Inbreeding accelerates reproductive senescence, but not survival senescence, in a precocial bird

Inbreeding depression is predicted to increase with age because natural selection is less efficient at purging deleterious alleles that are only expressed later in life. However, empirical results are scarce, and equivocal between studies. Here we performed controlled matings between related and unrelated individuals of domesticated Japanese Quail (Coturnix japonica) and monitored the performance of their offspring for all fitness components over their complete life course. We found rapid senescence in adult survival and egg-laying performance, and inbreeding depression at all life stages (reduced embryo viability, increased age at maturity, as well as reduced adult survival and reproduction). Inbreeding depression did not increase at later ages for survival, but did so for egg laying, thereby accelerating reproductive senescence. Moreover, the effect of inbreeding on egg laying persisted after correcting for lifespan, indicating that both survival and reproduction were independently affected by inbreeding. We suggest that in heterogeneous populations intra-generational purging may at earlier ages already select out the individuals that are homozygous for the specific alleles responsible for depressed survival, preventing the appearance of increased inbreeding depression in survival with age. Given that inbreeding affects reproduction independent of survival this should not apply to reproductive senescence or homogeneous populations, which may explain equivocal results between traits and studies.

Methods
All data represent raw survival and egg-laying data at various life stages over the complete lifespan of female Japanese Quail, collected from a captive population kept at the Institute of Avian Research in Wilhelmshaven, Germany.