Abstract The majority of chromosomes in Oreochromis niloticus, as with most fish karyotyped to date, cannot be individually identified owing to their small size. As a first step in establishing a physical map for this impor- tant aquaculture species of tilapia we have analyzed the location of the vertebrate telomeric repeat sequence, (TTAGGG) n , in O. niloticus. Southern blot hybridization analysis and a Bal31 sensitivity assay confirm that the vertebrate telomeric repeat is indeed present at O. nilot- icus chromosomal ends with repeat tracts extending for 4–10 kb on chromosomal ends in erythrocytes. Fluores- cent in situ hybridization revealed that (TTAGGG) n is found not only at telomeres, but also at two interstitial loci on chromosome 1. These data support the hypothe- sis that chromosome 1, which is significantly larger than all the other chromosomes in the karyotype, was pro- duced by the fusion of three chromosomes and explain the overall reduction of chromosomal number from the ancestral teleost karyotype of 2n=48 to 2n=44 observed in tilapia. Introduction Telomeres are the DNA sequences and proteins found at the ends of linear eukaryotic chromosomes (McEachern et al. 2000; Blackburn 2001). The basic structure and function of the telomere has been conserved throughout evolution (Brown 1989; Cross et al. 1989; Brown et al. 1990), reflecting its important role in stabilizing chromo- somes and blocking chromosomal end-to-end fusion and degradation (Blackburn and Szostak 1984; McEachern et al. 2000). Telomeric DNA is composed of tandem arrays of a species-specific 5–8 bp GT-rich repeated sequence, termed the telomeric repeat (Blackburn 2001). The length of these arrays varies from species to species, from as little as 36 bp in Oxytricha fallax (Pluta et al. 1982) to 50–150 kb in Mus musculus (Kipling and Cooke 1990; Starling et al. 1990; Ziljmans et al. 1997). Telomeric tract lengths are longer in germline tissue than in somatic tissue. For example, in humans, the telomeric repeat arrays in sperm cells are 10–15 kb in length, while in somatic tissue such as peripheral blood lymphocytes these arrays extend for 5–10 kb (Brown 1989; Cross et al. 1989; Brown et al. 1990; de Lange et al. 1990). Replication at the telomeres is achieved by the addi- tion of telomeric repeats to the 3′ end of the chromo- somes by a specialized reverse transcriptase called telomerase (reviewed in Greider 1996). Telomerase is a ribonucleoprotein that has been found in many eukary- otes (Greider 1996; Blackburn 2001). The RNA compo- nent of telomerase includes a short sequence that is com- plementary to the DNA on the G-rich strand of the telo- mere. This sequence acts as a template for the addition of the telomeric repeat units to the 3′ end of the telomere (Blackburn 1991). Vertebrates from fish to humans share a common telo- meric repeat sequence, (TTAGGG) n . In situ hybridiza- tion analysis of two orders of the class Pisces found that the (TTAGGG) n repeat sequence was present primarily near chromosomal ends and not interstitially (Meyne et al. 1989, 1990). However, Abuín et al. (1996b) found that the nucleolus organizing regions (NORs) in rainbow trout (Oncorhyncus mykiss) hybridized strongly to a (TTAGGG) n probe. We report here on the nature and dis- tribution of telomeric repeats in Oreochromis niloticus, the Nile tilapia. Oreochromis niloticus belongs to the family Cichlidae in the order Perciformes. African cichlids have been Edited by: P. Moens J.S.K. Chew · M.J. Dobson Department of Biochemistry & Molecular Biology, Dalhousie University, Halifax, Nova Scotia, Canada B3H 4H7 J.S.K. Chew · C. Oliveira · J.M. Wright ( ✉ ) Department of Biology, Dalhousie University, Halifax, Nova Scotia, Canada B3H 4J1 e-mail: jmwright@is.dal.ca Present address: C. Oliveira, Departamento de Morfologia, Instituto de Biociências, UNESP, Botucatu, São Paulo, Brazil Chromosoma (2002) 111:45–52 DOI 10.1007/s00412-002-0187-3 ORIGINAL ARTICLE Joyce S. K. Chew · Claudio Oliveira Jonathan M. Wright · Melanie J. Dobson Molecular and cytogenetic analysis of the telomeric (TTAGGG) n repetitive sequences in the Nile tilapia, Oreochromis niloticus (Teleostei: Cichlidae) Received: 7 November 2001 / Accepted: 8 January 2002 / Published online: 19 February 2002 © Springer-Verlag 2002