Lecture'21'April'120,'2015'Regional'Specification:''Anterior'posterior'pattern'formation'in'Drosophila.''Reading:''MBoC'1328D34;1336D41;'ECB3'282D84;'ECB4%274(75''Synopsis:''Drosophila*melanogaster'is'an'excellent'animal'model'for'studies'of'development'because'of'its'long'history'of'genetic'analysis,'its'rapid'development,'its'ease'of'culture'in'the'laboratory'and'its'detailed'anatomy.''A'key'feature'of'the'anatomy'is'that'the'body'of'the'larvae'is'made'up'of'14'segments,'each'of'which'can'be'identified'by'a'particular'pattern'of'hairs'(denticles).''The'segments'can'also'be'mapped'onto'a'fate'map'of'the'cellular'blastoderm'(Figure'22D27'MBoC).''The'early'development'of'Drosophila'(and'other'insects)'differs'from'that'of'C.*elegans'and'most'vertebrates'in'that'the'embryo'doesn’t'cleave'into'individual'cells.''Instead,'there'is'a'period'during'which'nuclei'proliferate'in'a'common'cytoplasm'(figure'22D28a'MBoC).''The'situation'in'which'multiple'nuclei'share'a'common'cytoplasm'is'referred'to'as'a'syncytium.''After'a'certain'number'of'divisions,'most'of'the'nuclei'move'to'the'periphery'of'the'cell,'forming'a'single'layer'of'nuclei,'each'of'which'is'barely'separated'from'its'neighbors'by'a'slight'invagination'(inpocketing)'of'the'plasma'membrane.''This'stage'of'development'is'called'the'syncytial%blastoderm.''After'a'few'additional'nuclear'divisions,'each'nucleus'becomes'completely'enclosed'by'membranes'in'a'process'called'cellularization.''[We'didn’t'discuss'the'pole'cells'–'an'interesting'story'that'you'can'learn'in'another'course.]''The'stage'of'development'after'cellularization'and'before'gastrulation'is'called'the'cellular%blastoderm'stage.'''Animal'development'relies'on'two'genomes:''the'genome'of'the'mother,'which'directs'the'production'of'the'oocyte,'and'the'embryonic'genome'that'results'from'the'fusion'of'the'haploid'sperm'and'egg'nucleus.''Depending'upon'the'animal,'the'maternal'genome'can'direct'more'or'less'of'the'early'development.''In'flies,'the'maternal'genome’s'contribution'is'significant'whereas'in'mammals'the'maternal'genome’s'contribution'is'minor.''''Genetic'analysis'combined'with'newly'developed'molecular'biology'tools'made'it'possible'to'learn'the'molecular'mechanisms'that'underlie'Drosophila'development.''The'story'begins'with'largeDscale'forward'genetics'searches'(“saturation'screens”)'to'identify'mutants'in'all'the'genes'that'play'a'role'in'setting'the'pattern'of'cuticles'(skins)'of'late'stage'embryos.'In'Nobel'Prize'winning'work,'two'researchers'sought'mutants'in'two'different'classes'of'genes:''1)'those'that'are'expressed'by'the'mother'during'oogenesis'(maternal'genes)'and'2)'those'that'are'expressed'by'the'embryo'during'embryogenesis'(embryonic'genes).''They'found'that'maternally'expressed'genes'that'could'mutate'to'altered'cuticle'pattern'along'the'anterior/posterior'axis'defined'three'groups'of'genes'based'on'their'mutant'phenotypes:'the'anterior'group,'the'posterior'group,'and'the'terminal'group'(see'more'information'below).'''The'screens'also'resulted'in'the'discovery'of'a'set'of'mutants'that'altered'the'cuticle'pattern'along'the'dorsal/ventral'axis,'but'we'won’t'cover'the'dorsal/ventral'system'in'this'course.''However,'it'is'important'to'know'that'every'cell'in'the'cellular'blastoderm'can'sense'its'position'with'respect'to'these'two'axes'and'will'integrate'the'two'signals'to'determine'its'fate'(slide'19).''For'example,'a'cell'in'the'dorsal'region'about'1/3'way'from'the'anterior'pole'will'interpret'that'position'and'know'that'it'should'form'the'dorsal'part'of'the'thorax'(e.g.'wing'in'an'adult'fly)'a'cell'1/3'of'the'way'from'the'anterior'pole'but'in'the'ventral'side'will'also'know'that'it'should'form'a'part'of'the'thorax'but'will'contribute'to'a'leg'instead'of'a'wing.''We'discussed'the'maternally'provided'anteriorDposterior'system'in'more'detail.''The'mutant'phenotypes'of'genes'expressed'in'oogenesis'revealed'three'classes'of'mutations'(Figure'22D30'MBoC)'mutants'lacking'the'most'anterior'segments'–'the'anterior%group'2)'mutants'lacking'the'most'posterior'segments'DD'the'posterior%group'3)'mutants'lacking'the'structures'at'the'ends'of'the'embryo'(terminal'structures)'–'the'terminal%group.''Over'the'years,'the'proteins'made'by'each'of'the'genes'have'been'identified'and'studied'at'the'molecular'level.''For'each'of'the'mutant'classes,'a'key'gene'has'been'identified'(Slide'12;'Figure'22D32'MBoC).''One'of'these'key'genes,'torso,'encodes'a'receptor'tyrosine'kinase'that'is'activated'only'at'the'poles'of'the'egg'during'oogenesis'by'signals'sent'from'a'subset'of'follicle'cells'that'surround'the'egg'(yellow'colored'cells'on'slide'17;'Figure'22D31'MBoC).'After'the'torso'receptor'is'activated'it'can'be'considered'a'cytoplasmic'determinant'because'the'signaling'downstream'of'it'will'be'localized'to'the'poles.'The'two'other'key'genes'encode'cytoplasmic'determinants:'Bicoid'for'the'anterior'group'and'Nanos'for'the'posterior'group.''The'mRNAs'for'these'two'genes'are'loaded'into'the'egg'in'a'polarized'way'–'Bicoid'at'the'anterior'tip'and'Nanos'at'the'posterior'tip.''After'the'egg'is'fertilized,'the'mRNA'is'translated'and'the'protein'diffuses'away,'but'the'mRNA'stays'in'place.''Thus,'the'protein'forms'a'gradient,'highest'at'the'site'where'the'mRNA'is'localized.''This'gradient'persists'through'most'of'the'syncytial'blastoderm'stage.''Bicoid'encodes'a'transcriptional'activator'that'functions'as'a'cytoplasmic'determinant'and'acts'as'a'morphogen%gradient.''Evidence'that'it'functions'in'a'graded'fashion'comes'first'and'obviously'from'the'observation'that'the'protein'forms'a'gradient'from'anterior'to'posterior'and'second,'and'more'important,'when'the'shape'of'the'gradient'is'changed'by'increasing'or'decreasing'the'number'of'copies'of'the'bicoid*gene,'it'changes'the'spatial'pattern'of'the'embryo'in'a'coordinated'manner'(see'slide'25).''Lower'than'normal'levels'of'Bicoid'moves'the'pattern'toward'the'anterior'of'the'embryo;'higher'than'normal'levels'of'Bicoid'moves'the'pattern'toward'the'posterior.''We'conclude'then,'that'the'egg'is'loaded'with'molecules'that'provide'broad'regional'information.''In'a'screen'for'cuticle'pattern'mutants'that'targeted'genes'expressed'by'the'embryonic'genome'during'embryogenesis,'three'classes'of'phenotypes'emerged.''First'were'mutants'in'a'set'of'genes'that'caused'large'gaps'in'the'pattern;'these'were'appropriately'called'the'gap%genes.''The'other'classes'of'mutant'were'surprising'because'instead'of'having'alterations'in'cont
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