PHYSICAL MAPPING OF THE GENOME OF Oreochromis niloticus BY FLUORESCENT IN SITU HYBRIDIZATION TO METAPHASE CHROMOSOMES.

Marine Gene Probe Laboratory, Department of Biology, Dalhousie University, Halifax, Nova Scotia Canada B3H 4J1

Recent genome mapping studies employing polymorphic DNA markers have generated a linkage map covering all 22 chromosomes of the important aquacultural species, Oreochromis niloticus, with potential to locate quatitative trait loci of benefit to strain enhancement (Kocher et al. 1998). To assist and complement these ongoing linkage studies, we are physically mapping cloned repetitive DNAs and tandemly-arrayed gene sequences by fluorescent in situ hybridization to metaphase chromosomes of the cichlid fish, O. niloticus. To date, we have localized: a satellite DNA sequence (SATA) to the centromeres of all chromosomes and a second satellite sequence (SATB) that appears restricted to the centromeric region of chromosome 4; two SINE (ROn-1 and ROn-2) and one LINE (LOn-1) sequences that show dispersed, and in some instances, punctate distribution over all chromosomes; TTAGGG(n) sequences that detect the telomeres of all chromosomes and two interstitial sites in chromosome 1 that suggests end-to-end fusion of three ancestral chromosomes, a result that may have significance for determining synteny in related cichlids; and the tandemly-arrayed genes for 5S and 18S ribosomal RNA, and histone gene clusters. A brief summary of our work on single-pass sequencing of expressed sequence tags from O. niloticus will be presented.