Guralnick, R.P.: Reconnecting cell and animal lineages: A synthetic approach to Spiralian evolution and development

Tracing the timing, position, and directionality of all the cell divisions from zygote up to late development is the study of cell lineages. During the late nineteenth and early twentieth century, American biologists at Woods Hole pioneered studies of cell lineages of spiral cleavers and used the data they collected in constructing evolutionary developmental scenarios. However, due to the lack of appropriate analytical frameworks, that data could not be rigorously used for either phylogenetic analysis or for testing major evolutionay-devlopmental questions. Here, I reanalyze all available cell lineage data using modern statistical techniques (like phylogenetic analysis). I show that: 1.) Cell lineage data reconstructs a phylogenetic hypothesis that similar but not identical to the patterns found in 18S and morphological analyses; 2.) 4d, the mesentoblast, is a unique cell that speeds up and slows down in both equal and unequal cleavers; 3.) Some cells that form in the same quartet at the same point in the cell lineage hierarchy have much lower variations across all Spiralia, arguing for architectural constraint or stabilizing selection.