Figure 8. Schematic illustrations of the Ds-Ft steepness model for leg regeneration. A: Normal allometric growth: the positional values (PVs) are denoted arbitrarily by the numbers 1 to 9, while the scalar values of the Ds-Ft gradient are indicated by orange in the Ds-Ft steepness model (Lawrence et al., 2008). The scalar value of the Ds-Ft gradient is minimum at the most distal value, PV = 9. The steepness of the gradient at each point, measured as a differential across each cell, correlates with the size along the PD axis in the leg. Growth would be predicted to cease when the slope of the gradient fell below a certain threshold value. B: Normal regeneration: After amputation at PV = 3 in the tibia (left side, the tibial stump is indicated by orange), blastemal cells detect positional disparity (PVs, 3/9) through the Ds-Ft signaling pathway, and then a steeply sloped Ds-Ft gradient is formed, which leads to intercalary growth until the re-establishment of positional continuity (yellow, PVs = 4–8), as epimorphic-like regeneration. The pre-existing stump (orange) grows allometorically, retaining the original positional and allometric information (PVs = 1–3). C: No regeneration: The pre-existing stump grows without restoring the missing portion, having the original positional and allometric information (PVs = 1–3). The phenotype was observed in the Gb'arm rdRNAi-ed leg (Nakamura et al., 2007). D: Morphallaxis-like regeneration in Gb'ft, Gb'ds, or Gb'd rdRNAi-ed nymphs after proximal amputation. No epimorphic-like regeneration takes place by suppression of proliferation of blastemal cells along the PD axis, while remodeling of the stump takes place as morphallaxis-like regeneration. The positional values are re-established in relation to the new tibia-tarsus boundary, in which information about the ultimate tibial size (allometric information) is lost. The normal Ds-Ft gradient, indicated by a dotted line, would shift down with the same slope so as to reset PV of the amputated surface to the most distal PV, or the minimum scalar value of the Ds-Ft gradient. The short leg size induced by rdRNAi-ed against Gb'ft is consistent with this model. E: Morphallaxis-like regeneration in the Gb'ft, Gb'ds, or Gb'd rdRNAi-ed nymphs after distal amputation. The dependence of leg size on the site of amputation in Gb'ft rdRNAi-ed legs is in agreement with the previous interpretation for Morphallaxis-like regeneration (Fig. 8D). F: “Long” phenotype in the Gb'ex rdRNAi-ed leg. Down-regulation of contact-dependent inhibition of proliferation results in the formation of a longer leg, due to overproliferation. In this case, the threshold value of the slope of the Ds-Ft gradient for the arrest of growth would be lower than the normal threshold value indicated by a dotted line, leading to formation of a longer leg. G–J: In the case of the intercalary transplantation, we can assume a steep gradient and a reverse-steep gradient would be presented in the junction in normal intercalary regeneration and reverse intercalary regeneration, respectively. Intercalary growth would be expected to cease when the slope of the linear gradient equals the pre-existing one. G: Normal intercalary regeneration: When a distal graft is transplanted to a proximal host, intercalary regeneration occurs so as to restore the missing portion (PVs:34567). After grafting, a steep Ds-Ft gradient would be formed at the junction. The cells of the graft proliferate to restore the pre-existing slope. H: In the case of Gb'ft rdRNAi-ed legs, no intercalary growth occurs, and the transplanted tibia becomes shorter. As shown in D, the Ds-Ft gradient would be expected to shift down with the same slope so as to make the continuous gradient in which the most distal position of the grafted tibia is the minimum scalar. I: Reverse intercalary regeneration: When a proximally amputated graft is transplanted to a distally amputated host, reverse intercalary regeneration occurs so as to maintain positional continuity (PVs:765). After grafting, a steep, reverse Ds-Ft gradient would be formed at the junction. The cells of the host proliferate to restore the pre-existing slope with the DP direction. J: In the case of Gb'ft rdRNAi-ed legs, no intercalary growth occurs, while the transplanted tibia becomes longer. As shown in D and E, the normal Ds-Ft gradient indicated by a dotted line, would shift up with the same slope so as to make the continuous gradient in which the most distal position of the grafted tibia is the minimum scalar. Thus, the rdRNAi phenotypes observed during leg regeneration are interpreted consistently with the Ds-Ft steepness model for regeneration (Bando et al., 2009).
Download figure to PowerPoint