Inbreeding depression and the maintenance of genetic load in Melitaea cinxia metapopulations

The effects of inbreeding on fitness and the maintenance of genetic load in metapopulations of the endangered Glanville fritillary butterfly (Melitaea cinxia) were examined in four laboratory experiments. In Finland M. cinxia occurs as a large metapopulation consisting of small local populations with fast turnover, whereas in southern France the species has a more continuous population structure. In the experiments, we compared the performance of crosses between full sibs, crosses between members of different families within populations, and crosses between individuals from different populations. These experiments were replicated using insects from two different regions, Finland and southern France, between which the frequency of natural inbreeding should differ substantially because of differing population structure. In Finnish butterflies, the rate of successful mating was lower among insects derived from small than from large natural populations, probably reflecting the effect of past inbreeding history. Mating between full sibs lowered egg hatching rate in all experiments. This reduction of egg hatching rate was more severe among French butterflies with a more continuous population structure than among Finnish butterflies with small naturally fragmented populations and with a history of repeated rounds of inbreeding in the past. This result suggests that recurrent inbreeding has led to partial purging of deleterious recessives from the Finnish metapopulation. Nonetheless, substantial genetic load still remains in this metapopulation, and we discuss possible reasons why this should be the case.