segmentation gene A segmentation gene is a gene involved in the early stages of pattern formation that define repeated units (metameres) in a segmented organism, usually the embryo An embryo is an initial stage of development of a multicellular organism. In org ...

is a generic term for a gene whose function is to specify tissue

pattern A pattern is a regularity in the world, in human-made design, or in abstract ideas. As such, the elements of a pattern repeat in a predictable manner. A geometric pattern is a kind of pattern formed of geometric shapes and typically repeated li ...

in each repeated unit of a segmented organism.

Animals Animals are multicellular, eukaryotic organisms in the biological kingdom Animalia. With few exceptions, animals consume organic material, breathe oxygen, are able to move, can reproduce sexually, and go through an ontogenetic stage in ...

are constructed of segments; however,

Drosophila ''Drosophila'' () is a genus of flies, belonging to the family Drosophilidae, whose members are often called "small fruit flies" or (less frequently) pomace flies, vinegar flies, or wine flies, a reference to the characteristic of many speci ...

segments also contain subdivided compartments. There are five gene classes which each contribute to the segmentation and development of the embryonic ''drosophila''. These five gene classes include the coordinate gene, gap gene, pair-rule gene, segment polarity gene, and homeotic gene. In embryonic ''drosophila'', the pair-rule gene defines odd-skipped and even-skipped genes as parasegments, showing 7 stripes in the embryo. In the next gene class, segment polarity gene, individual segments each have their own anterior and posterior pole, resulting in 14 segments. In the fruit fly ''Drosophila melanogaster'', segment polarity genes help to define the anterior and posterior polarities within each embryonic parasegment by regulating the transmission of signals via the

Wnt signaling pathway 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 is a portmanteau created from the names Wingless and Int-1. Wnt signaling ...

and

Hedgehog signaling pathway The Hedgehog signaling pathway is a signaling pathway that transmits information to embryonic cells required for proper cell differentiation. Different parts of the embryo have different concentrations of hedgehog signaling proteins. The pathwa ...

. Segment polarity genes are expressed in the embryo following expression of the

gap gene A gap gene is a type of gene involved in the development of the segmented embryos of some arthropods. Gap genes are defined by the effect of a mutation in that gene, which causes the loss of contiguous body segments, resembling a gap in the norm ...

s and

pair-rule gene A pair-rule gene is a type of gene involved in the development of the segmented embryos of insects. Pair-rule genes are expressed as a result of differing concentrations of gap gene proteins, which encode transcription factors controlling pair- ...

s. The most commonly cited examples of these genes are ''engrailed'' and ''gooseberry'' in ''Drosophila melanogaster''. The segment polarity is the last step in embryonic development and a repeated pattern where each half of each segment is deleted and a mirror-image is duplicated and reversed to replace that half segment; thus, forming a pattern element.

Segment polarity in Drosophila

Segmentation polarity occurs during the release of

morphogens A morphogen is a substance whose non-uniform distribution governs the pattern of tissue development in the process of morphogenesis or pattern formation, one of the core processes of developmental biology, establishing positions of the various ...

, which functions to differentiate patterns within sections.http://cuttlefish.bio.indiana.edu:7082/allied-data/lk/interactive-fly/aignfam/sgmtplty.htm#dafka The development of a pattern depends on the gradients of these morphogens.

''Engrailed''

In ''Drosophila'', the ''engrailed'' gene is expressed only in cells within the posterior section of every segment. Its role is to distinguish posterior from anterior sections of each segment. ''Engrailed'' expression is generally restricted to cells in the posterior compartment but research suggests it may have other functions.

''Gooseberry''

The ''gooseberry'' gene's role in segmentation was believed to be involved in segment-polarity class of segmentation genes required for the formation of larval segments because, during embryogenesis, half of the larval segments are replaced by the remain half segment, but in a reversed polarity, which suggested that ''gooseberry'' was a single gene. However, it is believed that this mechanism is controlled by two duplicated genes instead of one, which are called ''gooseberry (gsb)'' and ''gooseberry neuro (gsbn)''.

Development of the Central Nervous System (CNS)

Research into zygotes of ''Drosophila'' have indicated that several segment polarity genes are vital for segmentation involved in neuroblast formation and differentiation of cell into their neuroblast identity; thereby, developing the central nervous system. Research on the loss-of-function mutations in these genes of ''Drosophila'' suggests that segment polarity genes interactions are also responsible for neuroblast division, affecting the quantity of neuroblasts as well as their specificity.Bhat, K. M. (1999). Segment polarity genes in neuroblast formation and identity specification during Drosophila neurogenesis. Bioessays, 21(6), 472-485.

References

{{Reflist Embryology de:Segmentierungsgen#Segmentpolaritätsgene