Fig. 4.

“Gene family tree” of vertebrate Alx homeobox genes in chordate evolution. After the two whole genome duplications (WGD, two green arrows at bottom of diagram) in early vertebrate evolution, four Alx homeobox genes were generated. The “Alx2” gene (yellow line) may have been swiftly lost, whereas the Alx3 genes were lost independently in at least three lineages (shown as the stars). Further modification in the teleost fish lineage followed the third WGD. The diagram depicts the phylogenetic location of each Alx3 gene loss, but note that the timing of loss within each lineage has not been determined.