The seahorses, pipefishes and seadragons (Syngnathidae) are among the few vertebrates in which pregnant males incubate developing embryos. Syngnathids are popular in studies of sexual selection, sex‐role reversal, and reproductive trade‐offs, and are now emerging as valuable comparative models for the study of the biology and evolution of reproductive complexity. These fish offer the opportunity to examine the physiology, behavioural implications, and evolutionary origins of embryo incubation, independent of the female reproductive tract and female hormonal milieu. Such studies allow us to examine flexibility in regulatory systems, by determining whether the pathways underpinning female pregnancy are also co‐opted in incubating males, or whether novel pathways have evolved in response to the common challenges imposed by incubating developing embryos and releasing live young. The Syngnathidae are also ideal for studies of the evolution of reproductive complexity, because they exhibit multiple parallel origins of complex reproductive phenotypes. Here we assay the taxonomic distribution of syngnathid parity mode, examine the selective pressures that may have led to the emergence of male pregnancy, describe the biology of syngnathid reproduction, and highlight pressing areas for future research. Experimental tests of a range of hypotheses, including many generated with genomic tools, are required to inform overarching theories about the fitness implications of pregnancy and the evolution of male pregnancy. Such information will be widely applicable to our understanding of fundamental reproductive and evolutionary processes in animals.

中文翻译：

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合颌鱼类雄性妊娠的进化与生理

海马、尖嘴鱼和海龙 (Syngnathidae) 是少数几种能够让怀孕雄性孵化发育中胚胎的脊椎动物。Syngnathids 在性选择、性别角色逆转和生殖权衡的研究中很受欢迎，现在正在成为研究生殖复杂性的生物学和进化的有价值的比较模型。这些鱼提供了检查胚胎孵化的生理学、行为影响和进化起源的机会，独立于女性生殖道和女性荷尔蒙环境。这些研究使我们能够检查监管系统的灵活性，通过确定支持女性怀孕的途径是否也参与孵化男性，或者是否已经进化出新的途径来应对孵化发育中的胚胎和释放活幼体所带来的共同挑战。Syngnathidae 也是研究生殖复杂性进化的理想选择，因为它们表现出复杂生殖表型的多个平行起源。在这里，我们分析了合颌奇偶模式的分类分布，检查了可能导致男性怀孕出现的选择压力，描述了合颌生殖的生物学，并强调了未来研究的紧迫领域。需要对一系列假设进行实验测试，包括许多使用基因组工具生成的假设，以告知有关怀孕健康影响和男性怀孕演变的总体理论。