Inbreeding depression increases with maternal age in a seed-feeding beetle

Background: Interactions between inbreeding and maternal effects have received little attention, and the effect of maternal age on inbreeding depression in offspring has been almost entirely neglected. Maternal age affects allocation of resources and other materials to offspring, which can affect the fitness consequences of inbreeding. An interaction between inbreeding and maternal age thus has the potential to produce complex and long-lasting effects on population dynamics and evolutionary trajectories. Hypothesis: Inbreeding depression in offspring fitness traits increases with maternal age at reproduction. Organism: Two populations of the seed-feeding beetle, Callosobruchus maculatus (Coleoptera: Chrysomelidae: Bruchinae). Methods: We compared the effect of maternal age on egg development/hatch, larval survival, and larval egg-to-adult development time between offspring of sib-mated parents (offspring inbreeding coefficient: F = 0.25; parental F = 0) and offspring of outbred parents (F = 0 for both parents and offspring). Results: The magnitude of inbreeding depression (proportional reduction in fitness of inbred relative to outbred beetles, δ) increased with maternal age for all measured traits (the proportion of eggs that developed, egg hatch, larval hatch-to-adult survival, and larval egg-to-adult development time). The age effect was large (δ increased by as much as 8% per day of maternal age), although the majority of this increase occurred only in the oldest age classes. There was no difference between the two beetle populations in the magnitude of this inbreeding–maternal age interaction.