Mom and dad are not that different after all: Immune modulation as a prerequisite for the evolution of pregnancy.

Pregnancy, the post-fertilization period when embryos are incubated within the body, is a dynamic multistage process that has convergently evolved in many vertebrates. To increase independence from environmental fluctuations and protect offspring from predation, challenges had to be initially overcome. The most obvious, when considering such an intimate relationship between the parent and its semi-allogenic offspring, was the pressing need to dodge immunity-associated embryo rejection. In mammals, immunological tolerance was found to be dependent on the active modulation of the immune system. Even though supporting much of the current knowledge on vertebrate pregnancy, mammals lack extant transitional stages that could help reconstruct the evolutionary pathway of this fascinatingly complex reproduction mode. In this issue of Molecular Ecology, Parker et al. selected an untraditional model-the seahorse and pipefish family, whose species evolved male pregnancy across an almost continuous gradient of complexity, from external oviparity to internal gestation. By contrasting gene expression profiles of syngnathids with distinct brooding architectures, this study allowed for the observation of subtle evolutionary adaptations, while confirming the existence of remarkable similarities to "female" pregnancy (e.g., the evolution of male pregnancy in pouched species occurred alongside immune downregulation, and inflammation seems vital during early pregnancy stages). In a world where the debate on sex-roles takes centre stage, Parker et al.'s appeasing results hint at the fact that the strongly convergent evolution of vertebrate pregnancy was seemingly unaffected by which sex carries the burden of gestation.