SEQUENCE AND EMBRYONIC EXPRESSION OF THE AMPHIOXUS ENGRAILED GENE (AMPHIEN) - THE METAMERIC PATTERN OF TRANSCRIPTION RESEMBLES THAT OF ITS SEGMENT-POLARITY HOMOLOG IN DROSOPHILA

Vertebrate segmentation has been proposed as an evolutionary inheritan ce either from some metameric protostome or from a more closely relate d deuterostome, To address this question, we studied the developmental expression of AmphiEn, the engrailed gene of amphioxus, the closest l iving invertebrate relative of the vertebrates, In neurula embryos of amphioxus, AmphiEn is expressed along the anteroposterior axis as meta meric stripes, each located in the posterior part of a nascent or newl y formed segment, This pattern resembles the expression stripes of the segment-polarity gene engrailed, which has a key role in establishing and maintaining the metameres in embryos of Drosophila and other meta meric protostomes, Later, amphioxus embryos express AmphiEn in non-met americ patterns - transiently in the embryonic ectoderm and dorsal ner ve cord. Nerve cord expression occurs in a few cells approximately mid way along the rostrocaudal axis and also in a conspicuous group of ant erior cells in the cerebral vesicle at a level previously identified a s corresponding to the vertebrate diencephalon. Compared to vertebrate engrailed expression at the midbrain/hindbrain boundary, AmphiEn expr ession in the cerebral vesicle is relatively late, Thus, it is uncerta in whether the cerebral vesicle expression marks the rostral end of th e amphioxus hindbrain; if it does, then amphioxus may have little or n o homolog of the vertebrate midbrain, The segmental expression of Amph iEn in forming somites suggests that the functions of engrailed homolo gs in establishing and maintaining a metameric body plan may have aris en only once during animal evolution, If so, the protostomes and deute rostomes probably shared a common segmented ancestor.