The transition from fins to limbs was an important terrestrial adaptation, but how this crucial evolutionary shift arose developmentally is unknown. Current models focus on the distinct roles of the apical ectodermal ridge (AER) and the signalling molecules that it secretes during limb and fin outgrowth. In contrast to the limb AER, the AER of the fin rapidly transitions into the apical fold and in the process shuts off AER-derived signals that stimulate proliferation of the precursors of the appendicular skeleton1,2,3,4,5,6,7,8,9,10. The differing fates of the AER during fish and tetrapod development have led to the speculation that fin-fold formation was one of the evolutionary hurdles to the AER-dependent expansion of the fin mesenchyme required to generate the increased appendicular structure evident within limbs11. Consequently, a heterochronic shift in the AER-to-apical-fold transition has been postulated to be crucial for limb evolution11. The ability to test this model has been hampered by a lack of understanding of the mechanisms controlling apical fold induction11,12. Here we show that invasion by cells of a newly identified somite-derived lineage into the AER in zebrafish regulates apical fold induction. Ablation of these cells inhibits apical fold formation, prolongs AER activity and increases the amount of fin bud mesenchyme, suggesting that these cells could provide the timing mechanism proposed in Thorogood’s clock model of the fin-to-limb transition11. We further demonstrate that apical-fold-inducing cells are progressively lost during gnathostome evolution; the absence of such cells within the tetrapod limb suggests that their loss may have been a necessary prelude to the attainment of limb-like structures in Devonian sarcopterygian fish.
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The authors acknowledge E. McGlinn for expert advice, C. Wicking for comments on the manuscript, E. Tanaka for advice and technical support, Fishcore for expert zebrafish care, M. Morsch, V. Oorschot and the Monash electron microscopy facility for expert technical advice, Monash Micro Imaging for microscope availability and maintenance, S. Chow for technical assistance, M. Voz for reagents, T. Carney for fish lines and J. Joss for lungfish rearing and expertise. P.D.C. is supported by an NHMRC Principal Research Fellowship. N.J.C. is supported by The Snow Foundation and BitFury.org. This work was supported by an ARC Discovery Grant DP110101482. The Australian Regenerative Medicine Institute is supported by funds from the State Government of Victoria and the Australian Federal Government.
Reviewer Information Nature thanks C. Tabin and the other anonymous reviewer(s) for their contribution to the peer review of this work.
Extended data figures
Time-lapse recording of cell migration to the fin bud from the somites in TgBAC(pax3a:GFP) transgenic embryos
Time-lapse recording corresponding to images presented in Fig. 1a-c. Analysis of animals transgenic for TgBAC(pax3a:GFP) from 23 hpf – 33 hpf identifies that somite 4-6 contributes to the zebrafish pectoral fin mesenchyme (pfm) while somite 7 contributes to the posterior hypaxial muscle (phm) (n=6). Anterior is to the left, dorsal to the top, view is lateral.
Time-lapse recording corresponding to images presented in Fig. 1d-i. GFP positive cells originating in the pectoral fin bud mesenchyme migrate outside of the cluster of GFP positive cells in the fin bud mesenchyme and take on an elongated morphology within the AER. Anterior is to the left dorsal to the top, view is lateral.
Maximum projection followed by a surface rendering of the images presented in Fig. 2i-r. In this analysis somite 4 from an embryo transgenic for Tg(ubi:zebrabow) (red) is homotopically transplanted into a host embryo transgenic for Tg(actc1b:GFP) which marks the myotome in green. Contribution of cells derived from the grafted somite (red) to the AER is observed. Embryo is 48hpf, anterior is to the left dorsal to the top, initial view is lateral.
Laser mediated ablation of Tg(tbx6:gal4-vp16); Tg(UAS:Kaede) positive cells in the AER results in cell blebbing
Video recording of images presented in Extended Data Fig. 7i-l. A 405nm diode laser was used to ablate a region of interest specific for a cell in the AER containing non-photoconverted Kaede (green). After ablation non-photoconverted Kaede is absent and photoconverted Kaede (red) remains. Overall cellular morphology is disrupted and blebbing of dying cells is observed immediately post-irradiation. Anterior is to the left dorsal to the top, view is lateral.