Some Questions and Answers About the Role of Hox Temporal Collinearity in Vertebrate Axial Patterning

Abstract

The vertebrate anterior-posterior (A-P = craniocaudal) axis is evidently made by a timing mechanism. Evidence has accumulated that tentatively identifies the A-P timer as being or involving Hox temporal collinearity (TC). Here, I focus on the two current competing models based on this premise. Common features and points of dissent are examined and a common model is distilled from what remains. This is an attempt to make sense of the literature.

Mammalian-specific ectodermal enhancers control the expression of Hoxc genes in developing nails and hair follicles

“…While the impact of this temporal process upon the colinear distribution of Hox transcripts in space is still a matter of discussion (see, e.g., refs. [43][44][45][46], the mechanism underlying this phenomenon is accompanied by a progressive transition from a negative to a transcriptionally permissive chromatin structure (47). This phenomenon is however observed only during the first wave of activation of a given Hox cluster.…”

A Tribute to Lewis Wolpert and His Ideas on the 50th Anniversary of the Publication of His Paper ‘Positional Information and the Spatial Pattern of Differentiation’. Evidence for a Timing Mechanism for Setting Up the Vertebrate Anterior-Posterior (A-P) Axis

“…There are various morphogens that are thought to be involved in setting up the main body's A-P axis and the limb's P-D axis. Their roles in relation to the timing mechanism considered here have been discussed above and elsewhere [41,43]. However, this aspect deserves further attention.…”

“…Please note that each of these external pathways acts only over a certain limited developmental period and only over a certain limited part of the A-P axis. The (extensive) evidence for this mechanism is given in three recent reviews [19,41,43]. Space prohibits repeating it here again.…”

A Tribute to Lewis Wolpert on the 50th Anniversary of the Publication of His Paper ‘Positional Information and the Spatial Pattern of Differentiation’. Evidence for a Timing Mechanism for Setting Up the Vertebrate Anterior-Posterior (A-P) Axis

“…The (extensive) evidence for this mechanism is given in three recent reviews: Deschamps and Duboule, 2017, Durston, 2019cand Durston, 2019d. Space prohibits repeating it here again; Please note that many of the features of this mechanism are also relevant for limb P-D patterning: namely, retinoids, FGF, Hox genes, chromatin histone acetylation, collinear opening of Hox cluster chromatin, Hox-Hox interactions, action of different morphogens at different stages..…”