Organization of telomere sequences in birds: evidence for arrays of extreme length and for in vivo shortening

Telomeres are the specialized ends of chromosomes consisting of highly cons erved repeat (5'-TTAGGG-3')(n) sequences. Lack of information regarding the existence of an in vivo telomere clock function in birds, conflicting data regarding telomere array length in the chicken model, and the paucity of m olecular telomere information for other avian species led us to study telom ere array organization within and among 18 species and subspecies of birds. Most of the species contained between 2% and 4% telomere sequence per dipl oid genome. Arrays spanning 0.5-10 kb (Class I) and 10-40 kb (Class II) wer e observed in all of the species studied. Extremely long arrays, ranging fr om hundreds of kilobases to 1-2 Mb (Class III) were observed in all except two raptor species, the northern goshawk and American bald eagle. In chicke n, there was evidence for shortening of the Class II arrays in vivo, based on intraindividual comparisons of somatic versus germline tissues in birds of different ages; terminally differentiated erythrocyte arrays were, on av erage, 2.3 kb shorter than sperm (germline) arrays. This study provides the first evidence for the existence of telomere arrays significantly larger t han have been described for any vertebrate species to date and for developm entally programmed in vivo telomere shortening in the Aves taxa. The novel finding of megabase-sized telomere arrays may be an important feature of av ian karyotypes that contain a large number of very small genetic units, the microchromosomes. Copyright (C) 2000 S. Karger AG. Basel.