Multispecies data includes single-cell data across developmental stages for three species (Hippocampus erectus, Syngnathus schlegelii, Hippichthys heptagonus), single cell ATAC-seq, ChIP-seq, and RNA-seq datasets specific to Hippocampus erectus.

Seahorses and their relatives (syngnathids) are uniquely sex-role reversed. Their male pregnancy is facilitated by an evolutionary novel organ-the brood pouch that functions analogously to the mammalian uterus and placenta despite being originating from entirely different tissues. Here, through a set of comparative single-cell multiomics and genomics analyses, we investigated the transition from egg-laying to pregnancy during syngnathid evolution and compared the convergently evolved life-history of placental parental care with that in mammals. We show that a population of epithelial progenitors possess pouch-inducing potential and androgen, not female hormones as in other live-bearers, can trigger pouch ontogeny as evidenced by in vivo experiments. The functionality of the pouch is achieved through both recruiting cells (e.g., trophoblast-like cells) also found in mammalian uteri characterized by expressed orthologous genes, as well as co-opting specific cells for sticky eggs attached to specialized skin patches of males and expressed novel seahorse-specific genes (e.g., pastns, syn-lectins), which combined drove pouch diversity and complexity.