engrailed (gene)
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Biomineralization |
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engrailed is a homeodomain
Protein
engrailed (en) encodes the
Whilst the gene was traditionally understood to have served a role in segment polarization in the ancestral bilaterian, its association with shell formation in molluscs has produced an alternative hypothesis: that the ancestral role was associated with mineralization.[6] Even where this trait has been secondarily lost (such as in the onychophora) the gene is still expressed, marking the 'ghosts' of the shelly plates that the ancestral onychophora (i.e. lobopods) are thought to have borne.[4]
Arthropods
In the model organism, Drosophila melanogaster, engrailed acts as a segment polarity gene in early embryonic development. It is initially expressed in stages 8–11 of development in 14 isolated bands of cells along the embryo's anterior–posterior axis. The cells expressing engrailed define the anterior-most region of each parasegment. Once proper segments form, engrailed-expressing cells are found in the posterior-most region of each segment.[7]
engrailed homologs have also been found in many other arthropod species, including grasshoppers, milkweed bugs, centipedes, and beetles.
However, the ancestral role of engrailed was not in marking segmentation: it does not fulfill this role in Onychophora.[4]
Molluscs
Although it is not necessary for mineralization to occur, molluscs use engrailed to mark the boundaries of shell-forming fields (this has been demonstrated in cuttlefish,
In the scaphopods, engrailed is active in the development of the larval shell, but not the adult conch (a separate entity), suggesting a different evolutionary origin of the mature shell.[10] In cephalopods, engrailed appears to demark the shell field, but is not necessary for shell formation itself (skeletogenesis).[11]
It has been argued that engrailed was only co-opted to skeletal function in molluscs, and that its original function was related to segmentation, not biomineralization; whilst there is no consensus yet on which of these alternatives is correct, a role in biomineralization seems the more parsimonious.[12]