Integrative taxonomy reveals a new diploid linage in the relic genus Cyclocarya (Juglandaceae)

Abstract

Integrative taxonomy provides a powerful framework for delimiting species and uncovering cryptic diversity. Applying this approach to the relict genus Cyclocarya (Juglandaceae), long considered to contain only two ploidy‐defined species ( C. paliurus and C. serrata ) in the subtropical forests of East Asia, we identified a previously overlooked diploid lineage from Yunnan (Dian) and Guangxi (Gui), China (CDG). Morphometric analyses reveal that CDG is morphologically distinct in leaflet traits, exhibiting consistently long‐lanceolate and slender leaflets. Phylogenetically, CDG forms a clade with the tetraploid C. paliurus , separate from the diploid C. serrata . Coalescent‐based species delimitation and demographic modeling support the independent evolution of CDG and C. serrata since the Early Pleistocene (~1.43 Ma), with weak gene flow between them. To further characterize the demographic history and genetic diversity of this lineage, we analyzed runs of homozygosity (ROH) and individual heterozygosity. These analyses reveal low genetic diversity and extensive homozygous tracts in CDG, consistent with long‐term demographic contraction and range isolation. Importantly, the taxonomic recognition of this lineage is based on concordant evidence from cytology, morphology, phylogenomics, and coalescent‐based species delimitation. On this basis, we describe CDG as a new species, Cyclocarya dianguiensis . With only three known populations and fewer than 20 mature individuals, this species is assessed as Critically Endangered (CR) under IUCN criteria. Our study clarifies the phylogeny and taxonomy of Cyclocarya , and highlights the urgent need to conserve evolutionarily unique relict lineages in East Asia.