Pregnancy is almost always the role evolution assigned to females, not only in mammals but also in fish that, remarkably, have evolved live birth (vivipary) repeatedly from egg-laying ancestors. Yet, nothing in nature, exists without an exception to the rule. A rare deviation from the typical sex-roles is found in sygnathid fish, that includes the seahorses, pipefish, and their relatives where the male caries the eggs after fertilization until the babies hatch from his brood pouch (Figure 1). In 2016, we finally sequenced the first seahorse genome with the aim to learn about the genomic bases of biological features of these fascinating fish. One early finding was that the missing tbx4 probably explains why seahorses don’t have pelvic fins. More recently, we learned that the brood pouch, the crucial organ for carrying the young, varies widely in form and complexity between different sygnathids but shares in its more derived form remarkable structural and genetic similarities with the mammalian uterus. Our most recent study asked in more detail what the cellular basis of seahorse brood pouches and pregnancy is and how this compares to the mammalian placenta. We asked: How did male seahorses evolve what is essentially a womb? Did evolution repeat itself at the genetic, cellular, and organ levels to produce placenta-like structures in both seahorses and mammals independently? And in nature’s only “pregnant” males, what roles do hormones play in triggering the pouch’s development?
Our recent study published in Nature Ecology & Evolution deciphers the cellular and genetic blueprints behind this evolutionary marvel at the genomic, genetic, molecular and cellular levels. The results tell a story of ingenious biological innovation, driven by the co-option of existing genes and the de novo emergence of new ones that combined led to the male placental brood pouch. Through an extensive combination of cellular-molecular (e.g., comparative scRNA-Seq, scATAC-Seq, spatial transcriptome sequencing), we identified a population of brood pouch epithelial progenitor cells (BEPCs) with pouch-inducing potential in syngnathids. Using dynamic network biomarker (DNB) analysis, we found the androgen receptor gene co-expressed and showed a synchronous expression pattern with collagens during brood pouch organogenesis. Further in vivo experiments in seahorses and pipefish showed that females could be induced to grew male-like brood pouches after treatment with androgens. This experiment suggested that a typical “male” androgen hormonal gene regulatory pathway plays a key role in the ontogeny and evolution of a novel structure. A male hormone was shown to be responsible for inducing a male-like brood pouch in female seahorses!
It turns out that the independent evolution of a, in this case, male placental structure, for most viviparous animals, pregnancy is a female-driven process. Embryos develop inside the mother’s reproductive tract, receiving nutrients and oxygen through a placenta-a shared organ formed by both mother and embryo. The seahorse family, however, has overturned this paradigm. In these remarkable fish, it is the males that become pregnant. The emergence of male pregnancy in seahorses represents a fascinating case of convergent evolution, paralleling the development of the female placenta in eutherian mammals. This was accomplished both through the evolution of novel, seahorse-specific genes, but also by the recruitment of similar sets of genes and the evolution of new functionally equivalent types of seemingly homologous cells in seahorses that are also known from the female placenta of mammals. Our data suggests that specific cell repertoires that were initially used to make eggs sticky that attach to and are getting fertilized on specialized skin patches on the underside of male’s body might have been the first step during for the evolution of the syngnathid brood pouch. Next came the recruitment of orthologous cells with homologous functions and of orthologous genes (MHCIIs, mgat3, and nucb2), as well as newly evolved syngnathids-specific genes (pastns and syn-lectins), that may have facilitated the simplification (mostly by gene loss) of their immune system and pouch evolution that is unique to seahorse male pregnancy.
Pregnancy poses a fundamental immunological challenge. The father’s embryos are, of course, genetically half-foreign to him. Normally, an animal’s immune system would identify such “foreign” tissue and attack it. Seahorses have some of the most reduced immune systems of all fishes presumably to tolerate the presence of embryos in the male’s body. That seahorse immune systems are secondarily extremely simplified we could show through comparative genomic analyses with many other fish species. This study confirms that seahorses have lost the foxp3 gene, a master regulator of regulatory T cells (Tregs) that in other animals prevents immune attacks on fetal tissue. This loss suggests that seahorses have evolved a unique, foxp3-independent strategy for maintaining immune tolerance during pregnancy. It is akin to operating a sophisticated immune-safety system without its conventional central controller. We further observed that other immune-related genes remain active and dynamically regulated throughout male pregnancy, indicating that they may cooperate in novel compensatory pathways. Additionally, the known immunosuppressive effects of androgens may contribute to shaping a tolerant microenvironment within the brood pouch. Unraveling the precise mechanisms behind this immunological adaptation remains a compelling task for future research.
The story of the pregnant seahorse dad is a powerful testament to the creativity of evolution’s entangled bank that still provides surprises to science as it shows that there can be multiple paths to similar biological outcomes. The study of these marine super-dads provides more than just a better understanding of seahorses, it opened an unexpected window into fundamental biological principles and, to quote Darwin, how from so simple a beginning of sticky eggs endless forms most beautiful and wonderful have been, and are being, evolved.