1. INTRODUCTION
Defining how and where plant varieties will adequately respond to environmental variations is a central topic in plant science research. This is more preoccupying for orphan crops that are largely grown in marginal areas and neglected in the mainstream research agenda (Mabhaudhi et al. 2019). Availability of the genetic resources of those crops is still a challenge for many genebanks at national, regional, and international levels. This situation jeopardizes the sustainable utilization of the plant genetic diversity that can be useful for current and future food systems and secured nutrition (Govindarajet al. 2015; Litrico & Violle 2015; Singh et al. 2014). Such genetic resources are important for successful cultivars development and selection of economic and agronomic traits and could confer resilience to evolving climate.
In Kersting’s groundnut [Macrotyloma geocarpum (Harms) Maréchal and Baudet], a multipurpose staple orphan crop with high nutritional and economic values for smallholder farmers in West Africa (Adu-Gyamfi et al. 2011; Ajayi & Oyetayo 2009; Akohoué et al. 2019; Assogba et al. 2015; Obasi & Agbatse 2015), the need to solve the ecological suitability of the extant genetic resources arose despite the significant achievements made recently on the germplasm collection, conservation and crop selection. Kersting’s groundnut plays an important role in farming sustainability through its ability to fix atmospheric nitrogen in soil and enhance soil fertility (Mohammed et al. 2018). Furthermore, It serves in traditional medicine for local populations (Adu-Gyamfi et al. 2011; Assogba et al. 2015; Tamini 1995). However, the production of Kersting’s groundnut is declining rapidly and the genetic resources were rarely collected and safeguarded for future generation. In addition, environmental stresses are among the main causes for declining Kersting’s groundnut production from its cultivated areas (Akohoué et al. 2019; Coulibaly et al. 2020). Though Kersting’s groundnut has relatively good adaptation to low-input conditions (Achigan-Dako & Vodouhe 2006; Mergeai 1993), increased frequency of drought, intense precipitations, elevated temperatures, and increased salt and heavy metals in soils will often be accompanied by increased infestation by pests, and pathogens, are expected to limit the plant growth and productivity, and consequently the crop’s yield and production (Long et al. 2015).
Recent studies revealed a low variation within the species (Akohoue et al. 2020; Mohammed et al. 2018) that limits the extent of its genetic diversity and cultivated zones. Kersting’s groundnut counts six landraces set mostly within three agroclimatic zones; Northern-Guinean (NG), Northern-Sudanian (NS) and Southern-Sudanian (SS) of Benin, Burkina Faso, Ghana and Togo with the predominance of genetic resources and diversity in Southern-Sudanian zone (Akohoué et al. 2019; Coulibaly et al. 2020). Overall, the area of cultivation and adaptation of landraces differ among agroclimatic regions. The Black landrace was largely collected in the Northern-Sudanian environmental conditions and was widely preferred, cultivated and maintained by farmers (Coulibaly et al. 2020). The White landrace was widely grown in the Northern-Guinean transition zone of Benin (Akohoué et al. 2019; Assogba et al. 2015) while less cultivated in Burkina Faso (Coulibalyet al. 2020) and absent in other countries of West-Africa. The production of the Brown landrace was specifically limited to Ghana farming system (Coulibaly et al.2020).
Kersting’s groundnut landraces are the direct results of farmer selection, cultivation and maintenance over the centuries. This continual adaptation of the crop to smallholders farming conditions could continue to play a role in adapting production to climate change. Also, local adaptation of landraces could vary in their climatic response and requirement and therefore, may spread differentially under evolving environmental conditions (Schierenbeck 2017). To find the adequate referendum where the species can thrive, it has become crucial to approximate the potential distribution of the crop and its genetic resources.
Unfortunately, with the rapid evolution in climate conditions and the further introduction and adoption of new cash crops with high economic importance, local seed systems alone will likely be insufficient to ensure the endurance of the crop genetic resources and diversity. In these conditions, applying ecological research is required to inform conservation and management decisions in order to mitigate a species genetic erosion (Araújo et al.2005), as Kersting’s groundnut at National and Regional levels. Ecological niche modeling (ENM) can identify the environmental parameters that can impact a species’ distribution and project its potential distribution area onto new environmental surfaces to examine the effect of present or future environmental change (Araujo & Peterson 2012; Martínez-Meyer 2005).
Several statistical and mechanistic techniques proved effective in quantifying niches and spatial distribution of natural and cultivated species (Blonder 2018; Elith et al. 2006; Pironon et al. 2019; Ramirez-Cabral et al. 2016; Syfert et al. 2016). The basic modelling framework of species distribution models (SDMs) in general has been criticized on a number of gaps, such as ignoring heterogeneity in population and genetic structure in different parts of a species geographical range (Hampe & Petit 2005). However, many species are organized into differentiated genetic lineages across their geographical ranges (Hereford 2009; Leimu & Fischer 2008) and populations differ in their adaptive potential to respond to environmental change (Shaw & Etterson 2012). Studies proved that incorporating molecular data into SDMs represents an important step forward for modelling the effects of climate change on species geographical ranges (Alvarado-Serrano & Knowles 2014; Gotelli & Stanton-Geddes 2015; Ikeda et al. 2016).
In the case of Kersting’s groundnut (KG), much uncertainty remains concerning the ability of the crop to withstand the changing climate, suggesting that there is a clear need to comprehensively analyze the response of the crop diversity under new environmental conditions of the coming decades.
The present study was undertaken to predict the potential distribution of KG under present and future climate change scenarios. Recent molecular studies involving 281 individuals from Benin and Togo identified two major genetic clusters of KG and these two groups were distributed across Southern-Sudanian and Northern-Guinean agroclimatic zones (Akohoue et al. 2020). Kafoutchoni et al. (2021) also assessed the genetic structure of the species through GBS approach and Discriminant analysis of principal components (DAPC) and found eight genetically distinct groups from five origins. In this context, the following questions are of high interest: does the agroclimatic niches of KG vary with climate changes? Would KG genetic groups differ in their ability to respond to present and future climatic scenarios? This study examines the response of orphan crops to future climates by using genetic information and ecological niche modeling approach (gENMs) using KG as an example. Therefore, we combined KG population genomics data with ecological niche modeling: 1) to analyze the relationship between climate factors and species populations distribution in agroclimatic zones of Burkina Faso, Benin, Ghana, and Togo, and 2) to predict and examine areas that would be suitable for the species and genetic populations under the future scenarios. We hypothesized that: i) the distribution of KG remains stable under future climates, and ii) genetically distinct populations of orphan crops would respond differently to climate change.