
A segmentation gene is a
gene
In biology, the word gene has two meanings. The Mendelian gene is a basic unit of heredity. The molecular gene is a sequence of nucleotides in DNA that is transcribed to produce a functional RNA. There are two types of molecular genes: protei ...
involved in the early developmental stages of pattern formation. It regulates how cells are organized and defines repeated units in the
embryo
An embryo ( ) is the initial stage of development for a multicellular organism. In organisms that reproduce sexually, embryonic development is the part of the life cycle that begins just after fertilization of the female egg cell by the male sp ...
. Segmentation genes have been documented in three taxa:
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 ...
s (i.e.
insect
Insects (from Latin ') are Hexapoda, hexapod invertebrates of the class (biology), class Insecta. They are the largest group within the arthropod phylum. Insects have a chitinous exoskeleton, a three-part body (Insect morphology#Head, head, ...
s and
crab
Crabs are decapod crustaceans of the infraorder Brachyura (meaning "short tailed" in Greek language, Greek), which typically have a very short projecting tail-like abdomen#Arthropoda, abdomen, usually hidden entirely under the Thorax (arthropo ...
s),
chordate
A chordate ( ) is a bilaterian animal belonging to the phylum Chordata ( ). All chordates possess, at some point during their larval or adult stages, five distinctive physical characteristics ( synapomorphies) that distinguish them from ot ...
s (i.e.
mammal
A mammal () is a vertebrate animal of the Class (biology), class Mammalia (). Mammals are characterised by the presence of milk-producing mammary glands for feeding their young, a broad neocortex region of the brain, fur or hair, and three ...
s and
fish
A fish (: fish or fishes) is an aquatic animal, aquatic, Anamniotes, anamniotic, gill-bearing vertebrate animal with swimming fish fin, fins and craniate, a hard skull, but lacking limb (anatomy), limbs with digit (anatomy), digits. Fish can ...
), and
annelid
The annelids (), also known as the segmented worms, are animals that comprise the phylum Annelida (; ). The phylum contains over 22,000 extant species, including ragworms, earthworms, and leeches. The species exist in and have adapted to vario ...
s (i.e.
leech
Leeches are segmented parasitism, parasitic or Predation, predatory worms that comprise the Class (biology), subclass Hirudinea within the phylum Annelida. They are closely related to the Oligochaeta, oligochaetes, which include the earthwor ...
es and
earthworm
An earthworm is a soil-dwelling terrestrial invertebrate that belongs to the phylum Annelida. The term is the common name for the largest members of the class (or subclass, depending on the author) Oligochaeta. In classical systems, they we ...
s). In ''
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" ...
'', a common fruit fly, segmentation genes divide the embryo into 14 parasegments which are among the first compartments to form within the embryo. Rare variants in segmentation genes can cause changes in appearance of differing severity depending on its type. The genes can be classified into 3 groups:
Gap genes,
Pair-rule genes and
Segment polarity genes.
Gap genes
Gap genes are among the first genes expressed in the embryo. Here, expression refers to the
translation
Translation is the communication of the semantics, meaning of a #Source and target languages, source-language text by means of an Dynamic and formal equivalence, equivalent #Source and target languages, target-language text. The English la ...
of the gene. Gap genes were named as such because loss-of-function variants in gap genes resulted in large deletions (or gaps) in the neighbouring segments of the embryo. The expression of gap genes is regulated by maternally deposited factors called
maternal effect
A maternal effect is a situation where the phenotype of an organism is determined not only by the environment it experiences and its genotype, but also by the environment and genotype of its mother. In genetics, maternal effects occur when an orga ...
genes. Maternal effect genes encode factors like
messenger RNA
In molecular biology, messenger ribonucleic acid (mRNA) is a single-stranded molecule of RNA that corresponds to the genetic sequence of a gene, and is read by a ribosome in the process of synthesizing a protein.
mRNA is created during the ...
needed for early development such as
cell division
Cell division is the process by which a parent cell (biology), cell divides into two daughter cells. Cell division usually occurs as part of a larger cell cycle in which the cell grows and replicates its chromosome(s) before dividing. In eukar ...
. One of their main roles is to provide polarity and sense of direction to the embryo: which region will become the
anterior
Standard anatomical terms of location are used to describe unambiguously the anatomy of humans and other animals. The terms, typically derived from Latin or Greek roots, describe something in its standard anatomical position. This position pro ...
or the head region, and which region will become the
posterior or the tail region.
For instance, the mRNA of ''bicoid'', a maternal affect gene, is transported to the anterior region of the embryo and then spreads toward the posterior region.
This creates a concentration gradient where ''bicoid'' expression is highest in the anterior and gradually decreases towards the posterior.
''
Bicoid
Homeotic protein bicoid is encoded by the ''bcd'' maternal effect gene in ''Drosophilia''. Homeotic protein bicoid concentration gradient patterns the anterior-posterior (A-P) axis during ''Drosophila'' embryogenesis. Bicoid was the first prot ...
'' along with other maternal effect genes like ''nanos'' create multiple
concentration gradient
Fick's laws of diffusion describe diffusion and were first posited by Adolf Fick in 1855 on the basis of largely experimental results. They can be used to solve for the diffusion coefficient, . Fick's first law can be used to derive his second ...
s that regulate the expression of gap genes.
Gap genes are expressed in large sections of the embryo multiple parasegments wide. ''Kruppel'', for instance, is expressed in parasegments 4-6. There are at least 6 types of gap genes but the three that are well-known are ''
hunchback
Kyphosis () is an abnormally excessive convex curvature of the spine as it occurs in the thoracic and sacral regions. Abnormal inward concave ''lordotic'' curving of the cervical and lumbar regions of the spine is called lordosis.
It can ...
,'' ''knirps'', and ''
kruppel''.
Different concentration gradients of gap genes establish parasegment boundaries.
These parasegment boundaries help regulate or control the expression of pair-rule genes as well as segment polarity genes.
Lastly, the gap genes also play a role in later development such as giving rise to neurons along with formation of muscles and the gut.
Pair-rule genes
Pair-rule genes are genes that are expressed in alternating parasegments of the embryo for a total of 7-8 parasegments. The boundaries of parasegments are not determined by grooves that can be seen on the embryo but are compartments that show gene expression. One parasegment is made from the back half of a visible segment (not parasegment) and the front half of the visible segment behind it. An expression of a pair-rule gene in one parasegment is followed by a region of no expression in the following parasegment. For example, odd-skipped genes are expressed in alternating even-numbered parasegments (stripe 2, 4, and so on) while even-skipped genes are expressed in odd-numbered parasegments (stripe 1, 3, and so on). They were termed as such because loss-of-function variants in even-skipped genes can cause the disappearance of odd-numbered parasegments only leaving behind the even-numbered parasegments, hence, the name. Lastly, the pair-rule genes regulate the expression of segment polarity genes.
Segment polarity genes
Segment polarity genes are expressed in distinct regions within a parasegment. A parsegment is divided into anterior - the head -region, and the posterior - the tail - region. One segment polarity gene,
''engrailed'', is expressed in the anterior part of each parasegment while another, ''
wingless
In cellular biology, the Wnt signaling pathways are a group of signal transduction pathways which begin with proteins that pass signals into a cell through cell surface receptors. The name Wnt, pronounced "wint", is a portmanteau created from the ...
'', is expressed in the posterior region. Loss-of-function variants in ''engrailed'', for instance, can result in defects within the anterior portions of each parasegment. Lastly, certain segment polarity genes like ''wingless'' are involved in the planning and development of body parts such as the wings.
References
{{Reflist
Embryology