Genetic variation of the effects of spontaneous mutation on size at birth

Spontaneous mutation underlies all genetic variation, and thus influences the evolutionary dynamics of complex traits. Although much work has estimated mutation rates for fitness or at molecular scales, we have comparatively little information about mutational influences on other complex phenotypes. We conducted four mutation accumulation experiments with independent clones of Daphnia pulex, and then measured the effect of spontaneous mutation on size at birth, a complex trait whose connection to fitness depends on ecologically-mediated tradeoffs. Therefore it is unclear whether mutations, which are usually neutral or deleterious, should decrease or increase size at birth. In two experiments, individual instances of increased size at birth were common, whereas in the other two experiments, instances of decreased size at birth were common. Together, our data show that genetic background is an important determinant of the consequences of mutation for complex traits, and that mutation rates and direction can vary within species.