A chemosystematic investigation on the mono- and sesquiterpenoids in the genus Origanum (Labiatae) Melpomeni Skoula*, Panagiota Gotsiou, George Naxakis, Christopher B. Johnson Department of Natural Products, Mediterranean Agronomic Institute of Chania, Alsylion Agrokepion, P.O. Box 85, 73100 Chania, Greece Received 22 January 1999; accepted 5 May 1999 Abstract We have examined the volatiles of Origanum species native to Crete together with two naturally occurring hybrids. The three groups of Ietswaart's classi®cation were all represented. Group A, section Amaracus was represented by Origanum calcaratum and by Origanum dictamnus; group B, section Chilocalyx by Origanum microphyllum; section Majorana by Origanum onites; group C, section Origanum by Origanum vulgare ssp. hirtum. Two natural hybrids of O. vulgare ssp. hirtum, with O. microphyllum and Origanum onites L., O. x minoanum and O.x intercedens were also examined. The main volatiles found were compared with all the existing published analyses for Origanum and the taxonomic signi®cance of volatile oil composition was assessed. Our own results and the existing data together show convincingly that most Origanum species are rich either in sabinyl compounds or cymyl compounds but never both. We found that the hybrid between sabinyl-rich O. microphyllum and cymyl- rich O. vulgare ssp. hirtum contains mainly cymyl compounds, the sabinyl being suppressed. We suggest that one or more components of the cymyl pathway act throughout the Origanum genus, whenever present, to suppress the sabinyl pathway, which however is never completely absent. Generally, volatile composition is in accord with Ietswaart's classi®cation. Twelve of the 13 species in group A whose essential oil composition has so far been examined were found to be cymyl-rich and to lack signi®cant amounts of acyclic compounds or sesquiterpenoids. Group B contains a similar number of cymyl-rich and sabinyl- rich species but is the only group with members rich in sabinyl compounds. Group C is the only group rich in acyclic compounds and/or sesquiterpenoids. # 1999 Elsevier Science Ltd. All rights reserved. Keywords: Origanum; Labiatae; Oregano; Marjoram; Chemotaxonomy; Terpenoids; GC-MS; Hybrids 1. Introduction Within the genus Origanum, Ietswaart (1980), based on morphological criteria, recognised 3 groups, 10 sec- tions, 38 species, 6 subspecies and 17 hybrids. The members of the genus are mainly distributed along the Mediterranean region while 75% of them are restricted to the East Mediterranean. Eleven species occur in Greece, ®ve of them are found in Crete (Greuter, Burdet & Long, 1986). Origanum species are characterised by a wide range of volatile compounds, and in the present study we examined the volatiles of all taxa native to the island of Crete, and compare the pro®les with all the other species whose volatile composition has been reported, in order to attempt to establish the taxonomic signi®- cance of volatile oil composition. We also examined the composition in two naturally occurring hybrids. The Cretan Origanum taxa represent four dierent sections of the genus (Ietswaart, 1980). Group A, sec- tion Amaracus Bentham is represented by Origanum calcaratum Juss. (Syn. Amaracus tournefortii Bentham, Origanum tournefortii Aiton) endemic to Crete and the Aegean Islands and by Origanum dictamnus L. (Syn. Amaracus dictamnus Bentham) endemic to Crete. Group B, section Chilocalyx Ietswaart is represented by Origanum microphyllum Vogel (Syn. Majorana microphylla Bentham), endemic to Crete. Group B also includes section Majorana Bentham which is rep- resented by Origanum onites L. endemic to Sicily and East Mediterranean region. Group C, section Phytochemistry 52 (1999) 649±657 0031-9422/99/$ - see front matter # 1999 Elsevier Science Ltd. All rights reserved. PII: S0031-9422(99)00268-X * Corresponding author.