E.G. Conklin's support for the mosaic mode of development and understanding the role of maternal determinants

In 1905, the American embryologist, Edwin Grant Conklin published three landmark papers that substantially influenced the fields of developmental biology and evolution. They described the development of ascidian embryos, focusing on cleavage patterns and cell lineages. They also followed the distribution of colored substances localized in the oocyte that are later segregated into specific cleavage-stage cells and ultimately into different organs. Conklin observed that the cleavage patterns were invariant between embryos and that the oocyte substances were consistently distributed to the same lineages that gave rise to distinct organs. Accordingly, the first two papers concluded that cell fate was largely determined by the organization of the oocyte cytoplasm, components of which were differentially distributed by invariant cleavage patterns. In the third paper, he tested this idea by damaging some cells and following the fate of the remaining ones; he observed that the surviving cells formed the regions and organs that they would have originally produced, leading him to conclude that the differentiated animal was a mosaic of cells predetermined by the contents of the oocyte cytoplasm they inherited. Conklin's observations and conclusions motivated over a century of research using progressively more sophisticated techniques to map cell lineages and to screen for localized molecules. We review some of those studies to show how they have led to our current understanding of the essential role played by maternally derived molecules. Of course, we now know that normal development results from a complex combination of processes beyond maternal determinants, including gene regulatory networks, cell-cell interactions, secreted factors, epigenetic and environmental influences. In addition, it is clear that different species utilize a mosaic distribution of maternal molecules to different extents. However, without Conklin's contributions, this vital component of developmental control might have been overlooked, and the subsequent identity of a variety of maternal determinants delayed.