Alternative Lengthening of Telomeres in the Budding Yeast Naumovozyma castellii

The enzyme telomerase ensures the integrity of linear chromosomes by maintaining telomere length. As a hallmark of cancer, cell immortalization and unlimited proliferation is gained by reactivation of telomerase. However, a significant fraction of cancer cells instead uses alternative telomere lengthening mechanisms to ensure telomere function, collectively known as Alternative Lengthening of Telomeres (ALT). Although the budding yeast Naumovozyma castellii (Saccharomyces castellii) has a proficient telomerase activity, we demonstrate here that telomeres in N. castellii are efficiently maintained by a novel ALT mechanism after telomerase knockout. Remarkably, telomerase-negative cells proliferate indefinitely without any major growth crisis and display wild-type colony morphology. Moreover, ALT cells maintain linear chromosomes and preserve a wild-type DNA organization at the chromosome termini, including a short stretch of terminal telomeric sequence. Notably, ALT telomeres are elongated by the addition of ∼275 bp repeats containing a short telomeric sequence and the subtelomeric DNA located just internally (TelKO element). Although telomeres may be elongated by several TelKO repeats, no dramatic genome-wide amplification occurs, thus indicating that the repeat addition may be regulated. Intriguingly, a short interstitial telomeric sequence (ITS) functions as the initiation point for the addition of the TelKO element. This implies that N. castellii telomeres are structurally predisposed to efficiently switch to the ALT mechanism as a response to telomerase dysfunction.

Evolutionary analysis of chromosome end extension

There are substantial subtelomeric interstitial telomeric sequence (ITS) in the human genome, however the origin of these sequences is not well understood. We investigate the possibility that these ITS have arisen via a process of chromosome end extension to the telomere sequence. By analysing the relationship between subtelomeric duplication and ITS, we identify multiple ITS which were the ancestral chromosome telomeric capping sequence. Comparison of chromosome terminal sequence between 15 species reveals an ongoing evolutionary process of chromosome extension, with an average extension rate of 0.0020 bp per year per chromosome. Analysis of SNP data from 1000 genomes demonstrates reduced SNP diversity in subtelomeric regions, indicating that many terminal regions are younger than the remaining autosomal sequence.

Evolutionary analysis of chromosome end extension

There are substantial subtelomeric interstitial telomeric sequence (ITS) in the human genome, however the origin of these sequences is not well understood. We investigate the possibility that these ITS have arisen via a process of chromosome end extension to the telomere sequence. By analysing the relationship between subtelomeric duplication and ITS, we identify multiple ITS which were the ancestral chromosome telomeric capping sequence. Comparison of chromosome terminal sequence between 15 species reveals an ongoing evolutionary process of chromosome extension, with an average extension rate of 0.0020 bp per year per chromosome. Analysis of SNP data from 1000 genomes demonstrates reduced SNP diversity in subtelomeric regions, indicating that many terminal regions are younger than the remaining autosomal sequence.

Karyotypic variation of Glanidium ribeiroi Haseman, 1911 (Siluriformes, Auchenipteridae) along the Iguazu river basin

The Iguazu river is a tributary of the left margin of the Paraná river, isolated from this basin about 22 million years ago with the appearance of the Iguazu Falls. The Iguazu river is characterized by high endemism due to two factors: its rugged topography and the old isolation caused by formation of the Iguazu Falls. This study analyzed cytogenetically a population of Glanidium ribeiroi collected in a region at the final stretch of this basin, by Giemsa staining, C-banding, impregnation by silver nitrate, and FISH with probes of 5S rDNA, 18S rDNA, telomeric sequence [TTAGGG]n, and [GATA]n repeats. The diploid number was equal to 58 chromosomes. The heterochromatin was present in the terminal region of almost all chromosomes. The Ag-NORs were simple and presented interstitially on the short arm of the submetacentric pair 14, which was confirmed by FISH with 18S rDNA probe. The 5S rDNA-FISH marked only the submetacentric pair 16 on the long arm in interstitial position. The FISH with [TTAGGG]n probe presented all telomeres labeled as expected, with an absence of Interstitial Telomeric Sequence (ITS). The repetitive [GATA]n sequence was dispersed throughout the genome, with preferential location in the terminal region of all chromosomes. The data obtained are discussed herein with other species of Auchenipteridae, and other previously analyzed populations of G. ribeiroi from the Iguazu river, verifying differences among these populations, which should be mainly related to the rugged topography of this basin.