Posterior body elongation is a widespread mechanism propelling the generation of the metazoan body plan. The posterior growth model predicts that a posterior growth zone generates sufficient tissue volume to elongate the posterior body. However, there are energy supply-related differences between vertebrates in the degree to which growth occurs concomitantly with embryogenesis. By applying a multi-scalar morphometric analysis in zebrafish embryos, we show that posterior body elongation is generated by an influx of cells from lateral regions, by convergence-extension of cells as they exit the tailbud, and finally by a late volumetric growth in the spinal cord and notochord. Importantly, the unsegmented region does not generate additional tissue volume. Fibroblast growth factor inhibition blocks tissue convergence rather than volumetric growth, showing that a conserved molecular mechanism can control convergent morphogenesis through different cell behaviours. Finally, via a comparative morphometric analysis in lamprey, dogfish, zebrafish and mouse, we propose that elongation via posterior volumetric growth is linked to increased energy supply and is associated with an overall increase in volumetric growth and elongation.

1Axial elongation is a widespread mechanism propelling the generation of the 2 metazoan body plan. A widely accepted model is that of posterior growth, 3 where new tissue is continually added from the posterior unsegmented tip of 4 the body axis. A key question is whether or not such a posterior growth zone 5 generates sufficient additional tissue volume to generate elongation of the 6 body axis, and the degree to which this is balanced with tissue convergence 7 and/or growth in already segmented regions of the body axis. We applied a 8 multi-scalar morphometric analysis during posterior axis elongation in 9 zebrafish. Importantly, by labelling of specific regions/tissues and tracking 10 their deformation, we observed that the unsegmented region does not 11 generate additional tissue volume at the caudal tip. Instead, it contributes to 12 axis elongation by extensive tissue deformation at constant volume. We show 13 that volumetric growth occurs in the segmented portion of the axis and can be 14 attributed to an increase in the size and length of the spinal cord and 15 notochord. FGF inhibition blocks tissue convergence within the tailbud and 16unsegmented region rather than affecting volumetric growth, showing that a 17 conserved molecular mechanism can control convergent morphogenesis, 18 even if by different cell behaviours. Finally, a comparative morphometric 19 analysis in lamprey, dogfish, zebrafish and mouse reveal a differential 20 contribution of volumetric growth that is linked to a switch between external 21 and internal modes of development. We propose that posterior growth is not a 22 conserved mechanism to drive axis elongation in vertebrates. It is instead 23 associated with an overall increase in growth characteristic of internally 1 developing embryos that undergo embryonic development concomitantly with 2 an increase in energy supply from the female parent. 3 4