Steroidhormones control many aspects of reproduction, development, andhomeostasis in higher organisms.[2][3] In arthropods, steroid hormones play equal or even more vital developmental roles, especially in controlling the patterns ofgene expression between developmental stages.[1] 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 inDrosphilia.[4] 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.[5]
First elaborated by research groups led by Wieschaus and Nüsslein-Volhard in the early 1980s, the name was coined to collectively name a series ofDrosophila embryonic lethal mutations associated with defectiveexoskeleton formation. Early research showed that when one of the Halloween genes was mutated, fly embryos would die before the exoskeleton was created.[6] Mutants in the halloween gene series include thespook,spookier,phantom (orphm),disembodied (ordib),shadow (orsad), andshade 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.[5]
The mechanisms of transcriptional regulation of the Halloween genes appear to differ from one another.[7]
Thespook gene (Cyp307a1) is expressed in theprothoracic gland,[8] and in conjunction with the gene product ofspookier (Cyp307a2), converts 7-dehydrocholesterol to Δ4-diketol.[6]
Thephantom gene (Cyp306a1) encodes an encoding themicrosomal 25-hydroxylase. Strongexpression ofphm is restricted to the prothoracic gland cells of theDrosophila larval ring gland.[9] The gene product converts 2,22,25dE-ketodiol to 2,22dE-ketotriol.[6]
Thedisembodied gene (Cyp302a1) codes for a cytochrome P450 enzyme that adds ahydroxyl group to the carbon-22 position of 2,22,dE-ketotriol to make 2-deoxyecdysone.[10]dib mutants are defective in producing their cuticle and have severe defects in morphological processes such as head involution, dorsal closure and gut development.[11]
Theshadow gene (Cyp315a1) product produces ecdysone from 2-deoxyecdysone.[6]
Theshade gene (Cyp314a1) codes for anEcdysone 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.[12]
^Namiki T, Niwa R, Sakudoh T, Shirai K, Takeuchi H, Kataoka H (2005). "Cytochrome450 CYP307A1/Spook: A regulator for ecdysone synthesis in insects".Biochemical and Biophysical Research Communications.337 (1):367–74.doi:10.1016/j.bbrc.2005.09.043.PMID16188237.
^Warren JT, Petryk A, Marqués G, Parvy JP, Shinoda T, Itoyama K, Kobayashi J, Jarcho M, Li Y, O'Connor MB, Dauphin-Villemant C, Gilbert LI (2004). "Phantom encodes the 25-hydroxylase ofDrosophila melanogaster andBombyx mori: a P450 enzyme critical in ecdysone biosynthesis".Insect Biochemistry and Molecular Biology.34 (9):991–1010.Bibcode:2004IBMB...34..991W.doi:10.1016/j.ibmb.2004.06.009.PMID15350618.
^Gilbert LI. (2004). "Halloween genes encode P450 enzymes that mediate steroid hormone biosynthesis inDrosophila melanogaster".Molecular and Cellular Endocrinology.215 (1–2):1–10.doi:10.1016/j.mce.2003.11.003.PMID15026169.S2CID9215493.
^Chávez VM, Marqués G, Delbecque JP, Kobayashi K, Hollingsworth M, Burr J, Natzle JE, O'Connor MB (2000). "TheDrosophila disembodied gene controls late embryonic morphogenesis and codes for a cytochrome P450 enzyme that regulates embryonic ecdysone levels".Development.127 (19):4115–26.doi:10.1242/dev.127.19.4115.PMID10976044.
