Plastome phylogeography in two African rain forest legume trees reveals that Dahomey Gap populations originate from the Cameroon volcanic line.

Paleo-environmental data show that the distribution of African rain forests was affected by Quaternary climate changes. In particular, the Dahomey Gap (DG) - a 200 km wide savanna corridor currently separating the West African and Central African rain forest blocks and containing relict rain forest fragments - was forested during the mid-Holocene and possibly during previous interglacial periods, whereas it was dominated by open vegetation (savanna) during glacial periods. Genetic signatures of past population fragmentation and demographic changes have been found in some African forest plant species using nuclear markers, but such events appear not to have been synchronous or shared across species. To better understand the colonization history of the DG by rain forest trees through seed dispersal, the plastid genomes of two widespread African forest legume trees, Anthonotha macrophylla and Distemonanthus benthamianus, were sequenced in 47 individuals for each species, providing unprecedented phylogenetic resolution of their maternal lineages (857 and 115 SNPs, respectively). Both species exhibit distinct lineages separating three regions: 1. Upper Guinea (UG, i.e. the West African forest block), 2. the area ranging from the DG to the Cameroon volcanic line (CVL), and 3. Lower Guinea (LG, the western part of the Central African forest block) where three lineages co-occur. In both species, the DG populations (including southern Nigeria west of Cross River) exhibit much lower genetic diversity than UG and LG populations, and their plastid lineages originate from the CVL, confirming the role of the CVL as an ancient forest refuge. Despite the similar phylogeographic structures displayed by A. macrophylla and D. benthamianus, molecular dating indicates very contrasting ages of lineage divergence (UG diverged from LG since c. 7 Ma and 0.7 Ma, respectively) and DG colonization (probably following the Mid Pleistocene Transition and the Last Glacial Maximum, respectively). The stability of forest refuge areas and repeated similar forest shrinking/expanding events during successive glacial periods might explain why similar phylogeographic patterns can be generated over contrasting timescales.

Phylogenomic analyses reveal an exceptionally high number of evolutionary shifts in a florally diverse clade of African legumes.

Detarioideae is well known for its high diversity of floral traits, including flower symmetry, number of organs, and petal size and morphology. This diversity has been characterized and studied at higher taxonomic levels, but limited analyses have been performed among closely related genera with contrasting floral traits due to the lack of fully resolved phylogenetic relationships. Here, we used four representative transcriptomes to develop an exome capture (target enrichment) bait for the entire subfamily and applied it to the Anthonotha clade using a complete data set (61 specimens) representing all extant floral diversity. Our phylogenetic analyses recovered congruent topologies using ML and Bayesian methods. Anthonotha was recovered as monophyletic contrary to the remaining three genera (Englerodendron, Isomacrolobium and Pseudomacrolobium), which form a monophyletic group sister to Anthonotha. We inferred a total of 35 transitions for the seven floral traits (pertaining to flower symmetry, petals, stamens and staminodes) that we analyzed, suggesting that at least 30% of the species in this group display transitions from the ancestral condition reconstructed for the Anthonotha clade. The main transitions were towards a reduction in the number of organs (petals, stamens and staminodes). Despite the high number of transitions, our analyses indicate that the seven characters are evolving independently in these lineages. Petal morphology is the most labile floral trait with a total of seven independent transitions in number and seven independent transitions to modification in petal types. The diverse petal morphology along the dorsoventral axis of symmetry within the flower is not associated with differences at the micromorphology of petal surface, suggesting that in this group all petals within the flower might possess the same petal identity at the molecular level. Our results provide a solid evolutionary framework for further detailed analyses of the molecular basis of petal identity.

History of the fragmentation of the African rain forest in the Dahomey Gap: insight from the demographic history of Terminalia superba.

Paleo-environmental reconstructions show that the distribution of tropical African rain forests was affected by Quaternary climate changes. They suggest that the Dahomey Gap (DG)-the savanna corridor that currently separates Upper Guinean (UG, West Africa) and Lower Guinean (LG, western Central Africa) rain forest blocks-was forested during the African Humid Holocene period (from at least 9 ka till 4.5 ka), and possibly during other interglacial periods, while an open vegetation developed in the DG under drier conditions, notably during glacial maxima. Nowadays, relics of semi-deciduous forests containing UG and LG forest species are still present within the DG. We used one of these species, the pioneer tree Terminalia superba (Combretaceae), to study past forest fragmentation in the DG and its impact on infraspecific biodiversity. A Bayesian clustering analysis of 299 individuals genotyped at 14 nuclear microsatellites revealed five parapatric genetic clusters (UG, DG, and three in LG) with low to moderate genetic differentiation (Fst from 0.02 to 0.24). Approximate Bayesian Computation analyses inferred a demographic bottleneck around the penultimate glacial period in all populations. They also supported an origin of the DG population by admixture of UG and LG populations around 54,000 (27,600-161,000) years BP, thus before the Last Glacial Maximum. These results contrast with those obtained on Distemonanthus benthamianus where the DG population seems to originate from the Humid Holocene period. We discuss these differences in light of the ecology of each species. Our results challenge the simplistic view linking population fragmentation/expansion with glacial/interglacial periods in African forest species.

Development, characterization, and cross-amplification of microsatellite markers in the understudied African genus Anthonotha (Fabaceae).

PREMISE OF THE STUDY: Anthonotha macrophylla (Fabaceae) is a common tree species throughout the Guineo-Congolian forest that is sometimes confounded with other congeneric species; it is expected to be an interesting phylogeographical model to infer the history of the African dense forests. We developed 18 microsatellite markers from this species and tested their transferability in 15 congeneric species. METHODS AND RESULTS: A genomic library was obtained using the Illumina platform, and 18 polymorphic microsatellite loci were developed. The polymorphic microsatellites displayed two to 24 alleles (average: 11.9 alleles per locus, expected heterozygosity range: 0.18-0.91, mean: 0.64) in three populations of A. macrophylla from Benin, Liberia, and Cameroon. Cross-amplification in one to nine individuals of 15 congeneric Anthonotha species (A. acuminata, A. brieyi, A. cladantha, A. crassifolia, A. ferruginea, A. fragrans, A. gilletii, A. lamprophylla, A. mouandzae, A. noldeae, A. pellegrinii, A. pynaertii, A. stipulacea, A. wijmacampensis, and A. xanderi) showed successful amplification in six to 17 loci, making most of these markers useful at the generic level. CONCLUSIONS: This set of markers will be useful to study species delimitation and the genetic structure of Anthonotha species, and thus to better understand the history of tropical African rainforests.

Development and characterization of microsatellite markers in the African deciduous tree Terminalia superba (Combretaceae).

PREMISE OF THE STUDY: Microsatellites were designed and characterized in the African timber forest tree Terminalia superba (Combretaceae). Due to their high variability, these markers are suitable to investigate gene flow patterns and the structure of genetic diversity. METHODS AND RESULTS: From a genomic library obtained by next-generation sequencing, seven monomorphic and 14 polymorphic microsatellite loci were developed. The polymorphic microsatellites displayed two to 27 alleles (mean 11.4; expected heterozygosity range 0.283-0.940, mean 0.736) in one population from southeastern Cameroon. Genotypes were typical of an outbreeding diploid species, although null alleles explain a significant heterozygote deficit in three loci. Cross-amplification in three congeneric species (T. ivorensis, T. avicennioides, and T. mantaly) failed, suggesting that T. superba is rather divergent. CONCLUSIONS: This set of newly developed microsatellite markers will be useful for assessing the genetic diversity, population structure, and demographic history of T. superba in tropical African forests.