The significance of Hox gene collinearity

Abstract

Arthropods and vertebrates inherited their Hox clusters from an ancestral cluster of at least six genes already present in their last common ancestor, Urbilateria. Clustering and a common transcriptional direction are both likely features of the way that the gene complex first arose in a process of tandem gene duplication. Spatial collinearity (correspondence between ordering of Hox genes along the chromosome and their expression patterns along the head-tail axis) has been conserved in many animal groups and is likely to have been already present in Urbilateria. It is not known why the Hox cluster evolved with spatial collinearity. Four models are discussed. These vary in the significance they place upon Hox chromatin structure, and also on whether they propose that collinearity is primarily concerned with establishment or maintenance of Hox expression. Published proposals to explain spatial collinearity, which invoke enhancer sharing, chromatin closing or chromatin opening, are either problematic or can offer only partial explanations. In an alternative proposal it is suggested here that spatial collinearity evolved principally to maximise physical segregation, and thereby minimise incidence of boundaries, between active and inactive genes within the Hox cluster. This is to minimise erroneous transfer of transcriptional activity, or inactivity, between adjacent Hox genes.

ADHFe1: a novel enzyme involved in retinoic acid-dependent Hox activation

“…The trunk organizer is responsible for the formation and specification of embryonic regions caudal to the head domain. The vertebrate trunk domain is one of the main regions patterned by differential Hox gene activity (Durston, Zhu, 2015;Gaunt, 2015). Our results show that manipulation of ADHFe1 activity levels leads to abnormal expression of the Hox genes.…”

Ubiquitination-Independent Repression of PRC1 Targets during Neuronal Fate Restriction in the Developing Mouse Neocortex

“…Hox genes are typical PcG targets, with Ring1 having been found to temporarily repress the transcription of these genes in a largely Ub-dependent manner in mouse ES cells (Endoh et al, 2012). The persistent repression pattern of Hox gene loci is established earlier than the emergence of neocortical NPCs during mouse development (Akasaka et al, 2001;Schwartz and Pirrotta, 2007;Gaunt, 2015). As expected, the loss of Ring1B in Ring1A-deficient NPCs resulted in marked derepression of Hoxa1 and Hoxd8 expression in both early-stage (neurogenic) and late-stage (astrogliogenic) NPCs ( Figures 3A-3D).…”

Somites Without a Clock

“…Could the ectopic somites be occipital? We examined expression of Hox genes (20, 21) (Fig. 4, A to P): Hoxb3 (Fig.…”