Roth, O. et al. PNAS 117, 9431–9439 (2020)

Pregnancy puts a particular strain on the body, as the gestating animal’s immune system must come to tolerate the foreign DNA in its developing offspring. Much work has been done in mammalian mothers, but pregnancy has independently evolved in over 150 vertebrate lineages. These include seahorses and pipefish, in which many fathers take a turn at being pregnant.

New research looks at how these dads do it, finding evidence in the genomes of 12 different syngnathid species that paternal pregnancy is facilitated by a loss of major histocompatibility complex II. This collection of genes is an important component of the vertebrate adaptive immune system, and its loss to support pregnancy is despite the microbe-filled environment in which seahorses and pipefish live. The results suggest the vertebrate immune system may be more flexible than previously thought.