Euphytica 132: 185–190, 2003. © 2003 Kluwer Academic Publishers. Printed in the Netherlands. 185 Molecular cytogenetics in hop (Humulus lupulus L.) and identification of sex chromosomes by DAPI-banding G.I. Karlov 1 , T.V. Danilova 1 , C. Horlemann 2 & G. Weber 2,∗ 1 Timiryazev Agricultural Academy, Department of Biotechnology, Timiryazevskaya st. 44, 127550 Moscow, Russia; 2 Institute of Plant Breeding, Seed Science and Population Genetics, University of Hohenheim, Depart- ment of Plant Breeding and Biotechnology, D-70593 Stuttgart, Germany; ( ∗ author for correspondence, e-mail: weberg@uni-hohenheim.de) Received 1 November 2002; accepted 1 April 2003 Key words: fluorescence in situ hybridization, hop, Humulus lupulus (L.), karyotype, sex chromosomes, rDNA Summary Hop (Humulus lupulus L.) was karyotyped by Fluorescence In Situ Hybridization (FISH) with probes 18S-25S and 5S rDNA as well as DAPI banding and measurements of lengths of chromosome. Irrespective of the sex of the plants these probes revealed one signal near the telomere of chromosome 6 and two signals each on chromosomes 2 and 5. The shortest chromosome in a metaphase plate of a male plant was designated to be chromosomeY. It displayed neither signals with FISH nor DAPI bands. A middle sized metacentric chromosome with no telomeric but with an interstitial DAPI band on the short arm was designated as X. All autosomes displayed telomeric DAPI- positive bands. For the first time all mitotic hop chromosomes including the sex chromosomes were identified by methods of molecular cytogenetics. Abbreviations: DAPI – 4,6-diamino-2-phenylindole; FISH – fluorescence in situ hybridization; FITC – fluorescein isothiocyanate, IBA – Indole-3-butyric acid, SSC – standard saline citrate; LSD – least significant difference Introduction Hop (Humulus lupulus L.) is a dioecious perennial plant belonging to the family of Cannabinaceae. Its female flowers are mainly used as flavouring compon- ents in beer brewing. Male plants have no commercial value. Hop domestication in Europe was first docu- mented over 1000 years ago (Kohlmann & Kastner, 1976). Nevertheless, hop has received few attention from breeders. Furthermore, little has been published on its cytogenetics and genome (Wing, 1929; Ono, 1955; Neve, 1961; Haunold, 1991; Neve, 1991; Shep- hard et al., 2000). As a tool for breeders linkage maps of male and female plants were developed (Seefelder et al., 2000). Cultivated European hop (Humulus lupulus L.) is diploid (2n=2x=20) with 9 autosomal bivalents and two sex chromosomes (XX type in fe- male and heteromorphic X and Y type in male plants (Wing, 1929)). Cytological studies were predomin- antly restricted to the determination of chromosome numbers, a mere description of its karyotype, and the identification of sex chromosomes during meiotic metaphase 1 (Ono, 1955; Shephard et al., 2000). Fur- thermore, during mitosis the X- and Y-chromosomes have not been identified. Because of their homo- morphic structure mitotic metaphase chromosomes of hop have proved to be poorly distinguishable (Parker & Clark, 1991). Cytogenetic markers have not been identified for chromosomes of hop either (Pillay & Kenny, 1996). For chromosome identification, fluorescence in situ hybridization (FISH) with repetitive DNA probes may provide landmarks. For this purpose FISH has been successfully used in many plant species (Mukai, Endo & Gill, 1990; Mukai, Endo & Gill, 1991; Leitch & Heslop-Harrison, 1992; Linares et al., 1996; Lub- aretz et al., 1996; Kamstra et al., 1997; Galasso, Schmidt & Pignone, 2001; Sadder & Weber, 2001).