Amphioxus is a popular creature in evo-devo circles, as it is the closest living relative of vertebrates. This singular little fish-like creature (amphioxus literally means sharp at both ends) has a notochord but, unlike vertebrates, it has no skull and only a simple forerunner of the brain. The fact that amphioxus has no head is central to the transgenic studies reported by Peter Holland's group, who have recently demonstrated that the vertebrate head evolved by tweaking the more rudimentary developmental systems that specify the anterior-most structures in these invertebrates.

Hox genes encode a family of conserved transcription factors that specify the location of cells along the anterior–posterior axis of the embryo. Amphioxus is like the other invertebrates in that it has a single cluster of Hox genes, whereas all vertebrates have three to eight copies of each set.

Now, these authors show that the control regions of amphioxus Hox genes that pattern the anterior-most structures are recognized in vertebrates, where they can drive expression to specific head structures.

Seven cis-regulatory sequences, taken from three amphioxus Hox genes (AmphiHox1–3), were used to drive expression of a reporter gene (lacZ) in mouse and chick embryos. In both model vertebrates, the same four DNA fragments reproduced the expression of native vertebrate Hox genes in three head tissues: the developing hindbrain, the neural crest and the neurogenic placodes — the latter two of which are absent in amphioxus.

In addition, the expression of lacZ was sensitive to the concentration of retinoic acid, which is believed to be one of the morphogens that controls the spatial expression of individual Hox genes along the anterior–posterior axis.

Nature could have devised a whole new genetic network to create a head where previously there was none. Instead, it seems that she has economically crafted evolutionary innovations by manipulating the existing genetic machinery.

We are used to hearing about the conservation of gene coding regions, but probably less so about the conservation of their control regions, which, by contrast, we tend to think of as engendering change by altering the time and place of gene expression. This work is therefore remarkable in highlighting the strong selection pressures that have preserved genetic networks over the 500 million years that separate amphioxus from its vertebrate (and head-endowed) relatives.