''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 of genetic information from DNA to messenger RNA, by binding to a specific DNA sequence. The fu ...
involved in many aspects of multicellular development. First known for its role in
arthropod
Arthropods (, (gen. ποδός)) are invertebrate animals with an exoskeleton, a Segmentation (biology), segmented body, and paired jointed appendages. Arthropods form the phylum Arthropoda. They are distinguished by their jointed limbs and Arth ...
embryological development
Prenatal development () includes 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 genes, are a group of related genes that specify regions of the body plan of an embryo along the head-tail axis of animals. Hox proteins encode and specify the characteristics of 'position', ensuring that the cor ...
s, ''engrailed'' has been found to be important in other areas of development. It has been identified in many
bilaterian
The Bilateria or bilaterians are animals with bilateral symmetry as an embryo, i.e. having a left and a right side that are mirror images of each other. This also means they have a head and a tail (anterior-posterior axis) as well as a belly an ...
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 DNA sequence, around 180 base pairs long, that regulates large-scale anatomical features in the early stages of embryonic development. For instance, mutations in a homeobox may change large-scale anatomical features of the full ...
-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 t ...
, ''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 grc, ονυχής, , "claws"; and , , "to carry"), commonly known as velvet worms (due to their velvety texture and somewhat wormlike appearance) or more ambiguously as peripatus (after the first described genus, '' Peripatus ...
) 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 (the taxonomic 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" or "pomace fly". Starting with Ch ...
'', ''engrailed'' acts as a segment polarity gene
A segmentation gene is a generic term for a gene whose function is to specify tissue pattern in each repeated unit of a segmented organism. Animals are constructed of segments; however, Drosophila segments also contain subdivided compartments. The ...
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 grc, ονυχής, , "claws"; and , , "to carry"), commonly known as velvet worms (due to their velvety texture and somewhat wormlike appearance) or more ambiguously as peripatus (after the first described genus, '' Peripatus ...
.[
]
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
The gastropods (), commonly known as snails and slugs, belong to a large taxonomic class of invertebrates within the phylum Mollusca called Gastropoda ().
This class comprises snails and slugs from saltwater, from freshwater, and from land. The ...
, bivalves,[ polyplacophora,][ and scaphopods)][ but it has also been co-opted by the ]cephalopod
A cephalopod is any member of the molluscan class Cephalopoda (Greek plural , ; "head-feet") such as a squid, octopus, cuttlefish, or nautilus. These exclusively marine animals are characterized by bilateral body symmetry, a prominent head ...
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.
See also
* ''EN2'' (gene)
References
{{Genarch
Developmental genes and proteins
Evolutionary developmental biology