Evolutionary Developmental Biology: Gene Duplication, Divergence and Co‐option


The eukaryotic genome appears to have arisen by duplication of ancestral genomes and ancestral genes.

Keywords: HOX genes; gene clusters; pattern formation; Pax genes; junk DNA

Figure 1.

Correlations between divergent developmental plans affecting the gnathocaudal (cervicocaudal) body segments and states of individual HOX genes in a cluster/clusters. Each of four animal species is accompanied by its HOX gene composition. In all four animals, those HOX genes derived from the one that originally controlled gnathal development are shown as squares, whereas those derived from the original controller of thorax as well as abdomen are shown in circles. Triangles denote those derived from the AbdB‐like gene exclusively directed toward differentiation of the caudal end segment. (a) An unnamed Cambrian crustacean with about a dozen identical body segments (Lewis, ; Conway Morris, ). In this particular case, the presence of two HOX genes in a cluster probably sufficed for the gnathocaudal differentiation of body segments, the first being the common ancestor of Drosophila, Ubx and abdA, the second being an equivalent of AbdB. (b) A grasshopper‐like insect whose body segmentations are controlled by three HOX genes for differentiation of gnathal segments, two HOX genes each for differentiation of thorax and abdomen. (c) A brine shrimp (Artemia franciscana) as a representative crustacean and its HOX gene cluster (Averof and Akam, ). Because of a relative simplicity of its gnathal structure, only two HOX genes are assigned for differentiation of gnathal segments. (d) About 32 pairs of somites of a mouse embryo are shown as black beads below the central nervous system (shaded area). (e) Skeleton of a mammal‐like reptile Lycaenops (1/14 natural size) of the late Permian period some 200 million years ago. Shown below it are four clusters (A, B, C and D, from the top to the bottom) of HOX genes that originally numbered 13. Those subsequently disappeared as shown by dotted lines. The last five downstream HOX genes controlled development not of vertebrae but of front limb bones from scapula to phalangeal bones and these five HOX genes originated from an equivalent of AbdB (Holland et al., ; Condie and Capecchi, ; Davis et al., ).



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Ohno†, Susumo, and Holmquist, Gerald P(Oct 2001) Evolutionary Developmental Biology: Gene Duplication, Divergence and Co‐option. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1038/npg.els.0001062]