Table 1. Piwi orthologue expression.
| Phylum | Common name | Species | Known Piwi genes | PIWI protein expression | Reference |
|---|---|---|---|---|---|
| Porifera | Sponge | Ephydatia fluviatilis | EfPiwiA and EfPiwiB | Archeocytes (stem cells that differentiate into both somatic and germ cells) |
16 |
| Cnidaria | Jellyfish | Clytia hemisphaerica | Piwi | Somatic stem cells of the tentacle bulb (produce stinging cells characteristic of the cnidarians) |
17 |
| Jellyfish | Podocoryne carnea | Cniwi | Somatic stem cells of the tentacle bulb (see above); striated muscle cells capable of transdifferentiation |
18 | |
| Ctenophora | Comb jellyfish | Pleurobrachia pileus | PpiPiwi1 and PpiPiwi2 | Actively dividing adult somatic cells; germ line | 19 |
| Platyhelminthes | Planaria | Schmidtea mediterranea | smedwi-1, smedwi-2 and smedwi-3 | Neoblasts (totipotent stem cells that can repopulate all somatic and germline lineages) |
21, 22 |
| Saltwater flatworm | Macrostomum lignano | macpiwi | Neoblasts (see above) | 20 | |
| Mollusca | Sea slug | Aplysia californica | Piwi | Nervous system, heart and germ line | 80 |
| Arthropoda | Fruitfly | Drosophila melanogaster | piwi, aub and AGO3 | Gonad, brain, salivary gland | 2, 26, 92, 93 |
| Chordata | Sea squirt (ascidian) |
Botrylloides leachii and Botryllus schlosseri |
Piwi | Stem cell population (capable of whole-body regeneration) | 23, 24 |
| Mouse | Mus musculus | Miwi, Mili and Miwi2 | Diverse cancers (breast cancer, rhabdomyosarcoma, medulloblastoma); male germ line |
12-14, 33 | |
| Human | Homo sapiens | HIWI, HILI, HIWI2 and HIWI3 | Diverse cancers (breast cancer, cervical cancer, endometrial carcinoma, seminomas, hepatocellular carcinoma, gastric cancer, pancreatic adenocarcinoma, gastrointestinal stromal tumours, colon cancer, renal cell carcinoma); haematopoietic stem cells; and male germ line |
30, 32-36 |