''engrailed'' is a homeodomain

transcription factor In molecular biology, a transcription factor (TF) (or sequence-specific DNA-binding factor) is a protein that controls the rate of transcription (genetics), transcription of genetics, genetic information from DNA to messenger RNA, by binding t ...

involved in many aspects of multicellular development. First known for its role in

arthropod Arthropods ( ) are invertebrates in the phylum Arthropoda. They possess an arthropod exoskeleton, exoskeleton with a cuticle made of chitin, often Mineralization (biology), mineralised with calcium carbonate, a body with differentiated (Metam ...

embryological development Prenatal development () involves the development of the embryo and of the fetus during a viviparous animal's gestation. Prenatal development starts with fertilization, in the germinal stage of embryonic development, and continues in fetal deve ...

, working in consort with the

Hox gene Hox genes, a subset of homeobox, homeobox genes, are a gene cluster, group of related genes that Evolutionary developmental biology, specify regions of the body plan of an embryo along the craniocaudal axis, head-tail axis of animals. Hox protein ...

s, ''engrailed'' has been found to be important in other areas of development. It has been identified in many

bilaterian Bilateria () is a large clade of animals characterised by bilateral symmetry during embryonic development. This means their body plans are laid around a longitudinal axis with a front (or "head") and a rear (or "tail") end, as well as a left–r ...

s, including the arthropods, vertebrates, echinoderms, molluscs, nematodes, brachiopods, and polychaetes. It acts as a "selector" gene, conferring a specific identity to defined areas of the body, and co-ordinating the expression of downstream genes.

Protein

''engrailed (en)'' encodes the

homeodomain A homeobox is a Nucleic acid sequence, DNA sequence, around 180 base pairs long, that regulates large-scale anatomical features in the early stages of embryonic development. Mutations in a homeobox may change large-scale anatomical features of ...

-containing transcription factor protein Engrailed. Homologous Engrailed proteins are found in a diversity of organisms. When expressed in the

ectoderm The ectoderm is one of the three primary germ layers formed in early embryonic development. It is the outermost layer, and is superficial to the mesoderm (the middle layer) and endoderm (the innermost layer). It emerges and originates from the o ...

, ''engrailed'' is involved in the production of skeletal material. ''engrailed'', or genes with very similar sequences, are found in all bilaterian animals. ''engrailed'' plays a number of crucial roles in brain development across many species, including the determination of the hindbrain/midbrain border and aiding in neuronal axon guidance. This has led to the suggestion that the gene originally served a neurogenetic function in the ancestral bilaterian. It has been observed to express in the repeated units of arthropods, molluscs, onychophora, annelids, echinoderms and amphioxus. 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. Even where this trait has been secondarily lost (such as in the

onychophora Onychophora (from , , "claws"; and , , "to carry"), commonly known as velvet worms (for their velvety texture and somewhat wormlike appearance) or more ambiguously as peripatus (after the first described genus, ''Peripatus''), is a phylum of el ...

) the gene is still expressed, marking the 'ghosts' of the shelly plates that the ancestral onychophora (i.e. lobopods) are thought to have borne.

Arthropods

In the model organism, ''

Drosophila melanogaster ''Drosophila melanogaster'' is a species of fly (an insect of the Order (biology), order Diptera) in the family Drosophilidae. The species is often referred to as the fruit fly or lesser fruit fly, or less commonly the "vinegar fly", "pomace fly" ...

'', ''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. ''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 Onychophora (from , , "claws"; and , , "to carry"), commonly known as velvet worms (for their velvety texture and somewhat wormlike appearance) or more ambiguously as peripatus (after the first described genus, ''Peripatus''), is a phylum of el ...

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,

gastropods Gastropods (), commonly known as slugs and snails, belong to a large taxonomic class of invertebrates within the phylum Mollusca called Gastropoda (). This class comprises snails and slugs from saltwater, freshwater, and from the land. Ther ...

, bivalves, polyplacophora, and

scaphopod Scaphopoda (plural scaphopods , from Ancient Greek σκᾰ́φης ''skáphē'' "boat" and πούς ''poús'' "foot"), whose members are also known as tusk shells or tooth shells, are a class (biology), class of shelled Marine life, marine inve ...

s) but it has also been co-opted by the

cephalopod A cephalopod is any member of the molluscan Taxonomic rank, class Cephalopoda (Greek language, Greek plural , ; "head-feet") such as a squid, octopus, cuttlefish, or nautilus. These exclusively marine animals are characterized by bilateral symm ...

s in the production of evolutionary novelties such as the tentacles, eyes and funnel. This plasticity in gene function is characteristic of genes ancestrally associated with the nervous system, for instance the ''Hox'' genes, which are also associated with a wide range of derived organs in the cephalopods, but are involved in shell formation in gastropods. The gene has been sequenced in all groups of shelled molluscs, although for some time it eluded identification in the squid ''Loligo''. 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. In cephalopods, ''engrailed'' appears to demark the shell field, but is not necessary for shell formation itself (skeletogenesis). 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.

References

{{Genarch Developmental genes and proteins Evolutionary developmental biology

Protein

Arthropods

Molluscs

See also

References