Testing for pre- and post-copulatory inbreeding avoidance in the flour beetle Tribolium castaneum

Inbreeding depression poses a significant threat to fitness in many taxonomic groups, particularly those where close relatives are likely to come into contact. Studies have suggested that females may mate with multiple males to reduce inbreeding through post-copulatory processes of closely related males. In our mating trials, we examine potential pre- and post-copulatory inbreeding avoidance in the red flour beetle Tribolium castaneum using behavioural observations and subsequently, through the use of phenotypic mutant markers to assign parentage. As expected, mating rates were largely limited by females. Contrary to expectations, males courted and mated with related females more frequently than with unrelated females. Unrelated males sired a greater proportion of the offspring of doubly mated females, but this was explained by the higher survival of their offspring to adulthood. When the lower survival of inbred offspring was accounted for, there was no difference in the estimated fertilisation success of related and unrelated females. We show that last male sperm precedence declined over 5 weeks, suggesting that the cause of sperm precedence was primarily displacement of previously stored sperm, with evidence for a limited degree of sperm stratification within the spermatheca.