No evidence that life history strategies shape within-body mosaics of ageing

Tissues across the body may age at different rates, which creates within-body mosaic ageing. Life history strategies shape ageing patterns at the inter- and intra-specific level, but the impact of life history strategies on mosaic ageing patterns within individuals remains unknown. Reproductive tissues in individuals selected to prioritise reproduction may age faster than somatic tissues because of increased ‘wear and tear’. Alternatively, individuals that prioritise reproduction may specially protect reproductive tissues at the expense of somatic tissues to allow for increased reproductive function. Here, we used Japanese quail (Coturnix japonica) artificially selected for increased vs. reduced reproductive investment (egg size) to test how reproductive strategies modulate within-body variation in ageing patterns, by measuring telomere length as an ageing biomarker in blood, spleen and reproductive tissue (oviduct and testis). We did not find evidence that reproductive strategy shaped within-body mosaics of ageing: patterns of telomere length in somatic and reproductive tissues were similar in individuals selected for high and low reproductive investment, respectively, and strong positive correlations in telomere length across tissues were observed. Our results suggest that life history strategy does not affect tissue-specific ageing rate. Alternately, ‘wear and tear’ and ‘special protection’ mechanisms might act simultaneously and cancel each other out, resulting in similar telomere lengths across tissues. 

Methods
Individuals used in this study originated from a captive population of Japanese quail (Coturnix japonica) artificially selected for high (H-line) or low (L-line) maternal egg investment (two replicates each). In brief, in the first generation, females in the top and bottom 25% of egg size distribution (relative to body size) were selected for breeding for the replicated H-lines and L-lines, respectively. In subsequent generations, females within the top or bottom 50% were selected for breeding for the replicated H-line and L-line, respectively. Matings between relatives were prevented throughout the experiment. After four generations, H-line females laid eggs that were more than 1 SD bigger than egg of L-line females, while they did not differ in the number of eggs laid. So, H-line females invested overall more into reproduction. Blood, spleen and reproductive tissues were collected from females and males of this population and telomere lengths by TRF method were quantified.