Kondo and collaborators recently reported the absence of Hox temporal collinearity in Xenopus tropicalis. They found none in the initiation of accumulation of Hox transcripts (detected via RNA seq). And none in the initial expression sequence of primary unprorocessed transcripts (Identified by using qRT-PCR against introns or intron-exon boundaries). Nor in the initial acquisition by Hox gene DNA of a mark for active chromatin. These findings are in conflict with the idea that temporal collinearity has to do with the initiation of Hox gene transcription or with the opening of and a progression from repressed to active states in Hox chromatin. But collinear acquisition of the same active chromatin mark has been shown by others in murine 5’ Hoxd cluster genes.The reason for this difference is unknown . This careful study thus indicated that the initiation phase of Hox expression shows no temporal collinearity in X. tropicalis. A previous study in X. laevis from the same group also showed that the sequence of times for reaching (normalised) half maximal Hox expression showed no temporal collinearity. These conclusions are likely to be correct. These authors do however also conclude that “experimental evidence for the temporal collinearity hypothesis is not strong” There is however strong evidence that Hox temporal collinearity does occur in early vertebrate embryos. Below. I present and discuss 3 lines of evidence to resolve the present conflict I argue that Hox temporal collinearity actually does exist and that it is part of a central mechanism in early development.
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