Developmental Genetics of Amphioxus: Insights into the Origin and Evolution of the Vertebrate Body Plan

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The molecular machinery underlying development is remarkably similar among animals, and this provides a powerful tool for suqqesting body part homologies between distantly related animals. Dr. Holland comparing vertebrates with the cephalochordate, amphioxus-which, in many ways, approximates the invertebrate body plan from which the vertebrates evolved. Developmental genes of amphioxus are isolated and identified on the basis of sequence similarities with homologous genes of other animals, and the expression domains of these genes are used as phenotypic characters to indicate body part homologies. Hox genes and Engrailed gene are establishing the limits of the amphioxus hindbrain, and several genes (e.g. Wnts, Distal-less, PAX7, and NK2) are being used to map the regions of the amphioxus forebrain and midbrain. A similar rationale underlies comparison between vertebrate eyes and amphioxus photoreceptors (Pax6), between vertebrate nephrons and amphioxus nephridia (Pax2), and between the vertebrate thyroid and the amphioxus endostyle (Pax8, TTFL). In addition, developmental gene expression patterns after teratogen administration or microsurgery are identifying epigenetic networks common to vertebrates and amphioxus.