Abstract
Evidence for selective disadvantages of large body size remains scarce in general. Previous studies of the yellow dung fly Scathophaga stercoraria have demonstrated strong positive sexual and fecundity selection on male and female size. Nevertheless, the body size of flies from a Swiss study population has declined by ~10% 1993–2009. Given substantial heritability of body size, this negative evolutionary response of an evidently positively selected trait suggests important selective factors being missed. An episodic epidemic outbreak of the fungus Entomophthora scatophagae permitted assessment of natural selection exerted by this fatal parasite. Fungal infection varied over the season from ~50% in the cooler and more humid spring and autumn to almost 0% in summer. The probability of dying from fungal infection increased with adult fly body size. Females never laid any eggs after infection, so there was no fungus effect on female fecundity beyond its impact on mortality. Large males showed their typical mating advantage in the field, but this positive sexual selection was nullified by fungal infection. Mean fluctuating asymmetry of paired appendages (legs, wings) did not affect the viability, fecundity or mating success of yellow dung flies in the field. This study documents rare parasite-mediated disadvantages of large-sized flies in the field. Reduced ability to combat parasites such as Entomophthora may be an immunity cost of large body size in dung flies, although the hypothesized trade-off between fluctuating asymmetry, a presumed indicator of developmental instability and environmental stress, and immunocompetence was not found here.
Highlights
Systematic quantification of selection has become one of the hallmarks of modern biological research so as to acquire a thorough understanding of the process of natural selection and its evolutionary consequences
An episodic epidemic outbreak of the fungus Entomophthora scatophagae permitted assessment of natural selection exerted by this fatal parasite
At our Swiss study population, the entomophagous fungal parasite Entomophthora scatophagae, which has previously been described as a specific parasite of adult yellow dung flies at several sites in Europe and North America (Hammer 1941; Steinkraus and Kramer 1988; Maitland 1994; Steenberg et al 2001), showed high and generally fatal infection rates of up to 50% during the cooler and more humid periods of the year 2002
Summary
Systematic quantification of selection has become one of the hallmarks of modern biological research so as to acquire a thorough understanding of the process of natural selection and its evolutionary consequences. Phenomenological (i.e. non-behavioural) investigations of selection are the field method of choice to understand the evolution and population biology of single species and populations in an integrative manner, and to test hypotheses about the evolution of particular traits and patterns (e.g. sexual size dimorphism: Blanckenhorn 2007). The widespread yellow dung fly Scathophaga stercoraria (Diptera: Scathophagidae) is a classic model species for studies of natural and sexual selection (Parker 1979; Borgia 1982).
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