The Whitney Laboratory for Marine Bioscience
Congratulations to Assistant Professor of Biology at Whitney Laboratory, Dr. Christine Schnitzler, who co-authored a paper in Science Magazine titled "Transcription factor AP2 controls cnidarian germ cell induction."
Germ cells are the cells that create reproductive cells known as gametes (eggs and sperm) in animals. Somatic cells are all the cells not involved in reproduction, such as muscle cells, skin cells, or blood cells. In many animals, including flies, worms, and humans, the separation of germ cells from somatic cells is an irreversible, once-in-a-lifetime event that occurs during embryonic development. The introduced barrier between whether a cell becomes a somatic cell or a germ cell prohibits somatic cells from contributing to gamete production, and vice versa, thereby preventing somatic mutations, e.g cancers, to future generations.
By contrast, clonal animals, such as sponges and some cnidarians (corals and hydroids), do not have a barrier between somatic cells and germline cells. Instead, these animals maintain a population of adult stem cells throughout life that retain the ability to specialize either into somatic cells or into gametes. Other animals, such as sea urchins, snails, and annelid worms, specify their germ cells after embryogenesis, but it is unknown whether this process occurs only once or multiple times as in clonal animals.
Until now, the molecular mechanisms that control whether a cell becomes a germ cell or a somatic cell were understood in only a few germline-sequestering animals, but the genes that control germ cell fate in clonal species remained unknown.
In this study, the authors found that a single gene, Transcription factor AP2 (Tfap2), is sufficient to induce germ cell identity when expressed in adult stem cells in the clonal cnidarian Hydractinia. A related gene, Tfap2C, is known to be a major regulator of mammalian germ cell induction. Overall, this study shows that this transcription factor is an ancient regulator of animal germ cells shared by cnidarians and humans.
Clonal animals do not sequester a germline during embryogenesis. Instead they have adult stem cells that contribute to somatic tissues or gametes. How germ fate is induced in these animals and whether this process is related to bilaterian embryonic germline induction is unknown. We show that Transcription factor AP2 (Tfap2), a regulator of mammalian germlines, acts to commit adult stem cells, known as i-cells, to the germ cell fate in the clonal cnidarian Hydractinia symbiolongicarpus. Tfap2 mutants lacked germ cells and gonads. Transplanted wild type cells rescued gonad development but not germ cell induction in Tfap2 mutants. Forced expression of Tfap2 in i-cells converted them to germ cells. Therefore, Tfap2 is a regulator of germ cell commitment across germline-sequestering and germline-non-sequestering animals.