The origin of cultivated mangosteen (Garcinia mangostana L. var. mangostana): Critical assessments and an evolutionary-ecological perspective.

Mangosteen (Garcinia mangostana var. mangostana) is a popular tropical fruit, yet many aspects of its biology and evolutionary history are little known. Its origin remains contentious, although recent findings suggest G. mangostana L. var. malaccensis (Hook. f.) Nazre (synonym: G. malaccensis Hook. f.) as the sole progenitor. We review hypotheses on the origin of mangosteen and clarify points that have been affected by errors of fact and interpretation. The narrow focus and lack of detail in published results make their interpretation difficult. When possible, we support our interpretations with field observations and examination of herbarium specimens. We outline the main biological traits (e.g., dioecy, facultative apomixis, and polyploidy) of mangosteen and its wild relatives to infer traits that might have evolved during domestication of mangosteen. We find no clear indication that apomixis and polyploidy evolved during domestication. Polyploidy is known in the wild relatives, but apomixis has not yet been demonstrated. Also, we propose a testable new evolutionary-ecological framework that we call "Forest-Dusun Interface" to infer processes in the origin of mangosteen. Dusun (Malay) refers to subsistence orchards in this context. Lastly, we propose future studies to address identified knowledge gaps.

New microsatellite markers for Dacryodes edulis (Burseraceae), an indigenous fruit tree species from Central Africa.

Microsatellites were designed and characterized in the African fruit tree species Dacryodes edulis (Burseraceae). The fruits are commercialized throughout Central Africa and the species is present in forested environments as well as cultivated systems. The high variability of these markers makes them suitable to investigate the structure of genetic diversity in this important food tree species from Central Africa. From a genomic library obtained by next-generation sequencing, 21 new polymorphic microsatellite loci were developed. Tested on 95 individuals from four populations coming from three countries of the Congo Basin, the microsatellites displayed two to 20 alleles (mean 7.5; expected heterozygosity 0.003 to 0.937, mean 0.666). The transferability of microsatellites was effective for four other Dacryodes species (D. buettneri, D. igaganga, D. osika, D. pubescens). This set of newly developed microsatellite markers will be useful for assessing the genetic diversity and differentiation as well as gene flow patterns of D. edulis in tropical forests from Central Africa.

Relationships between population density, fine-scale genetic structure, mating system and pollen dispersal in a timber tree from African rainforests.

Owing to the reduction of population density and/or the environmental changes it induces, selective logging could affect the demography, reproductive biology and evolutionary potential of forest trees. This is particularly relevant in tropical forests where natural population densities can be low and isolated trees may be subject to outcross pollen limitation and/or produce low-quality selfed seeds that exhibit inbreeding depression. Comparing reproductive biology processes and genetic diversity of populations at different densities can provide indirect evidence of the potential impacts of logging. Here, we analysed patterns of genetic diversity, mating system and gene flow in three Central African populations of the self-compatible legume timber species Erythrophleum suaveolens with contrasting densities (0.11, 0.68 and 1.72 adults per ha). The comparison of inbreeding levels among cohorts suggests that selfing is detrimental as inbred individuals are eliminated between seedling and adult stages. Levels of genetic diversity, selfing rates (∼16%) and patterns of spatial genetic structure (Sp ∼0.006) were similar in all three populations. However, the extent of gene dispersal differed markedly among populations: the average distance of pollen dispersal increased with decreasing density (from 200 m in the high-density population to 1000 m in the low-density one). Overall, our results suggest that the reproductive biology and genetic diversity of the species are not affected by current logging practices. However, further investigations need to be conducted in low-density populations to evaluate (1) whether pollen limitation may reduce seed production and (2) the regeneration potential of the species.

Congruent phylogeographical patterns of eight tree species in Atlantic Central Africa provide insights into the past dynamics of forest cover.

Cycles of Quaternary climatic change are assumed to be major drivers of African rainforest dynamics and evolution. However, most hypotheses on past vegetation dynamics relied on palaeobotanical records, an approach lacking spatial resolution, and on current patterns of species diversity and endemism, an approach confounding history and environmental determinism. In this context, a comparative phylogeographical study of rainforest species represents a complementary approach because Pleistocene climatic fluctuations may have left interpretable signatures in the patterns of genetic diversity within species. Using 1274 plastid DNA sequences from eight tree species (Afrostyrax kamerunensis, A. lepidophyllus, Erythrophleum suaveolens, Greenwayodendron suaveolens, Milicia excelsa, Santiria trimera, Scorodophloeus zenkeri and Symphonia globulifera) sampled in 50 populations of Atlantic Central Africa (ACA), we averaged divergence across species to produce the first map of the region synthesizing genetic distinctiveness and standardized divergence within and among localities. Significant congruence in divergence was detected mostly among five of the eight species and was stronger in the northern ACA. This pattern is compatible with a scenario of past forest fragmentation and recolonization whereby forests from eastern Cameroon and northeastern Gabon would have been more affected by past climatic change than those of western Cameroon (where one or more refugia would have occurred). By contrast, southern ACA (Gabon) displayed low congruence among species that may reflect less drastic past forest fragmentation or a more complex history of vegetation changes. Finally, we also highlight the potential impact of current environmental barriers on spatial genetic structures.

Speciation slowing down in widespread and long-living tree taxa: insights from the tropical timber tree genus Milicia (Moraceae).

The long generation time and large effective size of widespread forest tree species can result in slow evolutionary rate and incomplete lineage sorting, complicating species delimitation. We addressed this issue with the African timber tree genus Milicia that comprises two morphologically similar and often confounded species: M. excelsa, widespread from West to East Africa, and M. regia, endemic to West Africa. We combined information from nuclear microsatellites (nSSRs), nuclear and plastid DNA sequences, and morphological systematics to identify significant evolutionary units and infer their evolutionary and biogeographical history. We detected five geographically coherent genetic clusters using nSSRs and three levels of genetic differentiation. First, one West African cluster matched perfectly with the morphospecies M. regia that formed a monophyletic clade at both DNA sequences. Second, a West African M. excelsa cluster formed a monophyletic group at plastid DNA and was more related to M. regia than to Central African M. excelsa, but shared many haplotypes with the latter at nuclear DNA. Third, three Central African clusters appeared little differentiated and shared most of their haplotypes. Although gene tree paraphyly could suggest a single species in Milicia following the phylogenetic species concept, the existence of mutual haplotypic exclusivity and nonadmixed genetic clusters in the contact area of the two taxa indicate strong reproductive isolation and, thus, two species following the biological species concept. Molecular dating of the first divergence events showed that speciation in Milicia is ancient (Tertiary), indicating that long-living tree taxa exhibiting genetic speciation may remain similar morphologically.

CpDNA-based species identification and phylogeography: application to African tropical tree species.

Despite the importance of the African tropical rainforests as a hotspot of biodiversity, their history and the processes that have structured their biodiversity are understood poorly. With respect to past demographic processes, new insights can be gained through characterizing the distribution of genetic diversity. However, few studies of this type have been conducted in Central Africa, where the identification of species in the field can be difficult. We examine here the distribution of chloroplast DNA (cpDNA) diversity in Lower Guinea in two tree species that are difficult to distinguish, Erythrophleum ivorense and Erythrophleum suaveolens (Fabaceae). By using a blind-sampling approach and comparing molecular and morphological markers, we first identified retrospectively all sampled individuals and determined the limits of the distribution of each species. We then performed a phylogeographic study using the same genetic data set. The two species displayed essentially parapatric distributions that were correlated well with the rainfall gradient, which indicated different ecological requirements. In addition, a phylogeographic structure was found for E. suaveolens and, for both species, substantially higher levels of diversity and allelic endemism were observed in the south (Gabon) than in the north (Cameroon) of the Lower Guinea region. This finding indicated different histories of population demographics for the two species, which might reflect different responses to Quaternary climate changes. We suggest that a recent period of forest perturbation, which might have been caused by humans, favoured the spread of these two species and that their poor recruitment at present results from natural succession in their forest formations.