Chromosome end maintenance is a fundamental phenomenon for all eukaryotes, sustaining telomeres as central structures in the process guarding the length of chromosome ends and concurrently being involved in maintaining stability and integrity of the whole genome. In the system of chromosome end maintenance, telomere function seems to be universal and essential and, as it is well documented in mammals, its impairment might be reflected by a wide range of cellular and physiological consequences. It, therefore, might evoke an idea of more or less unified telomere structure and maintenance in eukaryotes, possibly with a few exceptions. Indeed, telomere DNA sequence is seen to be greatly conserved for large taxonomic groups, as illustrated by vertebrates with a unified TTAGGG repeat, and telomerase has been found as a telomere maintenance mechanism in most tested eukaryotes. However, more detailed insight reveals how telomere structure and chromosome end maintenance might be in fact very different and flexible.
Although telomerase activity is known as the most common mechanism of telomere elongation, it is questionable how common this system really is in the whole scope of eukaryotes. Retroelement targeting to chromosome ends has been mentioned as a hallmark of telomere maintenance in Drosophila species, serving as the exemplary alternative to the common telomerase-based mechanism, albeit not acting as a universal system in the whole Drosophila genus and not alone in the insect order Diptera, where many species seem to depend on terminal gene conversion, rather than retroelement transposition. On the other hand, several species from distantly related eukaryotic taxa reveal classes of retrotransposable elements preferentially inserted into telomerase-added repeats at chromosome ends. In addition, rapidly evolving canonical telomere sequences in yeast reveal some plasticity in telomerase.
These findings document how chromosome end maintenance differs and might change and embrace new features in order to maintain telomere integrity and functionality. As different telomere systems have evolved, chromosome ends may have recruited various accessory factors that had been active in processes of DNA and RNA synthesis or repair. Alternatively, telomere-associated proteins might have adopted additional functions, for instance, to protect telomeres against genotoxic insults by, for example, oxygen free radicals. Thus, it cannot be a surprise that some telomeric proteins, including telomerase subunits, share functions with telomere-unrelated processes, such as DNA replication and damage response, response to oxidative stress, or even some cytoplasmic processes.
Realizing all this telomere flexibility and versatility it is tempting to ask about a role of telomeres in adaptive potential of organism to novel or stressful environments or a shift in physiological conditions. Alternatively, we can ask how environmental and physiological factors affect telomere biology.
Chromosome end maintenance is a fundamental phenomenon for all eukaryotes, sustaining telomeres as central structures in the process guarding the length of chromosome ends and concurrently being involved in maintaining stability and integrity of the whole genome. In the system of chromosome end maintenance, telomere function seems to be universal and essential and, as it is well documented in mammals, its impairment might be reflected by a wide range of cellular and physiological consequences. It, therefore, might evoke an idea of more or less unified telomere structure and maintenance in eukaryotes, possibly with a few exceptions. Indeed, telomere DNA sequence is seen to be greatly conserved for large taxonomic groups, as illustrated by vertebrates with a unified TTAGGG repeat, and telomerase has been found as a telomere maintenance mechanism in most tested eukaryotes. However, more detailed insight reveals how telomere structure and chromosome end maintenance might be in fact very different and flexible.
Although telomerase activity is known as the most common mechanism of telomere elongation, it is questionable how common this system really is in the whole scope of eukaryotes. Retroelement targeting to chromosome ends has been mentioned as a hallmark of telomere maintenance in Drosophila species, serving as the exemplary alternative to the common telomerase-based mechanism, albeit not acting as a universal system in the whole Drosophila genus and not alone in the insect order Diptera, where many species seem to depend on terminal gene conversion, rather than retroelement transposition. On the other hand, several species from distantly related eukaryotic taxa reveal classes of retrotransposable elements preferentially inserted into telomerase-added repeats at chromosome ends. In addition, rapidly evolving canonical telomere sequences in yeast reveal some plasticity in telomerase.
These findings document how chromosome end maintenance differs and might change and embrace new features in order to maintain telomere integrity and functionality. As different telomere systems have evolved, chromosome ends may have recruited various accessory factors that had been active in processes of DNA and RNA synthesis or repair. Alternatively, telomere-associated proteins might have adopted additional functions, for instance, to protect telomeres against genotoxic insults by, for example, oxygen free radicals. Thus, it cannot be a surprise that some telomeric proteins, including telomerase subunits, share functions with telomere-unrelated processes, such as DNA replication and damage response, response to oxidative stress, or even some cytoplasmic processes.
Realizing all this telomere flexibility and versatility it is tempting to ask about a role of telomeres in adaptive potential of organism to novel or stressful environments or a shift in physiological conditions. Alternatively, we can ask how environmental and physiological factors affect telomere biology.
