Temporal colinearity and the phylotypic progression: a basis for the stability of a vertebrate Bauplan and the evolution of morphologies through heterochrony

Abstract

Vertebrate Hox genes are essential for the proper organiz ation of the body plan during development. Inactivation of these genes usually leads to important alterations, or transformations, in the identities of the affected develop ing structures. Hox genes are activated in a progressive temporal sequence which is colinear with the position of these genes on their respective complexes, so that ‘anterior’ genes are activated earlier than ‘posterior’ ones (temporal colinearity). Here, an hypothesis is considered in which the correct timing of activation of this gene family is necessary in order to properly establish the various expression domains. Slight modifications in the respective times of gene activation (heterochronies) may shift expression domains along the rostrocaudal axis and thus induce concurrent changes in morphologies. It is further argued that temporal colinearity only occurs in cells with high mitotic rates, which results in a strong linkale between patterning and growth control and makes the pat terning process unidirectional, from anterior, proximal and early, to posterior, distal and late, a model referred to as the ‘Einbahnstrasse’. While the nature of the mechanism(s) behind temporal and spatial colinearities is unknown, it is proposed that such a mechanism relies on meta-cis interactions, that is it may necessitate gene conti guity. Such a mechanism would be based on DNA-specific, rather than gene-specific, features such as chromatin con figurations or DNA replication. The existence of such a meta-cis mechanism would explain the extraordinary con servation of this genetic system during evolution as its basic properties would be linked to that of the genetic material itself. Consequently, it is hypothesized that, in vertebrates, the resistance of this mechanism to evolutionary variations may be the reason for the existence of a short develop mental window of morphological invariance (the phylo typic progression).