Hybridization and Asymmetric Introgression after Secondary Contact in Two Tropical African Climber Species, Haumania Danckelmaniana and Haumania Liebrechtsiana (Marantaceae)

Premise of research.Interspecific hybridization is frequently observed between closely related plant species with overlapping distribution ranges, especially in zones of secondary contact after isolation in different refugia. Although well studied in temperate regions, interspecific hybridization is little documented in tropical regions, where it may be frequent in areas with high species diversity. To bridge this gap, we investigate hybridization in two closely related climber species from the genus Haumania (Haumania danckelmaniana and Haumania liebrechtsiana) originating from Gabon/Cameroon and the Democratic Republic of Congo, respectively, and co-occurring in Northern Gabon. The aim of this study was to genetically quantify and qualify the degree of hybridization between these species in their area of distributional overlap by including for the first time morphological intermediates.Methodology.We investigated 265 individuals from nine sites (seven sites containing both species) by genotyping seven microsatellites and sequencing one chloroplastic region (trnC-petN1r). Bayesian clustering analysis and simulated genotypes allowed the assignment of sampled genotypes to one or the other species or their intermediates (first-generation hybrid or backcrosses).Pivotal results.We found 13% and 41% of intermediates on the basis of morphological versus genetic data, respectively, indicating only a partial congruence. There was an equal amount of backcrosses to both parental species (15%). However, a comparison at the local scale of microsatellites and cpDNA sequence data revealed a unidirectional introgression from H. liebrechtsiana into H. danckelmaniana. At the scale of the nine sites, genetic differentiation was much lower among H. liebrechtsiana than among H. danckelmaniana populations.Conclusions.These results are in line with the previously proposed phylogeographic scenario for the genus Haumania and the theoretical expectation of chloroplast capture when an expanding species (here H. liebrechtsiana) invades the range of a local species (here H. danckelmaniana). Because the forest flora of Central Africa has undergone repeated cycles of fragmentation and expansion following past climate changes, we expect that similar scenarios of hybridization after secondary contact may be common.