evolutionary developmental biology Evolutionary developmental biology (informally, evo-devo) is a field of biological research that compares the developmental processes of different organisms to infer how developmental processes evolved. The field grew from 19th-century beginn ...

, homeotic genes are genes which regulate the development of anatomical structures in various organisms such as echinoderms, insects, mammals, and plants. Homeotic genes often encode

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 f ...

proteins, and these proteins affect development by regulating downstream gene networks involved in body patterning.

Mutation In biology, a mutation is an alteration in the nucleic acid sequence of the genome of an organism, virus, or extrachromosomal DNA. Viral genomes contain either DNA or RNA. Mutations result from errors during DNA or viral replication, m ...

s in homeotic genes cause displaced body parts (

homeosis In evolutionary developmental biology, homeosis is the transformation of one organ into another, arising from mutation in or misexpression of certain developmentally critical genes, specifically homeotic genes. In animals, these developmental ...

), such as antennae growing at the posterior of the fly instead of at the head. Mutations that lead to development of ectopic structures are usually lethal.

Types

There are several subsets of homeotic genes. They include many of the Hox and

ParaHox The ParaHox gene cluster is an array of homeobox genes (involved in morphogenesis, the regulation of patterns of anatomical development) from the Gsx, Xlox ( Pdx) and Cdx gene families. Regulatory gene cluster These genes were first shown to b ...

genes that are important for segmentation. Hox genes are found in bilateral animals, including ''Drosophila'' (in which they were first discovered) and humans. Hox genes are a subset of the

homeobox 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-g ...

genes. The Hox genes are often conserved across species, so some of the Hox genes of ''Drosophila'' are homologous to those in humans. In general, Hox genes play a role of regulating expression of genes as well as aiding in development and assignment of specific structures during embryonic growth. This can range from segmentation in ''Drosophila'' to

central nervous system The central nervous system (CNS) is the part of the nervous system consisting primarily of the brain and spinal cord. The CNS is so named because the brain integrates the received information and coordinates and influences the activity of all p ...

(CNS) development in vertebrates. Both Hox and ParaHox are grouped as HOX-Like (HOXL) genes, a subset of the ANTP class (named after the ''Drosophila'' gene,

Antennapedia ''Antennapedia'' (abbreviated ''Antp'') is a Hox gene first discovered in ''Drosophila'' which controls the formation of legs during development. Loss-of-function mutations in the regulatory region of this gene result in the development of ...

). They also include the MADS-box-containing genes involved in the

ABC model of flower development The ABC model of flower development is a scientific model of the process by which flowering plants produce a pattern of gene expression in meristems that leads to the appearance of an organ oriented towards sexual reproduction, a flower. There a ...

. Besides flower-producing plants, the MADS-box motif is also present in other organisms such as insects, yeasts, and mammals. They have various functions depending on the organism including flower development, proto-oncogene transcription, and gene regulation in specific cells (such as muscle cells). Despite the terms being commonly interchanged, not all homeotic genes are Hox genes; the MADS-box genes are homeotic but not Hox genes. Thus, the Hox genes are a subset of homeotic genes.

''Drosophila melanogaster''

One of the most commonly studied

model organism A model organism (often shortened to model) is a non-human species that is extensively studied to understand particular biological phenomena, with the expectation that discoveries made in the model organism will provide insight into the workin ...

s in regards to homeotic genes is the fruit fly ''

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 ...

''. Its homeotic Hox genes occur in either the Antennapedia complex (ANT-C) or the Bithorax complex (BX-C) discovered by

Edward B. Lewis Edward Butts Lewis (May 20, 1918 – July 21, 2004) was an American geneticist, a corecipient of the 1995 Nobel Prize in Physiology or Medicine. He helped to found the field of evolutionary developmental biology. Early life Lewis was born in Wi ...

. Each of the complexes focuses on a different area of development. The

antennapedia ''Antennapedia'' (abbreviated ''Antp'') is a Hox gene first discovered in ''Drosophila'' which controls the formation of legs during development. Loss-of-function mutations in the regulatory region of this gene result in the development of ...

complex consists of five genes, including proboscipedia, and is involved in the development of the front of the embryo, forming the segments of the head and thorax. The bithorax complex consists of three main genes and is involved in the development of the back of the embryo, namely the abdomen and the posterior segments of the thorax. During development (starting at the

blastoderm A blastoderm (germinal disc, blastodisc) is a single layer of embryonic epithelial tissue that makes up the blastula. It encloses the fluid filled blastocoel. Gastrulation follows blastoderm formation, where the tips of the blastoderm begins the fo ...

stage of the embryo), these genes are constantly expressed to assign structures and roles to the different segments of the fly's body. For ''Drosophila'', these genes can be analyzed using th
Flybase
database.

Research

Much research has been done on homeotic genes in different organisms, ranging from basic understanding of how the molecules work to mutations to how homeotic genes affect the human body. Changing the expression levels of homeotic genes can negatively impact the organism. For example, in one study, a pathogenic phytoplasma caused homeotic genes in a flowering plant to either be significantly upregulated or downregulated. This led to severe phenotypic changes including dwarfing, defects in the pistils, hypopigmentation, and the development of leaf-like structures on most floral organs. In another study, it was found that the homeotic gene '' Cdx2'' acts as a

tumor suppressor A tumor suppressor gene (TSG), or anti-oncogene, is a gene that regulates a cell during cell division and replication. If the cell grows uncontrollably, it will result in cancer. When a tumor suppressor gene is mutated, it results in a loss or re ...

. In normal expression levels, the gene prevents tumorgenesis and colorectal cancer when exposed to

carcinogen A carcinogen is any substance, radionuclide, or radiation that promotes carcinogenesis (the formation of cancer). This may be due to the ability to damage the genome or to the disruption of cellular metabolic processes. Several radioactive sub ...

s; however, when ''Cdx2'' was not well expressed, carcinogens caused tumor development. These studies, along with many others, show the importance of homeotic genes even after development.

References

External links

FlybaseHomeotic Genes and Body Patterns
{{genarch Genes Developmental biology

Types

''Drosophila melanogaster''

Research

See also

References

External links