Abstract
Species delimitation remains a difficult task in many groups of organisms. Even widespread and conspicuous tropical trees make no exception. Santiria trimera, an African rainforest tree, displays substantial morphological variability. While classical analysis of historical herbarium samples lead to the recognition of a single species, two morphotypes of S. trimera are regularly collected in Gabon and Equatorial Guinea, sometimes in sympatry: one form with stilt roots (SR) and another form without stilt roots (NSR). To assess whether these forms constitute distinct taxa and to understand evolutionary processes within African Santiria, we combined a morphological and a spatial analysis of both sympatric morphotypes in northern Gabon with a phylogenetic analysis of samples from West and Central Africa, including the island of Sao Tome. In Gabon, morphological traits, chloroplast DNA (cpDNA) and nuclear DNA (nuDNA) sequences congruently lead to the recognition of two well differentiated and reproductively isolated taxa corresponding to SR and NSR morphotypes. However, rare cases of the SR morphotype bearing DNA sequences typical of NSR individuals suggest that hybridization might sometimes occur. No evidence for habitat specialization of the two taxa was found. DNA sequences from Sao Tome formed monophyletic clades at both marker types and the nuDNA gene tree suggests that the Sao Tome population probably originated from Central Africa and subsequently underwent allopatric differentiation from continental populations. In Central Africa, S. trimera is composed of at least two distinct sympatric species following the Biological Species Concept and calls for further morphological and phenological studies and experimental crosses between them to specify their taxonomic status. Our results also show that classical taxonomic species delimitation may not always be congruent with the Biological Species Concept, calling for more population-based in situ morphological and molecular genetic analyses. (Resume d'auteur)
Highlights
Species delimitation remains a difficult task in many groups of organisms (Ellis et al 2006), notably in species rich eco systems like tropical rainforests, for at least two reasons
According to observations on adult trees from north Gabon, we distinguished two forms of S. trimera based on the presence or absence of stilt roots (Hladik & Hallé 1973, Florence & Hladik 1980, Reitsma 1988) and confirmed the congruence of this character with other morphological traits
Considering quantitative traits, the Stilt Roots (SR) morphotype is characterized by smaller leaflets and drupes, shorter petioles but a higher number of leaflet pairs than the No Stilt Root (NSR) morphotype
Summary
Species delimitation remains a difficult task in many groups of organisms (Ellis et al 2006), notably in species rich eco systems like tropical rainforests, for at least two reasons. Taxonomists generally define species as a group of individuals characterized by distinctive morphological features, often the only information available. Sometimes, this leads to arbitrary choices and it does not ensure that species delimitation necessarily matches the Biological Species Concept (BSC), a group of individuals. New species were identified within the widespread African tropical trees traditionally described as Carapa procera DC. (Dauby et al 2010) In these examples, the fact that distinct morphotypes were occurring in sympatry raises the “species problem” (de Queiroz 2005): (1) what are the necessary properties of the species? Some methods based on the phylogeny of chloroplast or nuclear DNA have allowed to clarify taxonomy within complexes of plant species (Murakami et al 1998) or genera (Weeks et al 2005, Miikeda et al 2006)
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