{{DISPLAYTITLE:''Bithorax'' complex}} {{Redirect|BXC}} [[File:Hoxgenesoffruitfly.svg|Homeobox gene expression in ''Drosophila melanogaster''|thumb|right|400px]]

The '''Bithorax complex (BX-C)''' is one of two ''Drosophila melanogaster'' homeotic gene complexes, located on the right arm of chromosome 3.<ref name=":0">{{cite journal | vauthors = McGinnis W, Levine MS, Hafen E, Kuroiwa A, Gehring WJ | title = A conserved DNA sequence in homoeotic genes of the Drosophila Antennapedia and bithorax complexes | journal = Nature | volume = 308 | issue = 5958 | pages = 428–33 | date = March 1984 | pmid = 6323992 | doi = 10.1038/308428a0 | bibcode = 1984Natur.308..428M | s2cid = 4235713 }}</ref> It is responsible for the differentiation of the posterior two-thirds (posterior thorax and each abdominal segment) of the fly by the regulation of three genes within the complex: ''Ultrabithorax (Ubx)'''', abdominal A (abd-A),'' and ''Abdominal B (Abd-B)''.<ref name=":1">{{cite journal | vauthors = Martin CH, Mayeda CA, Davis CA, Ericsson CL, Knafels JD, Mathog DR, Celniker SE, Lewis EB, Palazzolo MJ | display-authors = 6 | title = Complete sequence of the bithorax complex of Drosophila | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 92 | issue = 18 | pages = 8398–402 | date = August 1995 | pmid = 7667301 | pmc = 41164 | doi = 10.1073/pnas.92.18.8398 | bibcode = 1995PNAS...92.8398M | doi-access = free }}</ref><ref name=":2">{{cite journal | vauthors = Casanova J, Sánchez-Herrero E, Busturia A, Morata G | title = Double and triple mutant combinations of bithorax complex of Drosophila | journal = The EMBO Journal | volume = 6 | issue = 10 | pages = 3103–9 | date = October 1987 | pmid = 14650432 | doi = 10.1002/j.1460-2075.1987.tb02619.x | pmc = 553750 }}</ref><ref name=":3">{{cite journal | vauthors = Duncan I | title = The bithorax complex | journal = Annual Review of Genetics | volume = 21 | issue = 1 | pages = 285–319 | date = 1987-12-01 | pmid = 3327467 | doi = 10.1146/annurev.ge.21.120187.001441 }}</ref>

A complete deletion of this complex results in the posterior segments of the ''Drosophila'' embryo becoming copies of the second thoracic segment.<ref name=":4">{{cite journal | vauthors = Lewis EB | title = A gene complex controlling segmentation in Drosophila | journal = Nature | volume = 276 | issue = 5688 | pages = 565–70 | date = December 1978 | pmid = 103000 | doi = 10.1038/276565a0 | bibcode = 1978Natur.276..565L | s2cid = 2619820 }}</ref>

== History == The first homeotic mutation to be isolated was the ''bithorax (bx)'' mutant in 1915 by Calvin Bridges,<ref name=":5">{{cite journal | vauthors = Maeda RK, Karch F | title = The bithorax complex of Drosophila an exceptional Hox cluster | journal = Current Topics in Developmental Biology | volume = 88 | pages = 1–33 | date = January 2009 | pmid = 19651300 | doi = 10.1016/S0070-2153(09)88001-0 }}</ref> which was later worked on and better characterized as a gene cluster by Edward B. Lewis.<ref name=":4" /> Lewis found that BX-C contained genes that code for proteins controlling the levels of thoracic and abdominal development, and postulated that at least four of these genes were regulated by cis-regulatory elements, as well as a separate locus (Polycomb) that acted as a repressor.<ref name=":4" /> However, subsequent research has found that the complex only contains three genes that are differentially expressed in each parasegment.<ref name=":7">{{cite journal | vauthors = Maeda RK, Karch F | title = The ABC of the BX-C: the bithorax complex explained | journal = Development | volume = 133 | issue = 8 | pages = 1413–22 | date = April 2006 | pmid = 16556913 | doi = 10.1242/dev.02323 | doi-access = free }}</ref> Lewis’ work on BX-C earned him the Nobel Prize in Physiology or Medicine in 1995.<ref>{{Cite web|url=https://www.nobelprize.org/prizes/lists/all-nobel-laureates-in-physiology-or-medicine|title=All Nobel Prizes in Physiology or Medicine|website=NobelPrize.org|language=en-US|access-date=2019-11-27}}</ref>

== Gene information == The gene complex is located on the right arm of chromosome 3 alongside the other homeotic gene complex, the Antennapedia (Antp) complex.<ref name=":0" /> It is over 300 kb long and contains nine distinct cis-regulatory domains.<ref name=":1" /> The three transcription units, ''Ubx, abd-A,'' and ''Abd-B'', are spread out evenly on the complex.<ref name=":5" /> A mutant embryo with a triple knock-out of ''Ubx, abd-A,'' and ''Abd-B'' has a phenotype that is identical to that of an embryo with a complete BX-C deletion, suggesting that the three transcription units are the only functional protein-coding units in the complex.<ref name=":2" /> This was confirmed when the entire complex was eventually sequenced in 1995.<ref name=":1" /> Only about 1.4% of the BX-C sequence codes for proteins.<ref name=":1" />

Deletion of the entire complex results in death of the embryo late in development.<ref name=":4" /> It also shows changes in the segmentation of the embryonic cuticle, with all abdominal segments and the third thoracic segment resembling the second thoracic segment.<ref name=":4" /> Individual mutations within the complex also result in segmental changes, though not as dramatic as the entire deletion of the complex.<ref name=":6">{{cite book | vauthors = Bender W, Akam M, Karch F, Beachy PA, Peifer M, Spierer P, Lewis EB, Hogness DS | title = Genes, Development, and Cancer | chapter = Molecular Genetics of the Bithorax Complex in Drosophila Melanogaster | display-authors = 6 | journal = Science | volume = 221 | issue = 4605 | pages = 23–9 | date = July 1983 | pmid = 17737996 | doi = 10.1007/978-1-4020-6345-9_14 | publisher = Springer Netherlands | isbn = 9781402063435 }}</ref>

== Patterning == BX-C controls the differential patterning of the posterior thoracic segment (T3) and all 8 of the abdominal segments (A1-A8) of ''Drosophila''.<ref name=":42">{{cite journal | vauthors = Lewis EB | title = A gene complex controlling segmentation in Drosophila | journal = Nature | volume = 276 | issue = 5688 | pages = 565–70 | date = December 1978 | pmid = 103000 | doi = 10.1038/276565a0 | bibcode = 1978Natur.276..565L | s2cid = 2619820 }}</ref> During embryonic development, regulation of BX-C expression occurs on the level of units called parasegments. The parasegments are slightly misaligned with the anatomical segments, each comprising the posterior part of one segment and the anterior part of the adjacent segment.<ref name=":42" />

The parasegment-specific expression patterns of the BX-C genes are controlled by large and complex cis-regulatory regions.<ref name=":3" /><ref name=":7" /> 9 functionally distinct cis-regulatory sub-domains have been identified: abx/bx, bxd/pbx, iab-2, iab-3, iab-4, iab-5, iab-6, iab-7, and iab-8,9. They are arranged along the chromosome in the same order as the parasegments that they act in.<ref name=":1" /><ref name=":3" /> Loss-of-function mutations of one sub-domain will result in a target parasegment becoming a copy of the parasegment that is immediately anterior to it.<ref name=":4" /><ref name=":6" />

== References == {{reflist|32em}}

{{DEFAULTSORT:Bithorax Complex}} Category:Drosophila melanogaster genes