Cnidarian homeoboxes and the zootype


In a widely cited paper1, Slack et al. proposed that all animal phyla shared a particular pattern of gene expression, the “zootype”. The zootype hypothesis implies that at least six HOX-type homeobox-containing genes should be present in all metazoa. We have done a series of phylogenetic analyses which indicate that Cnidaria, thought to be the earliest-evolving animal phylum with the exception of the sponges2, lack several HOX genes that are present in Drosophila and vertebrates. Instead, these genes may have arisen by duplication after the origin of the Cnidaria.

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Figure 1: HOX gene cluster present in the common ancestor of all metazoans according to the zootype hypothesis, and HOX gene cluster present in the common ancestor of cnidarians and triploblasts according to our results, with Drosophila shown for comparison.


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Martínez, D., Bridge, D., Masuda-Nakagawa, L. et al. Cnidarian homeoboxes and the zootype. Nature 393, 748–749 (1998).

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