The genus Aethionema is sister to the core Brassicaceae (including Arabidopsis thaliana) and thus has an important evolutionary position for comparative analyses. Aethionema arabicum (Brassicaceae) is emerging as a model to understand the evolution of various traits. We generated transcriptome data for seven Ae. arabicum genotypes across the species range including Cyprus, Iran and Turkey. Combined flow cytometry and single nucleotide polymorphism (SNP) analyses identified distinct tetraploid (Iranian) and diploid populations (Turkish/Cypriot). The Turkish and Cypriot lines had a higher genome-wide genetic diversity than the Iranian lines. However, one genomic region contained genes with a higher diversity in the Iranian than the Turkish/Cypriot lines. Sixteen percent of the genes in this region were chaperonins involved in protein folding. Additionally, an analysis of glucosinolate profiles, chemical defence compounds of the Brassicaceae, showed a difference in diversity of indolic glucosinolates between the Iranian and Turkish/Cypriot lines. We showed that different Ae. arabicum individuals have different ploidy levels depending on their location (Iranian versus Turkish/Cypriot). Moreover, these differences between the populations are also shown in their defence compounds.