In recent years, modern methods for species delimitation have provided biologists with an increased ability to assess diversity more accurately. However, species delimitation still remains a challenge worldwide, especially in biodiversity hotspots such as tropical and subtropical Asia. 

In a study published in Molecular Phylogenetics and Evolution, researchers from the Xishuangbanna Tropical Botanical Garden (XTBG) explored species delimitation of Engelhardia, a genus of deciduous or evergreen trees in the walnut family (Juglandaceae), in tropical and subtropical Asia. 

The researchers used an integrative method based on multiple-locus genetic data and morphological analyses to delimit seven species within Engelhardia. 

In total, 716 individuals in 71 populations were genotyped using five chloroplast regions, one nuclear DNA region (nrITS), and 11 nuclear simple sequence repeats (nSSR). Phylogenetic trees were constructed and relationships among species were assessed. Molecular analyses were then combined with 14 morphological characteristics of 720 specimens to further explore the species boundaries of Engelhardia. 

Four species (E. hainanensis, E. apoensis, E. serrata, and E. roxburghiana) retain their current taxonomic status. E. fenzelii is resurrected from E. roxburghiana, and E. spicata is expanded to become a variable species complex to include E. spicata var. aceriflora, E. spicata var. colebrookeana, and E. rigida. Finally, E. serrata var. cambodica is re-ranked as an independent species. 

"Our study highlights the importance of mutual utilization and promotion of morphological and molecular data. We also suggest that the recognition of infraspecific taxa should be done with caution in order to simplify classifications and prevent confusion," said Prof. LI Jie, principal investigator of the study.