DEVELOPMENTAL GENE-EXPRESSION IN AMPHIOXUS - NEW INSIGHTS INTO THE EVOLUTIONARY ORIGIN OF VERTEBRATE BRAIN-REGIONS, NEURAL CREST, AND ROSTROCAUDAL SEGMENTATION

Amphioxus is widely held to be the closest invertebrate relative of th e vertebrates and the best available stand-in for the proximate ancest or of the vertebrates. The spatiotemporal expression patterns of devel opmental genes can help suggest body part homologies between vertebrat es and amphioxus, This approach is illustrated using five homeobox gen es (AmphiHox1, AmphiHox2, AmphiOtx, AmphiDll, and AmphiEn) to pro,ide insights into the evolutionary origins of three important vertebrate f eatures: the major brain regions, the neural crest, and rostrocaudal s egmentation. During amphioxus development, the neural expression patte rns of these genes are consistent with the presence of a forebrain (de tailed neuroanatomy indicates that the forebrain is all diencephalon w ithout any telencephalon) and an extensive hindbrain; the possible pre sence of a midbrain requires additional study. Further, during neurula tion, the expression pattern of AmphiDll as web as migratory cell beha vior suggest that the epidermal cells bordering the neural plate may r epresent a phylogenetic precursor of the vertebrate neural crest. Fina lly, when the paraxial mesoderm begins to segment, the earliest expres sion of AmphiEn is detected in the posterior part of each nascent and newly formed somite, This pattern recalls the expression of the segmen t-polarity gene engrailed during establishment of the segments of meta meric protostomes. Thus, during animal evolution, the role of engraile d in establishing and maintaining metameric body plans may have arisen in a common segmented ancestor of both the protostomes and deuterosto mes.