Trans-Phylum Single Cell Orthology Reveals Conserved Ovarian Cell States Between Sea Urchin and Human.

Oocytes are produced before birth in women and their abundance and quality decrease over time until the loss of ovarian functions at menopause. In contrast, animals such as sea urchins retain stem cells that enable a continuous, high fecundity production of quality oocytes throughout their lifespan. We hypothesize that the somatic cells required for these two different adult ovarian functions are distinct. Comparing sea urchin adult ovaries with human fetal ovaries may reveal greater conservation, since they are both in a state of active oocyte production. Here, we present the first integration of the sea urchin adult ovary with the human fetal and adult ovary single-cell RNA-seq datasets. Using SAMap, the resulting integration demonstrates high conservation of cell states and gene expression in both the somatic cells (such as immune and muscle cells) and the germ cells of the ovary. Whereas multiple cell states change over time in the human ovary during its transition from fetal to adult stages, the sea urchin adult ovary instead represents an intermediate state that preserves most of the cell states characteristic of both the fetal and adult human ovary. Comparing reproductive strategies and ovarian function in species separated by 540 million years since their last common ancestor could lead to new approaches to treat human reproductive senescence. Our results highlight the potential of the sea urchin as a powerful comparative model that lacks reproductive senescence to better understand the cellular transitions underlying the aging human ovary.