Lecture'20'April'15,'2015''Cell'autonomous'determination:'an'example'from'the'model'organism'C.#elegans''Reading:''Chapter'22'of'MBoC'pages'1321A1325.''''Synopsis:''This'lecture'does'not'closely'follow'the'MBoC'chapter,'but'the'reading'can'reinforce'and'perhaps'clarify'some'of'the'material'covered'in'class.''C.#elegans'is'an'excellent'model'to'address'some'fundamental'questions'in'developmental'biology.''This'small'soil'nematode'is'especially'well'suited'for'studies'of'determination'in'early'development.''The'completely'defined'lineage'makes'it'possible'to'interpret'the'consequences'of'experimental'and'genetic'perturbations.''The'nematode'early'embryo'provides'examples'of'both'cell$autonomous'and'conditional.determination.''In'this'lecture'the'focus'was'on'mechanisms'that'lead'to'cellAautonomous'determination,'using'the'determination'of'the'germ'line'as'an'example.'''The'complete'lineage'reveals'that'the'germ'line'arises'from'a'single'cell'(blastomere),'the'P4'cell,'at'about'the'16A24'cell'stage.''P0'(the'fertilized'egg'or'oneAcell'embryo)'produces'the'blastomere'P1,'which'produces'P2'which'produces'P3'which'produces'P4,'the'founder.cell'of'the'entire'germ'line.'(see'lecture'slides'5'and'6)'on'the'handout'currently'on'blackboard.''Although'we'also'know'quite'a'bit'about'how'other'founder'cell'fates'are'determined,'in'this'lecture'we'addressed'only'the'question:''How'is'the'germ'cell'fate'determined?''We'reviewed'the'concept'that'the'germ.line'is'different'from'the'soma'(all'of'the'cells'in'the'body'that'are'NOT'germ'line),'because'only'germ'line'DNA'can'be'passed'to'the'next'generation'through'the'ability'of'germ'line'cells'to'generate'the'gametes'(sperm'and'egg).''From'an'evolutionary'perspective,'one'can'think'of'the'soma'as'having'the'sole'function'of'passing'germ'cell'DNA'on'to'the'next'generation.''Observing'the'pattern'of'the'early'cleavages'led'researchers'to'the'hypothesis'that'many'of'the'cell'fates'of'the'founder'cells,'including'P4,'were'determined'cell'autonomously.''To'test'this,'they'isolated'the'founder'cell'in'question'using'the'technique'of'laser'ablation.''(The'laser'beam'is'focused'on'the'nucleus'of'the'cell'to'be'ablated'(killed)'through'the'microscope;'the'absorbed'heat'boils'the'nucleus.)'''Isolated)P 4)c e ll ''''''Experiment:))use)laser)beam)to)isolate)th e )P4 )c e ll If)it)is)deter m ined)cell)autonomously,)it)will)behave)like)a)germ)cell. ''Behaves)like)a)germ)line)cell!By'killing'the'somatic'sister'of'the'germ'line'cells'P1,'P2,'P3'and'P4'(see'diagram),'one'can'isolate'the'P4'cell'and'ask'whether'it'still'behaves'like'a'germ'cell.''The'answer'proved'to'be'“yes!”''Therefore'it'appears'that'the'germ'line'is'specified'cell'autonomously.''''If'so,'then'one'should'be'able'to'identify'cytoplasmic'determinants'of'the'germ'line'by'looking'for'molecules'that'are'unequally'distributed'by'asymmetric'divisions'into'the'germ'line'cells.'P.granules,'complex'aggregates'of'RNA'and'protein'(ribonucleoprotein'particles)'discovered'in'the'early'1980s,'satisfied'this'expectation.'''For'many'years,'P'granules'have'been'used'as'examples'of'cytoplasmic'determinants'(molecules'that'act'cell'autonomously'to'specify'the'developmental'fate'of'the'cells'that'contain'them).''Because'the'granules'segregate'exclusively'to'the'germ'line'cells,'it'is'a'reasonable'hypothesis'that'they'are'the'determinants'that'specify'that'fate.''Indeed,'one'gets'the'impression'from'Chapter'22'of'MBoC'that'P'granules'have'a'role'in'determining'the'germ'cell'fate'(although'the'authors'are'very'careful'not'to'state'that).''However,'until'recently,'no'one'had'been'able'to'test'this'hypothesis.''''In'December'2010,'a'paper'appeared'in'the'journal'Science'that'provided'evidence'showing'that,'contrary'to'what'many'believed,'P'granules'are'probably.not'germ'line'determinants.''In'this'paper'Gallo'and'colleagues'report'that'mutation'of'a'gene,.pptr$1,'which'encodes'a'regulatory'subunit'of'a'phosphatase,'results'in'a'failure'of'P'granules'to'become'localized'to'the'germ'line,'but'that'germ'cells'get'determined'in'spite'of'that'(see'the'movie'“PAgranules'and'H2B'in'WT'and'pptrA1.mov”'in'the'Lecture'20'folder'on'blackboard).''Thus,'the'authors'conclude'that'segregatio
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