Introgressive hybridization between the allotetraploid Coffea arabica and one of its diploid ancestors, Coffea canephora, in an exceptional sympatric zone in New Caledonia

The importance of introgressive hybridization in plant evolution has long been recognized. Nevertheless, information on gene flow between allopolyploids and their diploid relatives is very limited, even though gene flow could play a major role in polyploid establishment and evolution. Here, we investigated the processes governing hybrid formation and introgression between the allotetraploid Coffea arabica and one of its ancestral diploid progenitors, C. canephora, in a sympatric zone of New Caledonia. The occurrence of a large assortment of hybridization events between the 2 coffee species is clearly established. First-generation hybrids (FI) and post-F-1 hybrids were characterized. The involvement of unreduced gametes of C. canephora is suggested, because tetraploid F-1 hybrid plants were detected. Moreover, although bidirectional mating was observed, only unidirectional gene flow from C canephora to C. arabica was noted in post-F-1 hybrids. Most of the collected post-F1 hybrid plants exhibited a high level of introgression, and the frequency of introgression observed among the different analyzed loci was homogeneous, suggesting no significant counterselection against introgressions from C canephora. Overall, the New Caledonian central mountains appear to be a highly favourable environment for introgressive hybridization and a genetic diversity center for C. arabica.