No bull: dung-dwelling mushrooms show reproductive trait syndromes different from their non-coprophilous allies

The spores of most coprophilous mushrooms require passage through a mammalian gut. Guts and faeces constitute a chemically and microbially aggressive environment. Hence, the spores need to be armed, e.g. by melanisation and thick walls, possibly leading to large spores due to volume constraints. Conversely, litter is a less stressful substrate that may become colonised by mushrooms with less fortified spores. Compared with litter, dung pats are spatially constrained, which limits mycelial growth. Small mycelia can only produce small fruit bodies. Moreover, on quickly perishing faeces, fruiting takes place under fierce competition by microbes and dung-dwelling invertebrates. Therefore, coprophilous mushrooms are forced to mature fast, implying small fruit bodies as well. Competition in spatially less constrained litter substrates can be pronounced but should not lead to quick nutrient depletion as in dung, hence would allow for mushroom assemblages with on average larger fruit bodies. To find evidence for our assumptions, we compiled a database of fruit body and spore sizes of mushroom genera which contain coprophilous species, comprising 633 (including ca. 20% coprophilous) species across 18 genera worldwide. The data set was subjected to a phylogenetically informed statistical analysis. Our hypotheses were confirmed though the selective pressure of the faecal environment appears to be more forceful on spores considering the fact that the mean spore size differences are more pronounced than differences in mean fruit body size. It would be worthwhile to further elucidate this phenomenon and the coprophilous trait syndrome in general with molecular methods.