The Interactive Fly

Evolutionarily conserved developmental pathways

Segmentation and segment polarity - a conserved role for Engrailed

Vertebrate segmentation has been explained as an evolutionary inheritance from either some metameric protostome or from a more closely related deuterostome. AmphiEn is the engrailed homolog in amphioxus (Cephalochordata), an animal that is the closest living invertebrate relative to the vertebrate line. In neurula embryos of amphioxus, AmphiEn is expressed along the anteroposterior axis in metameric stripes, each located in the posterior part of a nascent or newly formed segment. This pattern resembles the expression stripes of Drosophila engrailed, which has a key role in establishing and maintaining the segments of Drosophila. Later in development, amphioxus embryos express AmphiEn in non-metameric patterns. Compared to vertebrate engrailed expression at the midbrain/hindbrain boundary, AmphiEn expression in the cerebral vesicle is relatively late. The segmental expression of AmphiEn in forming somites suggests that the functions of engrailed homologs in establishing and maintaining a metameric body plan may have arisen only once during animal evolution. If so, the protostomes and deuterostomes probably shared a common segmented ancestor (Holland, 1997).

REFERENCES

Holland, L. Z., et al. (1997). Sequence and embryonic expression of the amphioxus engrailed gene (AmphiEn): the metameric pattern of transcription resembles that of its segment-polarity homolog in Drosophila. Development 124: 1723-1732. Medline abstract: 97307778

date revised: 30 June 97

Developmental Pathways conserved in Evolution

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