Seahorse Male Pregnancy & Immune Adaptations

Seahorse Male Pregnancy

• Seahorses, together with sea dragons and pipefishes, belong to the Syngnathidae family of teleost fishes.
• Seahorses and other Syngnathidae species have a unique feature: male pregnancy.
• Male pregnancy in seahorses involves carrying fertilized eggs and developing embryos within a special brood pouch.
• The brood pouch forms in the abdomen, trunk, or tail, depending on the species, and resembles a human uterus with a placenta.

Characteristics of Seahorse Male Pregnancy

• Male pregnancy is an extremely costly investment, requiring physiological, metabolic, and anatomical changes in the parental organism.
• Male pregnancy has evolved independently around 150 times in different vertebrate lineages.
• One advantage of male pregnancy may be a division of reproduction costs between two parents, increasing the offspring number.
• While the male carries babies in the brood pouch, the mother can produce another batch of eggs.

Brood Pouch Development

• Different types of male pregnancy in various Syngnathidae fishes exemplify evolutionary gradation and progress in paternal involvement.
• In some species, eggs are attached to the skin surface without protection, while in others, the attached eggs are protected by skin flaps.
• In Syngnathus and Hippocampus, the brood pouch is fully closed, and developing embryos are integrated into paternal tissue and fully supplied through the vascularized placenta-like structure.

Immunologic, Metabolic, Cellular, and Molecular Processes

• Seahorses are a great model for studying various aspects of pregnancy, including molecular and cellular features of embryo–parent interactions, immune tolerance, and the effects of pollutants and environmental factors on pregnancy, embryo development, and offspring fitness.
• The development of immune tolerance of the parent toward the allogeneic embryos is a crucial aspect of seahorse male pregnancy.

Effects of Environmental Pollutants

• Seahorses are very useful for studying the effects of pollutants and environmental changes on pregnancy, embryo development, and offspring fitness.
• The effects of pollutants and environmental factors on pregnancy, embryo development, and offspring fitness are pressing issues that require further research.

Evolutionary History

• Seahorses evolved from pipefishes in the late Oligocene period.

• There are currently 47 known species of seahorses, which live in shallow marine habitats.

• The population of seahorses is in constant decline due to pollution, overfishing, and other human activities.### Epithelial Transformation and Pseudoplacenta Development

• During the development of the closed brood pouch, the external epithelium of the skin changes into the luminal (internal) epithelium of the brood pouch.

• Epithelial transformation correlates with the expression of haCTL IV lectin exclusively in the luminal epithelium.

• Epithelial transformation is accompanied by the aggregation of collagenous fibers, smooth muscle cells, and the development of capillaries below the epithelium, forming the pseudoplacenta that supports egg/embryo development.

Hormonal Regulation of Seahorse Male Pregnancy

• The hypothalamic–pituitary–gonadal (HPG) axis regulates reproduction stages and cycles in fishes.
• Gonadotropin-releasing hormone (GnRH) induces the pituitary gland to release gonadotropins, follicle-stimulating hormone (FSH) and luteinizing hormone (LH), into the bloodstream.
• Gonadotropin-inhibitory hormone (GnIH) inhibits gonadotropin secretion.
• In males, FSH induces spermatogenesis, and LH stimulates sperm maturation, and both gonadotropins stimulate the male gonad to produce androgens, such as testosterone and 11-ketotestosterone.

Gonadotropin-Inhibitory Hormone (GnIH)

• GnIH is involved in the regulation of male pregnancy.
• GnIH may decrease plasma testosterone levels during male pregnancy.
• GnIH is expressed in the hypothalamus and the brood pouch.

Sex Steroids and Male Pregnancy

• Androgens, such as testosterone, 11-ketotestosterone, and 11β-hydroxy androstenedione, are produced during male pregnancy.
• Sex steroids produced by gonads and responsible for sexual dimorphisms, such as testosterone (or ketotestosterone) in males and estrogen in females, behave differently during sex-role reversal.
• 11-ketotestosterone and 17α-hydroxy-20β-dihydroprogesterone are highest during pregnancy, suggesting their role in pregnancy regulation.

Neurohypophysial Hormones

• Neurohypophysial hormones, such as vasotocin and isotocin, regulate reproduction in vertebrates and some invertebrates.
• In seahorses, vasotocin and isotocin are expressed in the hypothalamus, gonads, gills, and brood pouch.
• Vasotocin induces the release of gonadotropins, stimulating estrogen secretion and causing premature birth.

Prolactin

• Prolactin is a polypeptide hormone produced by the acidophilic lactotroph cells of the pituitary gland.
• Prolactin affects its target tissues via the prolactin receptor (PRLR) in the target cells’ membranes.
• In Syngnathidae, prolactin is involved in male pregnancy, and its receptor is expressed in the brood pouch and all other tissues.

Retinoic Acid

• Retinoic acid (RA) is a metabolite of vitamin A1 (all-trans-retinol) involved in growth, embryonic development, differentiation, cancer, and immunity.

• RA signaling through the homeobox (Hox) and POU genes establishes the anterior–posterior axis and patterning of the embryo.

• In seahorses, RA is involved in brood pouch formation and pregnancy, and regulates antioxidant defenses.### Evolution of Immune Tolerance

• Early in evolution, acute endometrial inflammation occurred in some marsupials, which was a natural maternal immune reaction towards the attaching embryo.

• This inflammation was later transformed into the embryo-friendly process of implantation and placental pregnancy in mammals, by suppressing the most damaging parts of the inflammatory response.

Immune System Adjustments

• The trophoblast layer of the embryo does not express MHC II, which would otherwise present embryonic antigens to maternal T-helper cells and initiate an immune response.
• MHC I genes, which present antigens to maternal cytotoxic T cells, are downregulated in the trophoblast.
• Regulatory T cells (Tregs) recognize fetal antigens via maternal antigen-presenting cells (APCs) and induce tolerance towards the embryo.

Seahorses as a Model for Evolutionary Progress

• Male seahorses, with their gradation of pregnancy, from a simple attachment of eggs to the skin to internal gestation within the brood pouch, are perfect for reconstructing evolutionary progress in parental immunotolerance.
• Studies on seahorses have shown that the evolution of pregnancy coincided with either a complete loss or rearrangement of MHC II pathway genes and correlated with the expansion of the MHC I gene repertoire.

Immune Gene Repertoire

• MHC II molecules are found on professional antigen-presenting cells such as dendritic cells, mononuclear phagocytes, some endothelial cells, thymic epithelial cells, and B cells, which are crucial for initiating an immune response.
• MHC II binds peptides, which are derived from the proteolysis of self and non-self proteins, and presents them to antigen-specific CD4+ T cells.

Comparative Genomic Analyses

• Comparative genomic analyses across the Syngnathidae species showed that the complexity of the immune system gene repertoire decreased in evolution in parallel with the increase in paternal care.
• This is in contrast to a general tendency to increase immune complexity during evolution.

Loss of Spleen and Immune Defense

• Seahorses reduced not only the complexity of the immune genes but also eliminated the spleen during evolution.
• The loss of the spleen and important components of the immune defense does not impair the immunological response of seahorses against encountered microbes.

Immune Protection in Seahorse Pregnancy

• The seahorse brood pouch contains antimicrobial peptides such as hepcidin, which defend developing embryos against pathogens.
• Hepcidin is a small cysteine-rich antimicrobial peptide that regulates iron metabolism and functions in immune defense.
• Lysozymes, which process the antigen for presentation on MHC molecules and activate Toll-like receptors, are also involved in the defense against brood pouch pathogens.