Chromosome number variation in the genus <i>Acmella</i> (Asteraceae) from Thailand

Abstract

The genus Acmella Rich. ex Pers., a member of the Asteraceae family, possesses spicy and pungentflavored capitula. These capitula are widely used as vegetables and medicinal plants, prized for their anesthetic and anti-inflammatory properties. A preliminary survey uncovered morphological variations among Acmella plants in Thailand, complicating their taxonomic classification. This study investigates variations in Acmella plants distributed across several regions in Thailand using taxonomic and cytogenetic assessments, aiming to better understand their taxonomic position, genomic relationship, and the role of polyploidization within this genus. Taxonomy and conventional cytogenetic analysis of 24 accessions of Acmella collected from eight provinces in Thailand revealed five accepted species, one variety, and three undescribed taxa. This study identified and classified them into three groups based on chromosome numbers and ploidy levels. The tetraploid group comprised A. paniculata and A. uliginosa, with 2n=52; the hexaploid group comprised A. brachyglossa, A. ciliata, A. radicans var. radicans, and A. radicans var. debilis, with 2n= 78. However, the unidentified taxa, which comprised Acmella sp.1, Acmella sp.2, and Acmella sp.3, were placed into the pentaploid group, with 2n=65, and aneuploidy of 65. The chromosome size ranged from 0.70 to 4.50 µm. The type and position of satellite chromosomes varied among taxa, with certain types of chromosome rearrangement observed, especially in A. uliginosa. Overall, our findings support the allopolyploidization mechanisms in Acmella and suggest the origin of interspecific hybrids in the undescribed taxa from Thailand.