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

Abstract

long 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 has long attracted attention of the scientific community and was rapidly recognized 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 recognize the diversification of structures involved in male pregnancy as relevant for diagnosing syngnathid taxa. From an ancestral pipefish, which 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 interest in this family of quirky fish as an invaluable opportunity to shed additional light on the evolution of pregnancy.

Seahorse Male Pregnancy as a Model System to Study Pregnancy, Immune Adaptations, and Environmental Effects

“…The biggest challenge in pregnancy is how not to reject an allogeneic (nonself) embryo [45]. In mammalian pregnancy, immune tolerance toward the semi-allogenic embryo occurs through the downregulation of the major histocompatibility genes MHC I and MHC II.…”