Sperm as arbiters of environmentally induced paternal effects in a livebearing fish

Until recently, paternal effects – the influence of fathers on their offspring due to environmental factors rather than genes – were largely discarded or assumed to be confined to species exhibiting paternal care. It is now recognised that paternal effects can be transmitted through the ejaculate, but unambiguous evidence for them is scarce because it is difficult to isolate effects operating via changes to the ejaculate from maternal effects driven by female mate assessment. Here we use artificial insemination to disentangle mate assessment from fertilisation in guppies, and show that paternal effects can be transmitted to offspring exclusively via ejaculates. We show that males fed reduced diets produce poor quality sperm and that offspring sired by such males (via artificial insemination) exhibit reduced body size at birth. These findings may have important implications for the many mating systems in which environmentally induced changes in ejaculate quality have been reported.