Disentangling the paradox of inbreeding and telomere dynamics

Inbreeding can have both profound and subtle fitness consequences, yet the physiological and molecular mechanisms underlying inbreeding depression remain difficult to study in the wild. Telomeres are repetitive DNA sequences protecting chromosome ends, and telomere length (TL) changes are widely used biomarkers of health, ageing and fitness in animal populations. However, studies have reported inconsistent associations between TL and inbreeding, including negative, positive or null relationships. Here we synthesize evidence across eukaryotes and reveal a contrasting pattern. When comparing species or strains with assumed different histories of inbreeding owing to domestication or captive breeding, most studies inferred positive associations between TL and inbreeding. However, when comparing TL with measured individual levels of inbreeding within natural populations, including humans, most studies found negative associations between TL and inbreeding. Within both comparison types, there is very little evidence to evaluate the magnitude and reliability of the influences of domestication and inbreeding, respectively, on TL. We discuss how the longer telomeres of domesticated and laboratory organisms may reflect correlated effects of captive breeding and artificial selection rather than inbreeding. Furthermore, if inbreeding shortens telomeres in natural populations, this has implications for interpreting telomere biomarkers in evolutionary and conservation contexts. This article is part of the theme issue 'Developmental circuits in ageing'.