Intrachromosomal telomeric sequences (TTAGGG)n were analyzed in the two members of the family Giraffidae, the giraffe and the okapi. The giraffe has a diploid chromosome number of 2n = 30, whereas the okapi chromosome number varies from 2n = 46 to 2n = 45 and 2n = 44 due to a “recent” Robertsonian fusion event. The interstitial telomeres that we detected in these species are of two types: (1) In the okapi, a long interstitial telomeric element is present at the fusion site of the rob(4;26). The nature of this interstitial telomeric element suggests that it is a remnant of the telomeres of the ancestral chromosomes that participated in the fusion event. (2) In the giraffe, short stretches or degenerate telomeric sequences which are part of the satellite DNA are present at intrachromosomal sites. The results of this study provide insights into the origin of interstitial telomeric sequences in the Giraffidae.

The mechanism(s) for the origin of jumping translocations (JTs) are unknown. To assess the possible involvement of telomeric sequences in the jumping process, metaphases of a patient with hydrops fetalis having a JT were analyzed for the presence of interstitial telomeres. Telomere DNA sequences were detected at the junction sites of the donor and the recipient chromosomes. Interstitial telomeric sequences have so far only been detected in JTs involving chromosome 15q in patients with Prader-Willi syndrome. Our finding of interstitial telomeric sequences in a JT with a chromosome different from chromosome arm 15q in a patient without Prader-Willi syndrome implies that telomere sequences may be common to all telomeric JTs. The possible role of telomeric sequences as a cause of the observed chromosomal mosaicism is discussed.

All human X-linked genes known so far, except for the Xp/Yp pseudoautosomal genes, are conserved as a single linkage group on the murine X chromosome. We show that the interleukin-9 (IL-9) receptor gene (IL9R), which is located within the human Xq/Yq homology region, maps to the murine chromosome 11. The Xq/Yq pseudoautosomal region (Xq PAR) thus represents a second region on the human X chromosome which is not X linked in mice. Furthermore, we show that IL9R is absent on the Y of great apes. IL9R is thus exceptional among X/Y genes in that it is X linked in some mammals, but autosomal or pseudoautosomal in others. Genes located on the X and the Y generally escape X inactivation. An exception to this rule is SYBL1, a gene located in Xq PAR. SYBL1 is X inactivated and is inactive on the Y chromosome. In contrast, we show that IL9R expression does occur from the Y, the active and the inactive X chromosomes. This finding raises the question of how the transcriptional regulation of genes within Xq PAR occurs and how the X inactivation status of IL9R has evolved following the autosome to X and the X to X/Y translocation. The evolutionary analysis of the IL9R gene, which is located at 10 kb from the telomere, and its pseudogenes at several telomeres, also provides insight into the evolution of these loci and of subtelomeric regions in general.