Describing parentage and relatedness patterns in the horned passalus (Odontotaenius disjunctus) through genotyping-by-sequencing

In cooperative breeders, genetic monogamy increases the indirect benefits gained by older offspring that delay dispersal and help rear younger siblings. To assess the magnitude of these indirect benefits, the genetic mating system of the socially monogamous bess beetle (Odontotaenius disjunctus) was investigated for the first time using genotyping-by-sequencing. Bess beetles live and feed in decaying logs, and helpers may gain additional direct benefits from prolonged family formation by forgoing dispersal and breeding within their natal log. To determine if beetles exhibit this extreme philopatry, relatedness between breeding adults within and between logs was compared. Extra-pair paternity was high in this population, with an average of 43% of offspring in a brood sired by extra-pair males, resulting in an average sibling relatedness (r) of 0.23. Within-log breeder relatedness was not higher than between-log breeder relatedness, suggesting that individuals disperse before breeding. Despite the overall low observed relatedness between siblings, helping might still be favored if helping carries few costs, as might be the case if resource processing, a byproduct of feeding, is a major benefit of family-formation for bess beetles. In addition, more heterozygous males that were more genetically dissimilar to their mates were cuckolded less, suggesting that the kin-selected benefits of genetic monogamy and limited dispersal may be outweighed by the costs of inbreeding, driving both promiscuous mating and adult dispersal.