A fundamental problem for the evolution of pregnancy, the most specialized form of parental
investment among vertebrates, is the rejection of the nonself-embryo. Mammals achieve
immunological tolerance by down-regulating both major histocompatibility complex
pathways (MHC I and II). Although pregnancy has evolved multiple times independently
among vertebrates, knowledge of associated immune system adjustments is restricted to
mammals. All of them (except monotremata) display full internal pregnancy, making …

A fundamental problem for pregnancy to evolve nant species were identified in both genera
of the syngnathids: The is inadvertent rejection of the embryo when being recognized as
pipefishes (Syngnathus) displayed loss of several genes of the MHC foreign tissue by the
vertebrate's adaptive immune system.[...] Morphology of brood pouches of subfamily
Nerophinae (A), and of the genera Syngnathus (B), and Hippocampus (C) and display of the
placenta-like structures in syngnathids (only Syngnathus and Hippocampus).[...] This is A …