Left-right asymmetries in Spiralia

In spite of the great diversity of forms found in nature, most metazoans are organized along an anterior-posterior and a dorsoventral axis. These two orthogonal axes define the left and right sides of animals. While most animals show overall bilateral symmetry, in some cases, specific features or organs display consistent left and right asymmetries that can be critical for their normal functioning. Descriptions of these asymmetries and the mechanisms behind them are available mainly for deuterostomes, whereas information for other groups like spiralians is rather scarce. Here I review previous and recent data, highlighting the relevance of the cleavage program in establishing the dorsoventral axis and therefore the left and right sides of spirally cleaving embryos of groups like platyhelminthes, nemerteans, mollusks, and annelids. In addition, I summarize morphological asymmetries detected throughout embryogenesis and in the adult forms of these groups as well as the mechanisms involved in these processes. This information contributes to our understanding of how bilateral asymmetries are regulated in spiralians and provides additional insights into the evolution of left-right asymmetries through the evolutionary history of bilaterians.