The fin-to-limb transition represents one of the major vertebrate morphological innovations associated with the transition from aquatic to terrestrial life and is an attractive model for gaining insights into the mechanisms of morphological diversity between species1. One of the characteristic features of limbs is the presence of digits at their extremities. Although most tetrapods have limbs with five digits (pentadactyl limbs), palaeontological data indicate that digits emerged in lobed fins of early tetrapods, which were polydactylous2. How the transition to pentadactyl limbs occurred remains unclear. Here we show that the mutually exclusive expression of the mouse genes Hoxa11 and Hoxa13, which were previously proposed to be involved in the origin of the tetrapod limb1, 2, 3, 4, 5, 6, is required for the pentadactyl state. We further demonstrate that the exclusion of Hoxa11 from the Hoxa13 domain relies on an enhancer that drives antisense transcription at the Hoxa11 locus after activation by HOXA13 and HOXD13. Finally, we show that the enhancer that drives antisense transcription of the mouse Hoxa11 gene is absent in zebrafish, which, together with the largely overlapping expression of hoxa11 and hoxa13 genes reported in fish3, 4, 5, 6, 7, suggests that this enhancer emerged in the course of the fin-to-limb transition. On the basis of the polydactyly that we observed after expression of Hoxa11 in distal limbs, we propose that the evolution of Hoxa11 regulation contributed to the transition from polydactyl limbs in stem-group tetrapods to pentadactyl limbs in extant tetrapods.
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Extended data figures and tables
Extended Data Figures
- Extended Data Figure 1: Absence of antisense transcription 3′ to the Hoxa11 promoter in the Hoxa11eGFP/eGFP limb and evidence that Hoxa11as-b transcripts produced in trans have no effect on Hoxa11 expression. (168 KB)
a, b, Detection of Hoxa11as-b transcripts in wild-type limb buds at E12.5 (a), and whole-mount in situ hybridization to detect gfp antisense transcripts in Hoxa11eGFP/eGFP limb buds at E12.5 (b). c–e, Hoxa11 expression in wild-type limb buds (c), and Hoxa11as-b (d) and Hoxa11 (e) expression in Prx1-Hoxa11as limb buds. Original magnification, ×31.5.
- Extended Data Figure 2: Deletion of the distal enhancer in Hoxa11 intron using CRISPR-Cas9. (150 KB)
a, Scheme of the wild-type and targeted (Hoxa11ΔInt) loci. Sites targeted by the single-guide RNAs (sgRNA_1 and sgRNA_2) for the CRISPR-Cas9-mediated deletion of the distal enhancer. The blue rectangles indicate the position of the DNA probe used to confirm the deletion by Southern blot in b. b, Lane 1 shows the 6-kb KpnI band resulting from the CRISPR-Cas9-mediated deletion. Lane 2 was loaded with wild-type DNA. c, PCR reaction using a forward primer located upstream of sgRNA_1 and a reverse primer located downstream sgRNA_2 shows the presence of a 300 bp (ΔInt 300 bp) fragment expected for the Hoxa11ΔInt allele. d, The sequence of the 300-bp PCR fragment confirms the CRISPR-Cas9-mediated deletion of the Hoxa11 intronic region containing the distal enhancer (only the sequence encompassing the deletion breakpoints is shown).
- Extended Data Figure 3: The distal enhancer located in the Hoxa11 intron is bound by HOXA13 and HOXD13 in distal limb cells and its activity is increased by HOXA13 in 293T cells. (147 KB)
a, Integrative genomics viewer (IGV) screenshot showing HOXA13 and HOXD13 ChIP–seq data at the Hoxa11 locus. These ChIP–seq data were obtained using chromatin from distal forelimb buds of wild-type E11.5 mouse embryos (R. Sheth et al., manuscript submitted). b, Transfection assay shows HOXA13 dependent activation of Hoxa11 intron driving reporter gene expression. Two-tailed Tukey’s multiple comparisons test was performed. Error bars indicate s.d (n = 3). RQ, relative quantification.
- Extended Data Figure 4: Individual inactivation of Hoxa13 or Hoxd13 is not sufficient to fully abrogate antisense transcription in distal limbs. (327 KB)
a, b, Whole-mount in situ hybridization, using probe A (see Fig. 1) to detect all antisense transcripts, on Hoxd13−/− (a) and Hoxa13−/− (b) mouse limb buds at E11.5. Antisense transcription in distal limbs remains robust in both mutants but a clear reduction is seen in the distal Hoxa13−/− limbs. Original magnification, ×31.5.
- Extended Data Figure 5: Inactivation of both Hoxa13 and Hoxd13 disrupts antisense transcription overlapping with the Hoxa11 exon 1. (765 KB)
a–d, Hoxa11as-b expression (probe B in Fig. 1) in limb buds (a, b) and tail buds (c, d) from wild-type (a, c) and Hoxa13−/− Hoxd13−/− (b, d) E12.5 mouse embryos. Whole-mount in situ hybridization shows that Hoxa11as-b expression in tail buds (internal control) is similar in both the wild-type (c) and double-mutant (d) embryos, whereas there is almost no expression remaining in Hoxa13−/− Hoxd13−/− limb buds (b). Original magnification, ×31.5.
- Extended Data Figure 6: Generation of the RosaHoxa11 knock-in mouse line. (126 KB)
a, Targeting of the endogenous Rosa26 locus (top three lines). The wild-type Rosa26 locus is shown below (middle). Regions used as homologous arms for the recombination in ES cells are indicated by brown rectangles labelled 5′ and 3′, respectively. Scheme of the targeted locus after homologous recombination in ES cells and after Cre-mediated recombination is shown at the bottom. The position of the internal (IP) and external (EP) probes and restriction sites used for Southern blot analysis are indicated on both the wild-type and targeted locus. b, c, Southern blots of ES cells clones using the internal probe (b) and external probe (c) to detect the targeted allele (lane 1). d, Southern blot of wild-type (lane 2) and heterozygous (lane 1) mice. A, AscI; E, EcoRV; P, PacI; S, SwaI.
- Extended Data Figure 7: The conditional gain of Hoxa11 using the Hoxa13Cre allele results in the formation of supernumerary digits. (1,211 KB)
a, b, Autopod of RosaHoxa11/+ (a) and RosaHoxa11 Hoxa13Cre (b) at E15.5. Anterior is up. The Rosa26 locus and Hoxa13Cre allele being on the same chromosome (Chr6), the gain-of-function phenotype was assessed with only one copy of the RosaHoxa11 allele. c–e, Autopod skeletons of Prx1Cre; RosaHoxa11/Hoxa11 mice at P0 from four distinct mutants (anterior is up). The number of digits varies from 6 to 7, with often a small post-axial extra-digit (posterior). The extra-digit phenotype is fully penetrant upon Cre-activation of two copies of the RosaHoxa11 allele (n = 10). Original magnification, ×20.d, Quantification of Hoxa11 expression level by quantitative reverse transcriptase PCR (RT–qPCR) on RNA extracted from E11.5 forelimb, relative to both Gapdh and Tbp mRNA of Prx1Cre; RosaHoxa11/Hoxa11 embryos. Two-tailed t-test was performed. Error bars indicate s.d (n = 4).
- Extended Data Figure 8: hoxa11 and hoxa13 are expressed in overlapping domains in zebrafish fins. (774 KB)
a–d, Expression of hoxa13b (a, b) and hoxa11b (c, d) in zebrafish fins at 60 hpf (a, c) and 72 hpf (b, d). Dotted lines indicate the boundary between the endochondral disc and the fin fold. Original magnification, ×400.
- Extended Data Figure 9: Absence of antisense transcription at the hoxa11a and hoxa11b loci in zebrafish fins. (686 KB)
a, b, Whole-mount in situ hybridization with probes designed to detect putative antisense transcription at hoxa11a (a) and hoxa11b (b). c–f, No antisense transcription is detected, whereas expression of hoxa11a (c), hoxa11b (d), hoxa13a (e) and hoxa13b (f) is observed in zebrafish fins at the same stage. Asterisks correspond to the staining from the fin on the other side of the embryo. Original magnification, ×63.