Meeting Abstract
In many animal groups the anterior develops first, and the body elongates progressively by growth at the posterior pole. This may have been present in the last common ancestor of bilaterians, but the diversity of extant mechanisms has made this difficult to support. So far most attempts to find molecular similarities between posterior growth in vertebrates and other animals have focused on somitogenesis/segmentation processes. There are a few commonalities, but overall little molecular similarity has been found. We propose that teloblastic growth—where elongation occurs by production of a band of progeny cells from a set of posterior stem cells—may be a more deeply conserved mode of posterior growth in animals. The Notch/dl signaling pathway is a potential point of similarity between different taxa with teloblastic growth, and other forms of posterior growth. In the model mollusc Ilyanassa, we have found that N/dl signaling is required for pattering the 4d teloblastic lineage, a broadly conserved aspect of spiralian development. First, we show that delta and Su(H) are specifically expressed in a subset of cells in and around the 4d lineage. We then demonstrate using specific gene knockdowns, that loss of either delta or notch activity in the whole embryo, or specifically in the 4d lineage, prevents the differentiation of all structures derived from 4d. In ongoing work, we are examining the role of N/dl signaling relative to other factors that are required in this lineage, including Nanos, dpp, retinoic acid and caudal-like. These results establish N/dl as a key regulator of teloblastic growth in a mollusc. Together with existing pharmacological results from leech, we argue that the role of this pathway in teloblastic growth is a conserved aspect of spiralian development, and may hint at posterior growth mechanisms that are conserved between more distantly related groups of animals.
