as some CWR may be lacking in some countries in a region. West Africa, being a region dominated by agricultural nations, will find the implementation of the conservation plan from this inventory useful, as it will enhance the region's global relevance in agricultural productivity. As reported by Maxted et al (2008) and Engels and Thormann (2020), collaboration by neighboring nations could enhance the extensive and effective conservation of CWR genetic diversity. It is therefore the collective responsibilities of the neighboring nations where this CWR diversity is found to regionally conserve it Kell et al, 2017;Allen et al, 2019).

Conclusion
This study shows that West Africa harbours CWR diversity that can contribute significantly to sustainable agricultural development in the region. Kell et al (2015) noted that countries should widen their utilization of CWR across national boundaries and all nations are inter-independent in the quest for food security. Similar to an existing CWR inventory for the North African region (Lala et al, 2018), the CWR checklist, prioritization and prioritized inventory presented in the study will help in the development of a CWR conservation plan for West Africa. The conservation and utilization of CWR in this inventory for crop improvement has the potential to significantly reduce the over-dependence on synthetic agrochemicals and fertilizers in the region, which negatively impacts on its biodiversity and agricultural productivity. There is an urgent need to take a systematic and pragmatic approach in the conservation and sustainable utilization of CWR diversity in West Africa to ensure food security.

Supplemental data
Supplemental Table 1: Related crop and concept level of priority CWR for West Africa.
Supplemental Table 2: Endemicity and International Union for Conservation of Nature and Natural Resources (IUCN) category of priority CWR.
Supplemental Table 3: Gross production value of socioeconomically valuable crops in West Africa.

Author contributions
Nigel Maxted and Joana Magos Brehm designed the study. All authors were involved in checklist compilation and prioritization. Michael Nduche wrote the manuscript while all authors discussed the results, commented on the manuscript, read and approved the final manuscript.