Skip to main page content
Access keys NCBI Homepage MyNCBI Homepage Main Content Main Navigation
, 94 (25), 13695-700

Regulation of Number and Size of Digits by Posterior Hox Genes: A Dose-Dependent Mechanism With Potential Evolutionary Implications

Affiliations

Regulation of Number and Size of Digits by Posterior Hox Genes: A Dose-Dependent Mechanism With Potential Evolutionary Implications

J Zákány et al. Proc Natl Acad Sci U S A.

Abstract

The proper development of digits, in tetrapods, requires the activity of several genes of the HoxA and HoxD homeobox gene complexes. By using a variety of loss-of-function alleles involving the five Hox genes that have been described to affect digit patterning, we report here that the group 11, 12, and 13 genes control both the size and number of murine digits in a dose-dependent fashion, rather than through a Hox code involving differential qualitative functions. A similar dose-response is observed in the morphogenesis of the penian bone, the baculum, which further suggests that digits and external genitalia share this genetic control mechanism. A progressive reduction in the dose of Hox gene products led first to ectrodactyly, then to olygodactyly and adactyly. Interestingly, this transition between the pentadactyl to the adactyl formula went through a step of polydactyly. We propose that in the distal appendage of polydactylous short-digited ancestral tetrapods, such as Acanthostega, the HoxA complex was predominantly active. Subsequent recruitment of the HoxD complex contributed to both reductions in digit number and increase in digit length. Thus, transition through a polydactylous limb before reaching and stabilizing the pentadactyl pattern may have relied, at least in part, on asynchronous and independent changes in the regulation of HoxA and HoxD gene complexes.

Figures

Figure 1
Figure 1
Digit length is additively determined by the dose of Hoxd-11, Hoxd-12, Hox-13, and Hoxa-13 gene products. The length of forelimb digit IV (arrowheads), including the metacarpal bone, was taken as a reference measure in mice of different genotypes. Compound mutant genotypes are indicated on the left, black rectangles indicating the loss of one dose of the corresponding gene (shown at the top). In this view, the wild-type hand (A) had 10 doses of active products. From B to G doses were progressively removed by using the corresponding genotypes: (B) HoxDDel/+; (C) HoxDDel/+;Hoxa-13+/−; (D) Hoxd-13/HoxDDel; (E) HoxDDel/Del; (F) Hoxd-13−/−/;Hoxa-13+/−; (G) HoxDDel/Del;Hoxa-13+/−. (H) The length of digit IV was measured, expressed as fraction of wild-type digit length, and plotted against the Hox dose (see Materials and Methods for the calculation of the respective dose per gene). In this way, the length of digit IV varied as a linear function of the dose, regardless of the nature of the combination. (AG) Anterior is up, posterior is down. I–V indicate digit number with, by convention, digit I being the thumb. In mutants with more or less than five digits, the phalanx pattern makes individual homologization impossible, but the digit found at the position corresponding to wild-type digit IV was always the longest.
Figure 2
Figure 2
Dose-dependent variation of the dactyly, as illustrated by the feet of mice of various genotypes. The posterior parts of both complexes are schematized below with the genes whose functions were removed in black. (A) Wild type. (B) HoxDDel/+. (C) HoxDDel/Del. (D) Hoxd-13−/−;Hoxa-13+/−. (E) HoxDDel/Del;Hoxa-13+/−. The progressive ectrodactyly goes together with a transition from pentadactyly (A and B) to polydactyly (C) to oligodactyly (E). Digits I (anterior to the left) and V are indicated as well as the most posterior digit IV or VI (small arrowheads in C and D). The large arrowheads in C and D point to a digit I specific alteration produced in absence of Hoxd-13 (2), which identifies this digit. Thus, although the phalanx pattern does not allow homologies with other digits, the extra digit belongs to the II-V domain.
Figure 3
Figure 3
Relationship between the sizes of the penian bone, the baculum, and digit IV in HoxD and HoxA mutant juvenile male mice. (A) Forelimb digit IV (Left) and the penis (Right) of 11-day-old juvenile sibling males were isolated, cleared, and compared. The genotypes are indicated on the right. An extreme reduction of the baculum was observed in mice of the HoxDDel/Del;Hoxa-13+/− genotypes, i.e., mice with the shortest digits. In such specimen, the baculum was ill-formed and barely half of the expected length and thickness. (B) As control for the overall size of the mice, the scapula of the same two genotypes are shown. Although a noticeable reduction of the scapula in HoxDDel/Del;Hoxa-13+/− animals indicate their general shorter statures, this reduction was much less drastic than that observed for the penian bone, which was almost absent.
Figure 4
Figure 4
Absence of crossregulatory and autoregulatory interferences as judged by a HoxD reporter gene in HoxA and HoxD compound mutant mice. Hoxd-11/lacZ reporter gene expression (4) in pairs of forelimbs (A and B) and hindlimbs (C and D) derived from mice of four different genotypes schematized in between as in Fig. 2. The presumptive digits are labeled from I (anterior to the left) to V. Although the morphology of these developing limbs vary with the genotypes, they all strongly expressed the HoxD reporter gene in both the distal and proximal domains. This expression indicated that neither the HOXD, nor the HOXA13 proteins are necessary for the function of the Hoxd transgene in limbs. (E) Hoxd-11/lacZ reporter gene expression in a forelimb of HoxDDel/Del;Hoxa-11−/− mouse 5 days after birth. Expression of the Hoxd-11/lacZ reporter gene was detected in both the autopod (digits) and around the zeugopod (radius and ulna), indicating that HOXA11 is not required for the activation of HoxD genes in the developing limbs. The forearm of the limb shown under E is truncated because of the absence of both Hoxd-11 and Hoxa-11 functions (7). A, anterior; P, posterior; r, radius; u, ulna; h, humerus.
Figure 5
Figure 5
Scheme showing the relationships between the function of the HoxA and HoxD complexes in distal limbs and the dactyly. Hox complexes are shown either in light gray, when not functional, or bold boxes when functional. In the absence of HoxA and HoxD expression, a complete adactyly is observed. This situation may reflect an ancestral step in which neither complexes had been recruited in distal limbs. Activation of the HoxA complex genes in distal limb, in absence of HoxD function (pathway on the top) coincides with the appearance of a series of truncated digit-like bony elements with a clear polydactyly. The subsequent recruitment of Hoxd genes in the digit domain could lengthen all the digits while reducing their number to the pentadactyl formula. Alternatively, activation of the Hoxd genes first (pathway in the bottom) would have generated an oligodactyl limb, with potentially long digits, and the subsequent activation of the Hoxa genes would have shortened digits and extended their number to five. Paleonthological and developmental evidence support the first pathway (see the text).

Similar articles

See all similar articles

Cited by 50 PubMed Central articles

See all "Cited by" articles

Publication types

LinkOut - more resources

Feedback