**1. Introduction**

The genus *Thymus* (Lamiaceae) consists of approximately 300 species of herbaceous perennials and sub-shrubs, distributed throughout the world and predominantly found in the Mediterranean basin [1,2]. They are widely used as spices, herbal tea, and insecticide in addition to flavor and fragrance materials. Among these, *Thymus quinquecostatus* (Bak-ri-hyang) is a scrubby subshrub and an important aromatic plant in Korea. Two varieties of *T. quinquecostatus* such as *T. quinquecostatus* Celak and *T. quinquecostatus* var. *japonica* are found in Korea [3]. In traditional systems of medicine, *T. quinquecostatus* is used for the treatment of cough, inflammation, preventing excessive intestinal gas, and diaphoresis [3–5]. Recent scientific studies reported that *T. quinquecostatus* has antioxidant, antimicrobial, insecticidal, immunological, antidiabetic, and antitumor properties [6–8]. The essential oil of *T. quinquecostatus* is extensively used in cosmetic industries for fragrance purposes. Owing to its medicinal and aromatic properties, it is also broadly used in pharmaceutical and food industries [9–11]. The most abundant components in *T. quinquecostatus* essential oil are thymol, γ-terpinene, and *p*-cymene [8].

Currently, thyme seeds are commercially available in the market. However, most thyme cultivars are not ye<sup>t</sup> chemically or genetically characterized. In particular, *T. quinquecostatus* cultivar is often mixed with other thyme cultivars during cultivation and in nature. Therefore, it is important to validate the methods for the identification of the Korean native thyme cultivar, *T. quinquecostatus*. The essential oil components were commonly used to examine variations between populations [12–14]. It was reported that the genetic constitution and environmental conditions highly influenced the yield and essential oil composition of various plant species [1]. The chemical composition of essential oils might be further altered due to cross hybridization, morphogenesis, polyploidization, extraction methods, drying conditions, stages of harvesting, etc. According to the type, the major components represent 60–95% of the total essential oil. The essential oil composition might vary with the cultivar type. In *T. vulgaris*, seven di fferent chemotypes, such as thymol, carvacrol, geraniol, linalool, thujanol-4, terpinen-4-ol, and 1,8-cineole are described [15,16]. However, the identification of the factors responsible for the chemical polymorphism registered within species is the most challenging aspect of the essential oil analysis.

In addition, the morphological similarity and anatomical features of thyme cultivars create a problem for the correct identification. The genetic variation of plants was also a ffected by evolution in both inter- and intra-species. Since the distinctness of *Thymus* species from another is always challenging to identify, several characters might need to be considered. DNA-based molecular markers were used for the successful detection of this genetic variation, in the process of evolution, gene flow, and population diversity in many plant species. In recent decades, a number of molecular techniques were used to assess genetic diversity in plants. Among them, the PCR-based random amplified polymorphic DNAs (RAPD) were used for the identification of cultivar and genetic relationships, among and within plant species. Similarity banding pattern was scored for calculation of genetic relatedness [13]. Furthermore, RAPD analysis did not need any previous knowledge regarding the target sequence on the genome of the species [15]. When compared with other molecular markers, RAPD markers can produce a high percentage of polymorphism in plants with very similar genetic characteristics. RAPD analysis has various advantages, including easiness, rapidity, requiring a small amount of genomic DNA as a template, and the possibility for detecting dissimilarity in the coding and noncoding areas of the genome [17–19]. Previously, several authors have reported the use of RAPD markers to study the genetic diversity, phylogenetic relationship, and its combination with the analysis of essential oil composition in various *Thymus* species [1,13,15,20,21]. RAPD data are also important to solve the taxonomic issues within and among plant species. Furthermore, the physiological and morphological variations, essential oil composition, ploidy level, and the relationship between the chemical and genetic evaluations of the *Thymus* species were assessed [15].

In recent times, several companies have commercialized thyme seeds. Hence, Korean native thyme, *T. quinquecostatus* (Bak-ri-hyang) is always mingled with commercial thyme species during cultivation and marketing. In addition, there are no studies on the RAPD evaluation of *T. quinquecostatus* and most studies focus on its essential oil composition. In this context, the present study aimed to evaluate the chemical and genetic variations among *T. quinquecostatus* cultivars from Korea and six commercial *T. vulgaris* cultivars, by using di fferent RAPD markers and essential oil profiles, in order to distinguish Korean native thyme, Bak-ri-hyang from commercial thyme cultivars.
