Combining morphology and population genetic analysis uncover species delimitation in the widespread African tree genus Santiria (Burseraceae)

Tariq Stévart,Franck Kameni Monthé,Elie C Nzigou Doubindou,Olivier J Hardy,Davy U Ikabanga,Bertrand M’Batchi,Alain Souza,K Guillaume Koffi,Gilles Dauby

doi:10.11646/phytotaxa.321.2.2

Abstract

Taxonomic classification based on morphology alone can prove difficult. This is the case of the polymorphic forest tree species Santiria trimera in Africa, whose classification has remained controversial for over a century. Studies combining chloroplast and nuclear DNA sequences show the existence of several phylogenetic clades in this taxon, with some occurring in sympatry in western Central Africa suggesting the existence of at least two species. By combining genetic and morphological markers, we aim to assess the species delimitation in the Santiria species complex. Morphological trait (trunk, leaflet, flower and fruit characteristics) analysis using 223 standing individuals and 103 herbarium samples were combined with genetic analyses using 479 individuals genotyped at eight microsatellite markers. Genetic clusters were identified using Bayesian assignment in order to delimit species following the Biological Species Concept and to identify distinctive characters from morphometric analyses in retrospect. Three genetic clusters were identified and found to occur in sympatry. The type of inflorescence and the colour of unripe fruit were the most discriminant morphological traits among those genetic clusters, while many quantitative traits showed overlapping distributions between genetic clusters and explain the difficulty encountered by previous botanists to resolve the taxonomy of Santiria. The combination of genetic and morphological data suggests the presence of three species within the taxon Santiria trimera from western Central Africa. This work should guide a taxonomic revision within the genus Santiria in Africa.

Highlights

The species concept is central to biology, but it has received various definitions (Le Guyader 2002)
We identified three sympatric genetic clusters, corresponding to three species, following both the biological and the typological species concepts, with more reliable morphological diagnostic traits than those proposed by Koffi et al (2010, 2011)
Stilt roots and leaflet size:—Three morphotypes were defined by Koffi et al (2010, 2011, 2012) based on the absence of stilt roots (NSR), or the presence of stilt roots combined with small leaflets (SRsl) or with large leaflets (SRll)

Summary

Introduction

The species concept is central to biology, but it has received various definitions (Le Guyader 2002). The species concept based on typology is defined as a group of individuals that share a set of morphological traits, with particular traits that distinguish them from other species This concept is based on nomenclatural types that subsequently are associated with a set of specimens with similar morphological characteristics and bear diagnostic traits (Candolle 1813, Le Guyader 2002). This is the concept usually considered by botanists who establish species identification keys based on morphological characters. Another major concept is the biological species concept, which defines species as reproductively isolated units (Mayr 1963). Species delimitation based solely on morphological markers is challenging in highly diverse ecosystems, such as tropical forests where many related species with complex evolutionary histories can co-occur and for which good material is seldom available (Duminil et al 2006, Duminil & Di Michele 2009)

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