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Drosophila Body Plan

-Treated flies with chemical mutagens to look for mutations in which embryonic segments were transformed or went missing in order to identify complete set of genes required for proper segmentation
-From screens, identified maternal effect, Gap, pair-rule and segment polarity genes
-One isolated mutant strain produced embryos containing two tail segments=bicaudal
-Over years, nearly all identified genes in original screens have been shown to be evolutionarily conserved all the way to mammals


Bithorax Complex in Drosophila

-Edward Lewis awarded Nobel Prize in Medicine for his contributions to understanding of how homeotic genes established body plan
-Conducted genetic analysis of Bithorax Complex-set of three genes (Ultrabithorax, Abdominal-A and Abdominal-B) that specify fates of thoracic and abdominal segments in Drosophila


Mutations Reducing Ultrabithorax Activity in 3rd Segment

-lead to homeotic transformation of third thoracic segment into second thoracic segment.
-End result=animal with two thoraxes and four wings
-Note true flies differ from other insects like butterflies or dragonflies in number of wings
-Due to differential regulation of Ultrabithorax


Forced Expression of Ultrabithorax in 2nd Segment

-If Ultrabithorax is forcibly expressed within second thoracic segment wing is converted into haltere


Antennapedia Complex

-gene cluster contains 5 homeotic genes
-labial (lab), proboscipedia (pb), Deformed (Dfd), Sex comb reduced (Scr), and Antennapedia (Antp)
-Loss or mis-expression of any member of this complex results in homeotic transformation of entire segments


Antp Expression

-usually expressed in segments that give rise to legs and is absent from segment that gives rise to antenna
-Loss of Antp expression leads to transformation of leg into antenna
-Expression of Antp within antennal segment results in transformation into fully formed leg


pb Expression

-expressed in segments that give rise to proboscis (mouth part)
-Loss of pb leads to transformation of proboscis into set of legs
-Fly that lacks pb and simultaneously has Antp expressed within antenna will have ten instead of six legs


Hox Genes

-Genes that comprise Bithorax and Antennapedia clusters are called Hox genes
-Each encodes a protein that can bind to DNA and either activate or repress transcription
-All eight Hox proteins contain homeodomain



-These domains are 60 amino acids in length and are folded into three alpha helix structures
-Third helix contains major groove of DNA double helix (called recognition helix)
-Amino acids which fall within the third helix make physical contact with the nucleotides within the major groove and provide binding specificity
-Additional specificity achieved by interactions between amino acids 3 and 5 and nucleotides within minor groove
-Mutations that change amino acids can completely alter binding specificity of homeodomain
-Homeodomain is a very common DNA binding motif


Hox Genes with Homeodomains

-All Hox genes contain homeodomain but all homeodomain containing proteins are not Hox proteins
-Ex: Pax6/Ey protein has two DNA binding motifs-paired and homeodomain
-Not Hox protein despite presence of Homeodomain
-Loss of this protein results in loss of eye but not a change in tissue identity
-In order for gene to be considered Hox gene, loss of that gene must be accompanied by a change in identity of a cell population or entire tissue


Co-linearity of Hox Gene Position

position of each Hox gene on chromosome is correlated with its expression pattern within embryo and identity of adult structure it controls
-All eight genes located on third chromosome
-Relative position of each Hox gene on third chromosome is co-linear with its expression pattern in the embryo


Example of Co-linearity of Hox Genes

-Ex: lab gene is positioned more anteriorly than Dfd gene on third chromosome.
-Similarly within embryo lab gene is expressed in cells that lie more anterior to those
that express Dfd gene
-Relationship is true for seven of eight Hox genes. pb is the exception


Similarity of Hox Genes

-Comparison of eight Hox genes indicates there are significant similarities between them
-Not only true of homeodomain, but also in other sections of proteins as well.


Model to Account for Similarity

-One model has been proposed to account for this high level of sequence conservation is that the ancient Hox cluster actually consisted of a single gene and that over the course of evolution, a series of duplication events has led to the birth of the remaining Hox genes
-One additional idea is that the positions of the Hox genes on chromosome actually reflect the order in which the genes were duplicated
-Ex: proposed that lab gene was duplicated to give us pb gene which was further duplicated to give us Dfd and so on


Prediction of Model

-organisms with relatively few Hox genes have a simpler body
plan than those with many Hox genes
-Generally true


Hox Genes in Flies and Mammals

-Eight genes found in Drosophila genome also present within mammalian genome
-Much like Drosophila clusters, mammalian organization of Hox clusters is co-linear with their expression pattern within embryo
-Since Antp and Bx complexes are found in both flies and mammals it indicates that these genes evolved prior to the split between the two organisms.


Number of Hox Genes Found in Genome (Duplication)

-Each Hox gene is found in a single copy within Drosophila genome
-However in mammalian genome each gene is found at least four copies suggesting that additional duplication events have occurred after the split in lineages that gave rise to flies and vertebrates
-Additionally Abd-B gene has undergone unique set of duplications-15 copies of those genes
-Some Hox Genes have less than 4 copies (Abd-A)-possible that chromosomal deletions have occurred


Mammalian Homeotic Transformations: Vertebrae in Mice

-Mutations that disrupt mammalian Hox genes also lead to homeotic transformations
-Ex: in mice best studied Hox genes are the ones that affect vertebrate development
-Mutations in different Hox genes lead to transformation of one vertebrae type into another
-Transformations can be relatively mild because there are several copies of each Hox gene thus there is functional redundancy amongst the genes
-More severe defects can be observed if multiple Hox genes are simultaneously removed


Mammalian Homeotic Transformations: Ovaries in Humans

-Homeotic mutation seen in humans
-Ex: ovarian teratoma
-Two examples in diagrams: portion of ovary has been transformed into epidermal and cranial tissue and is growing hair inside the ovary
-Other has a set of teeth in it