Abstract
In vertebrates, maternal transfer of immunity via the eggs or the placenta provides offspring with crucial information on prevailing pathogens and parasites. Males contribute little to such trans-generational immune priming, either because they do not share the environment and parasite pressure of the offspring, or because sperm are suggested to be too small for transfer of immunity. In the teleost group of Syngnathids (pipefish, seahorses and sea dragons) males brood female eggs in a placenta-like structure. Such sex-role-reversed species provide a unique opportunity to test for adaptive plasticity in immune transfer. Here males in addition to females should influence offspring immunity. We experimentally tested paternal effects on offspring immunity by examining immune cell proliferation and immune gene expression. Maternal and paternal bacterial exposure induced the offspring's immune defence five weeks after hatching, and this effect persisted in four-month-old offspring. For several offspring immune traits double parental exposure (maternal and paternal) enhanced the response, while for another group of immune traits, the trans-generational induction already took place if only one parent was exposed. Our study shows that sex-role reversal in connection with male pregnancy opens the door for bi-parental influences on offspring immunity, and may represent one additional advantage for the evolution of male pregnancy.