20-hydroxyecdysone, a key regulatory hormone involved in cuticle development in insectsThe halloween genes are a set of
gene
In biology, the word gene (from , ; "... Wilhelm Johannsen coined the word gene to describe the Mendelian units of heredity..." meaning ''generation'' or ''birth'' or ''gender'') can have several different meanings. The Mendelian gene is a b ...
s identified in ''
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 ...
'' that influence
embryonic development. All of the genes code for
cytochrome P450 enzymes in the ecdysteroidogenic pathway (biosynthesis of
ecdysone
Ecdysone is a prohormone of the major insect molting hormone 20-hydroxyecdysone, which is secreted from the prothoracic glands. It is of steroidal structure. Insect molting hormones (ecdysone and its homologues) are generally called ecdysteroids. ...
from
cholesterol
Cholesterol is any of a class of certain organic molecules called lipids. It is a sterol (or modified steroid), a type of lipid. Cholesterol is biosynthesized by all animal cells and is an essential structural component of animal cell mem ...
). Ecdysteroids such as
20-hydroxyecdysone
20-Hydroxyecdysone (ecdysterone or 20E) is a naturally occurring ecdysteroid hormone which controls the ecdysis (moulting) and metamorphosis of arthropods. It is therefore one of the most common moulting hormones in insects, crabs, etc. It is als ...
and
ecdysone
Ecdysone is a prohormone of the major insect molting hormone 20-hydroxyecdysone, which is secreted from the prothoracic glands. It is of steroidal structure. Insect molting hormones (ecdysone and its homologues) are generally called ecdysteroids. ...
influence many of the
morphological, physiological, biochemical changes that occur during molting in insects.
[
Steroid hormones control many aspects of reproduction, development, and ]homeostasis
In biology, homeostasis (British also homoeostasis) (/hɒmɪə(ʊ)ˈsteɪsɪs/) is the state of steady internal, physical, and chemical conditions maintained by living systems. This is the condition of optimal functioning for the organism and ...
in higher organisms. In arthropods, steroid hormones play equal or even more vital developmental roles, especially in controlling the patterns of gene expression between developmental stages. The forerunner of steroid hormones is cholesterol that vertebrates can synthesize. In contrast, insects need to take up cholesterol in their diet. In 20E biosynthesis, a series of hydroxylation takes place and the genes that encoded the enzymes for catalyzing the hydroxylation were first identified in ''Drosphilia''. Insects have been around since about 500 million years before the first mammal and have continued to be evolutionarily successful. This suggests that exogenous cholesterol was required for the mechanism for steroid hormone biosynthesis.
First elaborated by research groups led by Wieschaus and Nüsslein-Volhard in the early 1980s, the name was coined to collectively name a series of ''Drosophila'' embryonic lethal mutations associated with defective exoskeleton
An exoskeleton (from Greek ''éxō'' "outer" and ''skeletós'' "skeleton") is an external skeleton that supports and protects an animal's body, in contrast to an internal skeleton (endoskeleton) in for example, a human. In usage, some of the ...
formation. Early research showed that when one of the Halloween genes was mutated, fly embryos would die before the exoskeleton was created. Mutants in the halloween gene series include the ''spook'', ''spookier'', ''phantom'' (or ''phm''), ''disembodied'' (or ''dib''), ''shadow'' (or ''sad''), and ''shade'' genes.
The mutant homozygous embryos appear phenotypically normal until mid-embryonic development where the embryos exhibit abnormal developmental characters. Some abnormal characteristics include undifferentiated cuticle, a failure of head involution, dorsal closure, compact appearance and abnormal looping of the hindgut. These embryos die before reaching the end of embryogenesis.
Descriptions
The ''spook'' gene (Cyp307a1) is expressed in the prothoracic gland
The prothoracic glands are either of a pair of endocrine glands located in the prothorax of certain insects that regulate molting. They have an ectodermal origin and secrete ecdysteroids, such as ecdysone and 20-hydroxyecdysone
20-Hydroxyecdyson ...
, and in conjunction with the gene product of ''spookier'' (Cyp307a2), converts 7-dehydrocholesterol to Δ4-diketol.[
The ''phantom'' gene (Cyp306a1) encodes an encoding the ]microsomal
In cell biology, microsomes are heterogeneous vesicle-like artifacts (~20-200 nm diameter) re-formed from pieces of the endoplasmic reticulum (ER) when eukaryotic cells are broken-up in the laboratory; microsomes are not present in healthy, liv ...
25-hydroxylase. Strong expression
Expression may refer to:
Linguistics
* Expression (linguistics), a word, phrase, or sentence
* Fixed expression, a form of words with a specific meaning
* Idiom, a type of fixed expression
* Metaphorical expression, a particular word, phrase, o ...
of ''phm'' is restricted to the prothoracic gland cells of the ''Drosophila'' larval ring gland. The gene product converts 2,22,25dE-ketodiol to 2,22dE-ketotriol.[
The ''disembodied'' gene (Cyp302a1) codes for a cytochrome P450 enzyme that adds a ]hydroxyl
In chemistry, a hydroxy or hydroxyl group is a functional group with the chemical formula and composed of one oxygen atom covalently bonded to one hydrogen atom. In organic chemistry, alcohols and carboxylic acids contain one or more hydro ...
group to the carbon-22 position of 2,22,dE-ketotriol to make 2-deoxyecdysone. ''dib'' mutants are defective in producing their cuticle and have severe defects in morphological processes such as head involution, dorsal closure and gut development.
The ''shadow'' gene (Cyp315a1) product produces ecdysone from 2-deoxyecdysone.[
The ''shade'' gene (Cyp314a1) codes for an Ecdysone 20-monooxygenase responsible for adding a hydroxyl group to the 20C-position of ecdysone to make 20-hydroxyecdysone, the final step in the biosynthetic pathway.]
References
{{Cytochrome P450
Developmental genes and proteins
Drosophila melanogaster genes