Evolution and development (Evo-Devo) is becoming a fashionable preoccupation in modern biology. This is due to the fact that evolutionists discover development, while developmental biologists discover evolution. I believe that a major reason for this common interest originated in the unexpected discovery that the early stages of embryonic development and especially the establishment of the antero-posterior axis were homologous features in Drosophila and Vertebrates. More precisely, Drosophila geneticists progressively identified a series of genes (called homeotic) which were responsible for the fate of successive body segments and for their differentiation into head, thorax and abdomen. Surprisingly, there was a colinearity between the order of these homeotic genes on the third chromosome, and of their expression along the antero-posterior axis of the embryo. When these genes were cloned, it was rapidly found that they also existed in Vertebrates, and surprisingly in the same order on the chromosome (Gehring 1998). More precisely these genes, called Hox genes, form four paralogous clusters on four different chromosomes. The existence of these four clusters confirms the hypothesis put forward three decades ago (Ohno 1970) that, during Vertebrate evolution, two successive tetraploidisations took place.
The similar order of the genes in Vertebrates and Drosophila (and in several other phyla) implies for example that the same genes are expressed in the head of both taxa. It was tempting at the beginning to assume that the head of a fruit fly and the head of a mouse might be homologous organs. However this conclusion would be wrong. Drosophila and Vertebrates are distantly related Metazoa. Vertebrates belong to chordates within Deuterostomia while Drosophila belongs to Ecdysozoa within Protostomia (De Rosa et al. 1999). We have seen that in chordates, an ancestral state was very likely deprived of a differentiated head, and that the Vertebrate head was a new acquisition depending on the neural crest. Besides Arthropods, Ecdysozoa comprise other phyla which, like nematodes, do not possess a differentiated head (De Rosa et al. 1999). The absence of a head is likely a primitive state; in other words, the insect head is also likely to be a new acquisition, rather than being homologous to the vertebrate head.
The fact that the same genes are expressed during head development in both Drosophila and Vertebrates must be considered as an analogy and an evolutionary convergence. Why this convergence used the same genes to produce analogous organs is unclear. One interpretation is that evolution was constrained by a limited set of developmental genes and was obliged to reutilize the same genes for building new organs and new functions. It is also possible that the functional order of the homeotic genes along the chromosome was an additional constraint, preventing the building of a head with completely unrelated genes.
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