Dr. Christina Bauch

Telomeres

Telomeres are nucleoprotein structures at chromosome ends, consisting of evolutionarily conserved DNA sequence repeats ( (TTAGGG)n for vertebrates) and the shelterin protein complex. Functional telomeres prevent chromosome degradation and maintain genome stability. Telomeres of a critically short length have been associated with premature ageing syndromes in humans and reduced survival probability across taxa. In somatic cells telomeres shorten due to incomplete replication during cell division, which can be accelerated by DNA- and protein-damaging factors and attenuated or counteracted by maintenance processes. Telomeres generally shorten with age and to a higher extent early in life. Telomere shortening rate differs between individuals, for example in relation stress exposure, resource-based life history trade-offs (e.g. reproduction vs. self-maintenance) or environmental conditions, and it has thus been suggested to reflect somatic costs of exposure to these challenges. In jackdaws we show that telomere dynamics have only a low heritable component and differences among individuals are mainly driven by environmental effects. Telomere length on the other hand is highly heritable (e.g. shown in free-living jackdaws and to a very similar extent in common terns) and older fathers produce offspring with shorter telomeres in jackdaws (comparing within and among fathers) and in common terns. Note that in humans and chimpanzees older fathers have offspring with longer telomeres, thus, show the opposite effect, at least on the population level as it has not yet been investigated within fathers.

Measuring telomere length and dynamics

Telomere length is often measured in the blood as the sampling method is relatively low invasive, allows repeated measurements and consequently the assessment of telomere dynamics. As telomere length correlates between tissues of an organism, telomere length in the blood can be seen as an appropriate representative for the individual. A telomere length value reported in the literature is usually an average telomere length. Telomere length varies between chromosomes of the genome and between cells within samples, and each sample is therefore characterized by a telomere distribution.

jackdaws Corvus monedula

We use the terminal restriction fragment analysis (TRF) to measure telomeres. In brief, high quality DNA is isolated, digested - except for telomeric sequence - and resolved by pulsed-field gel electrophoresis. Using a protocol where DNA double-strands are not separated, the DNA single strand overhang is hybridized with labelled probes, exposed to a screen and the telomere length distribution of a sample is visualised.

Gel picture showing individual telomere length distributions in jackdaws. Only terminally located telomeric sequences are marked.

Molecular size markers on lanes 1-2 & 28-29 were added for size calibration of telomere length.

Telomere length distribution within the genome in different bird species

great tits Parus major

common terns Sterna hirundo

cory's shearwaters Calonectris borealis

Magnitude and distribution of telomere lengths in the genome differ significantly between species, resulting in species-specific patterns.

Great tits show ultra-long, so-called class III telomeres, that are visible as banding patterns. The distribution of shorter class II telomeres is visible as a smear (lower half of the picture).

Common terns show class II telomeres and in addition some class III telomeres.

Cory's shearwaters show a similar distribution pattern as jackdaws without detectable ultralong telomeres.

Note that the gel electrophoresis settings are adjusted to the species and therefore the gel pictures are not 1:1 for the different species, but compare the size ladder in the picture detail.

Longer telomeres within a genome lose more base pairs with age and are better predictors of survival and other fitness components than shorter telomeres.

Publication on the topic:

Within the genome, long telomeres are more informative than short telomeres with respect to fitness components in a long‐lived seabird