New insights on spatial genetic structure and diversity of Coffea canephora (Rubiaceae) in Upper Guinea based on old herbaria

Backgrounds and aims - Previous studies showed that robusta coffee (Coffea canephora Pierre ex A.Froehner), one of the two cultivated coffee species worldwide, can be classified in two genetic groups: the Guinean group originating in Upper Guinea and the Congolese group in Lower Guinea and Congolia. Although C. canephora of the Guinean group is an important resource for genetic improvement of robusta coffee, its germplasm is under-represented in ex situ gene banks and its genetic diversity and population structure have not yet been investigated. Methods - To overcome the limitations of living collections, we explored old herbarium specimens collected in Guinea and Cote d'Ivoire and conserved at the Museum National d'Histoire Naturelle, Paris. First, we reviewed the history of collection missions in both countries and how the C. canephora herbaria from the Museum were assembled. Then, using 23 nuclear microsatellite markers, factorial and model-based Bayesian analyses, we investigated the genetic diversity of 126 specimens and 36 controls, analysed their distribution among the Congolese and Guinean groups, and estimated admixture proportions for each individual. Key results - For the first time, we detected population genetic structure within the Guinean group of C. canephora. The Guinean genotypes can be assigned to five sub-groups with distinct geographic distribution especially in Guinea where two sub-groups (Maclaudii and Game) are characterized by a low level of admixture due to geographical isolation. Conclusions - We showed how combining a literature review and genetic data from old herbarium specimens can shed light on previous observations made by botanists and guide further actions to better preserve native coffee plants in forest remnants of West Africa.