Air yam is a climbing plant cultivated for food in parts of Africa and Asia, used in traditional medicine, and grown ornamentally. Its extensive geographic distribution is accompanied by considerable variation in appearance and chromosome number. Earlier molecular studies have revealed genetic diversity and geographic structure, but most focused on germplasm characterization or breeding rather than classification. Historically, var. kacheo and f. domestica were recognized largely through visible traits. Whether those differences reflect distinct evolutionary histories remained unclear, particularly because chloroplast DNA alone cannot fully represent the history of plants shaped by hybridization and chromosome multiplication.
A study (DOI: 10.48130/tp-0026-0037) published in Tropical Plants on 23 October 2026 by Min Chen's & Jia-Yu Xue's team, Nanjing Agricultural University, reports that f. domestica shows stronger genomic differentiation than var. kacheo, challenging how their existing taxonomic ranks reflect evolutionary divergence.
Researchers compared leaves, stems, bulbils, and underground tubers across the three groups. They counted chromosomes in root-tip cells and used flow cytometry to measure relative nuclear DNA content. Sequencing covered 17 accessions: 13 of var. bulbifera and two each of var. kacheo and f. domestica. The team assembled complete chloroplast genomes, reconstructed evolutionary trees using chloroplast sequences and nuclear single-nucleotide polymorphisms, and assessed population structure, genetic differentiation, and allele sharing. Morphological comparisons distinguished var. kacheo by its ridged stems with membranous wings. Forma domestica had broadly heart-shaped leaves, irregularly ridged bulbils, and scarcely developed underground tubers. Chromosome counts revealed 40, 60, or 80 chromosomes among representative var. bulbifera accessions, compared with 80 in the examined var. kacheo accession and 60 in f. domestica. Flow cytometry supported these differences through corresponding changes in DNA content. The 17 chloroplast genomes were highly conserved in organization, measuring approximately 153,000 DNA bases. Nevertheless, their sequence differences consistently placed f. domestica in a distinct, early-diverging lineage, a result also recovered from nuclear DNA. For var. kacheo, the two genomic histories disagreed: chloroplast analysis placed its accessions among var. bulbifera, whereas nuclear analysis grouped them into a separate lineage. Population analyses reinforced the unequal differentiation. Genetic differentiation from var. bulbifera, measured by FST, reached 0.2801 for f. domestica but only 0.0752 for var. kacheo. Tests also detected excess allele sharing between the two focal groups, consistent with possible historical genetic exchange, although its direction and timing remain unresolved. Together, these results show why distinctive plant structures cannot, by themselves, establish the depth of evolutionary separation.
The study supports treating f. domestica as a stronger candidate for varietal recognition, while the evidence for var. kacheo more closely resembles that expected for a morphological form. These comparisons remain working hypotheses rather than formal name changes. Broader geographic sampling and analyses that better account for chromosome-copy differences are needed before revising the classification. By showing where appearance, chromosomes, and genomic histories agree—and where they conflict—the research offers a practical framework for investigating boundaries within air yam and other plants with variable chromosome numbers.
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References
DOI
10.48130/tp-0026-0037
Original Source URL
https://doi.org/10.48130/tp-0026-0037
Funding information
The authors declare financial support was received for the research, authorship, and/or publication of this article. This research was funded by the National Natural Science Foundation of China (32200194 and 32671990).
About Tropical Plants
Tropical Plants (e-ISSN 2833-9851) is the official journal of Hainan University and published by Maximum Academic Press. Tropical Plants undergoes rigorous peer review and is published in open-access format to enable swift dissemination of research findings, facilitate exchange of academic knowledge and encourage academic discourse on innovative technologies and issues emerging in tropical plant research.