Selection on morphological traits and fluctuating asymmetry by a fungal 2 parasite in the yellow dung fly

11 Evidence for selective disadvantages of large body size remains scarce in general. Previous 12 phenomenological studies of the yellow dung fly Scathophaga stercoraria have demonstrated 13 strong positive sexual and fecundity selection on male and female size. Nevertheless, the body 14 size of flies from a Swiss study population has declined by almost 10% from 1993 to 2009. 15 Given substantial heritability of body size, this negative evolutionary response of an evidently 16 positively selected trait suggests important selective factors being missed (e.g. size-selective 17 predation or parasitism). A periodic epidemic outbreak of the fungus Entomophthora 18 scatophagae allowed assessment of selection exerted by this parasite fatal to adult flies. Fungal 19 infection varied over the season from ca. 50% in the cooler and more humid spring and autumn 20 to almost 0% in summer. The probability of dying from fungal infection increased with adult 21 body size. All infected females died before laying eggs, so there was no fungus impact on 22 female fecundity beyond its impact on mortality. Large males showed the typical mating 23 advantage in the field, but this pattern of positive sexual selection was nullified by fungal 24 infection. Mean fluctuating asymmetry of paired appendages (legs, wings) did not affect the 25 viability, fecundity or mating success of yellow dung flies in the field. This study demonstrates 26 rare parasite-mediated disadvantages of large adult body size in the field. Reduced ability to 27 combat parasites such as Entomophthora may be an immunity cost of large size in dung flies, 28 although the hypothesized trade-off between fluctuating asymmetry, a presumed indicator of 29 developmental instability and environmental stress, and immunocompetence was not found 30 here. 31 32