LINDBERG, D. R. *; GURALNICK, R. P.: The Evolution of Early Development in Gastropod Molluscs

Cell lineage data for 30 exemplar gastropod taxa representing all higher taxa and the outgroups Scaphopoda and Polyplacophora were examined for phylogenetic signal. Most cell lineages show phyletic trends of accelerating or retarding their formation relative to one or more outgroups and other more basal ingroup taxa. PAUP* analyses under three sets of character type assumptions produced increasing resolution of relationships within the cell lineage data set. The topology of the strict consensus trees for unordered, ordered, and dollo assumptions recovers early gastropod evolution as represented by the grade Patellogastropoda through Neritopsina, but only the ordered and dollo assumptions resolve some of the relationships within the caenogastropod + heterobranch clade. Directionality of character transformations is clearly present and important in this data set, but developmental rate characters are also subject to homoplasy, and while the overall direction does not appear to vary, there is considerable variation in rate of change both within and between taxa. Lastly, we treated cell fate maps as branching morphologies using warp analysis. The bending energies from thin-plate spline analyses were also used as branch lengths between OTU and HTUs in the Ponder & Lindberg gastropod tree. Long branch affects associated with chitons are readily apparent, and while not as severe as the chitons, the scaphopod branch is also noticeably longer than most gastropod OTUs. Moreover, the longest branches (greatest deformation in the developmental pathways) are found in the youngest taxa (the caenogastropod + heterobranch clade), while within the earlier patellogastropod-neritopsine grade branch lengths are shorter with the vetigastropods having the longest branch.