An Integrative Phylogenomic Approach to Species Delimitation in the Taxonomically Challenging Precious Woods Genus Dalbergia (Fabaceae)

Abstract

The genus Dalbergia L.f. (Fabaceae) includes valuable timber species, many of which are threatened by overexploitation. Conservation and establishment of sustainable management rely on viable taxonomy and correct species identification, whether to assess species diversity, to identify population trends, or to verify timber identity declarations. Species identification of Dalbergia is particularly challenging in Madagascar due to a large number of similar species and reliance on flowers and fruits for identification, which are often absent on living specimens. We used recently collected DNA material of Malagasy Dalbergia species in combination with older herbarium specimens (including types) to ascertain whether species can be delimited using a phylogenomics approach based on target enrichment of thousands of nuclear genetic loci. All except two species could be assigned to two genetic supergroups of Malagasy taxa that show pronounced differences in flower and fruit characters. We further identified eleven well-supported clades within those supergroups, most of which are clearly associated with different eco-geographic regions of Madagascar. Each clade contains several described species, each of which can be distinguished using our target enrichment approach, but not with traditional DNA barcoding. Results further indicate that several infraspecific and synonymous taxa recognized in the most recent revision of the group are genetically, eco-geographically and morphologically distinct, and should be elevated to the rank of species. Moreover, DNA sequence results and morphology strongly support the recognition of over twenty new Malagasy Dalbergia species, the majority of which produce large trees. Our results demonstrate that the current taxonomy of Malagasy Dalbergia does not adequately account for the level and patterns of species diversity observed, and they provide insights that are highly informative for a taxonomic revision being conducted on this important genus, which is essential for the development of forensic timber identification strategies that depend on correctly identified reference collections.