Male Pregnancy and the Evolutionary Importance of Immunological Tolerance in Syngnathid Fishes
Pregnancy has evolved upwards of 150 times across vertebrates and represents the reproductive strategy of the highest investment. Morphological and immunological tolerance adaptations evolved to facilitate the connection between embryo and parent during pregnancy, preventing embryonic rejection from occurring and maintaining an immunologically homeostatic environment. However, knowledge concerning the co-evolutionary relationship between pregnancy and the immune system is overwhelmingly restricted to mammalian species. In order to understand how pregnancy could have evolved to such a diversity as well as providing further insight into the specific adaptations that facilitated pregnancy evolution, research needs to transcend the mammalian system. The syngnathid fish group, which includes the seahorses, pipefishes and seadragons, lays claim to the only evolutionary instance of male pregnancy in the animal kingdom. A number of different brooding forms have evolved within the lineage, ranging from pouchless external egg carrying skin integument (Nerophinae), muscular flap derived, semi-sealed pouches (Syngnathus) and the specialised sealed pouches with muscularised opening and closing capabilities (Hippocampus). However, the evolutionary oddities within the syngnathid group extend beyond their reproductive strategies. The functional loss of the Major Histocompatibility Complex class II pathway (MHC II) in the Syngnathus genus and related pathway components in the seahorse (Hippocampus) has raised questions regarding their immunological proficiency. Furthermore, the relevance of this loss to immunological tolerance levels and its potential influence on the evolution of advanced male pregnancy has been stipulated. In turn, this thesis explored the male pregnancy transcriptome at various stages of gestation in multiple species, while also assessing the role of the adaptive immune system and immunological tolerance in male pregnancy evolution.