Two different evolutionary origins of stem cell systems and their molecular basis.

We propose two major evolutionary origins of stem cell systems in the animal kingdom. Adult pluripotent stem cell systems are found in many invertebrates and probably evolved as components of asexual reproduction. Lineage-specific stem cell systems probably evolved later and include neural and hematopoietic stem cell types. We propose that these two types of stem cell systems evolved independently. The vasa-like genes regulate reproductive stem cells, but not lineage-specific stem cells, which may be regulated by gcm genes. Here, we review the evidence for the molecular basis for the evolutionary origin of these two different stem cell systems.