How a small marine invertebrate reveals stem cell secrets


A research group led by Prof Lucia Manni of the Department of Biology at the University of Padua, in collaboration with researchers at Stanford University, investigate the molecular and morphological mechanisms determining the sexual or asexual developmental pathway of the small marine invertebrate, Botryllus schlosseri, an organismfound widely throughout the lagoon of Venice.

Hundreds of genetically equal individuals (zooids) can form a colony of Botryllus schlosseri through the asexual reproduction from a single larva. These colonies develop into a small flower-shaped bud, where each petal then forms into an individual adult. The stem cells of the colony allow each adult to form more buds through asexual reproduction, which then begins the cycle of growing new individual organisms. In this way, the colony multiplies. However, individuals within the colony then face its sexual development. Several embryos can develop at the same time within the colony thanks to other stem cells contributing to organ formation. While the starting points are quite different, the results from the two developmental pathways are the same; they both produce indistinguishable individuals.

Prof Manni explains, “Our study investigates the molecular and morphological mechanisms that lead to the construction of the same individual through different developmental pathways. To do so we built an atlas of sexual and asexual development (Tabula compositi chordati Botrylli) that follows the systematic development of organs such as the nervous system, blood cells, and gonads. We were very surprised to find that only 30% of the genes that drive development are shared both pathways. However, organs develop in the same order, and many tissue-specific genes are used at the same time within the two pathways.”

These results have shown that convergent morphology does not necessarily imply convergent molecular mechanisms. However, they pointed out that tissue-specific stem cells and transcription factors that guide the expression of other genes play a crucial role in producing the same body shape along different developmental pathways. This little creature can help us to unravel the secrets of stem cellsand to understand better the mysterious and fascinating process that leads to the formation of organisms.