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 relation 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. (2022) 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. (2022) 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.
Syngnathids (seahorses, pipefishes, pipehorses and seadragons) belong to a diverse family of bony fish with more than 300 species, primarily recognized by its unique mode of reproduction: male pregnancy. This novelty of the syngnathid lineage long attracted attention from the scientific community and was rapidly recognised as significant to the
evolutionary radiation of the syngnathid lineage. In fact, one of the first attempts at reconstructing the phylogenetic relationships within syngnathids, long before the advent of genomics, was partially grounded on the general architecture of brooding structures (Herald, 1959). Curiously, recent efforts, backed by current methodologies (Hamilton et al., 2017; Stiller et al., 2022), still recognise the diversification of structures involved in male pregnancy as relevant for diagnosing syngnathid taxa. From an ancestral pipefish, that probably presented a rather simple brooding structure where eggs were exposed, increasingly more complex structures evolved up to the sealed seahorse brood pouch (Figure 1). This apparent gradient of complexity, easily observable in extant syngnathid species (and almost absent in mammals), renewed the interest in this family of quirky fish into as an invaluable opportunity to shed additional light on the evolution of pregnancy.