Inbreeding depression and family variation in a social insect,Bombus terrestris(Hymenoptera: Apidae)

Abstract

In bumble bees, only a few colonies dominate reproduction at the end of a season such that successful families are likely to experience episodic inbreeding. We compared the fitness of 33Bombus terrestriscolonies resulting from 188 brother–sister matings (“inbred”) to that of 43 outbred colonies (from 183 outbred matings). Large among‐family variation appears to be a key feature that characterizes all main steps in the life cycle ofB. terrestris. Hibernation survival and colony foundation of queens varied strongly among maternal lines (P>0.001). Mating with a brother significantly reduced a queen's hibernation survival and colony foundation success by varying degrees (6% vs 23%). The effect on hibernation survival was not found when including the date of mating into the analysis: queens that mated and hibernated early in the season survived hibernation more frequently than those that mated later (P=0.001) irrespective of mating treatment. Furthermore, maternal family lines differed significantly in their response to inbreeding with respect to colony size, caloric investment into sexuals, and the Shaw‐Mohler fitness index. Some family lines even showed outbreeding depression. On average, inbred colonies were significantly smaller (coefficient of depression δ=0.20) but there was no inbreeding depression for reproductive output or cumulative fitness. Thus, as a population,B. terrestrisis likely to cope with inbreeding episodes. However, among‐family variation in the effects of inbreeding intensifies the differential representation of families in the population.