The Yin and Yang of Evo/Devo

nervous system, and possibly even segmentation. In Michael Akam University Museum of Zoology fact, something rather like an annelid. Downing Street Peter Holland's report on a " Sister of Hox " gene clus-Cambridge CB2 3EJ ter added a new twist to this question. Holland (Univer-United Kingdom sity of Reading, U.K.) has been trying to make sense of several homeobox genes that have sequences closely related to the Hox genes, but which are not in the Hox clusters—the caudal, Xlox/Pdx, and Gsh gene families. For the last ten years, the evolutionary interests of devel-Phylogenetic analysis links these genes specifically with opmental biologists have focused on universality— the Hox genes of anterior, middle, and posterior paral-conserved genes, doing the same job in different phyla. ogy groups respectively, suggesting that they arose It is hardly surprising then that this theme dominated after the gene duplications that gave rise to the multiple the " Development and Evolution " meeting sponsored genes of the Hox cluster itself. They have previously by the Juan March Foundation and held November 3–5, been viewed as genes that jumped out of the Hox clus-1997. Peter Lawrence (MRC, Cambridge) chose to em-ter. Holland has now found that all three are linked in the phasize a different perspective, though, in his opening Cephalochordate Amphioxus, defining a gene cluster remarks: " How have animals evolved to be so different " which he calls the Sister of Hox cluster (N. M. Brooke (Figure 1). These complementary themes are the Yin and et al., unpublished data). The conserved characteristic Yang of evolutionary developmental biology. I suspect of the genes in this sister cluster is that they are involved that over the next decade we shall see the emphasis with gut patterning. In Amphioxus, they are expressed moving from one to the other. colinearly in the endoderm, while the vertebrate Xlox Making an animal is not just " painting by genes " — homolog, Pdx specifies the pancreas (Offield et al., mapping out the body plan with conserved transcription 1996). Distinct Hox and " Sister of Hox " genes are found factors. That is where the process starts, defining do-in protostomes and deuterostomes, but probably not in mains and making the borders that are key elements in Cnidaria, so the duplication that generated the two sister patterning. But the next step is just as important—filling clusters must have occurred some time between the in the …