Genetics of Behavior and Development I Lecture 34 Adult C elegans nematode worm With embryos With larvae The Adult Nematode worm Caenorhabditis elegans Hermaphrodite MALE After egg fertilization the embryo is laid at 6h and it hatches at 12h to enter the first larval stage L1 The C elegans Life Cycle only 50 h Adult C elegans nematode worm Pattern of early cell divisions from the C elegans zygote to form six Founder Cells With embryos Fertilized egg zygote With larvae P1 P0 ABa P2 ABp AB Fertilized egg zygote Full lineage of the entire C elegans body First cell division 0h hatch zygote Intestine L1 Neurons 0h hatch Intestine Pharynx Neurons L2 L3 50h Adult Germ line L1 L2 L4 Subsequent divisions Full lineage of the entire C elegans body zygote Pharynx EMS L3 Vulva Epidermis Somatic gonad L4 50h Adult Vulva Epidermis Somatic gonad Germ line 2002 Nobel Prize in Physiology or Medicine Important features of the C elegans cell lineage It is the best documented example of a highly reproducible pattern of cell division for any organism The pattern of cell divisions starting from a wild type C elegans fertilized egg is nearly always the same About 10 rounds of cell division create the adult worm that has many of the same tissues that we do John Sulston Any one particular cell undergoes relatively few divisions Sydney Brenner One hemaphrodite worm can produce 500 000 progeny in a week on a petri dish Bob Horvitz MIT Powerful model organism to identify the regulators that control cell lineage in both time and space How would you identify the regulatory mechanisms that control the development of cell lineages Look for mutants that display aberrant lineages and characterize the genes of course Sydney Brenner and subsequently others mutagenized C elegans and screened for funny looking worms What does funny mean altered body shapes e g dumpy small long uncoordinated or otherwise behaving strangely One class of mutants turned out to be lacking a body part namely the vulva through which the embryos are laid these animals are unable to lay eggs These egg laying mutants were picked for study with the idea that there were obvious problems with the execution of at least some cell lineages maybe there would be a problem with other lineages Full lineage of the entire C elegans body Full lineage of the entire C elegans body zygote 0h hatch zygote Intestine Pharynx Germ line L1 0h hatch Intestine Pharynx L1 Neurons L2 Neurons L2 L3 L3 Vulva L4 Epidermis 50h Adult Somatic gonad Vulva L4 Epidermis 50h Adult Somatic gonad Zoom in on particular cell lineages in this case the T lineage L1 L2 Ask whether the development of this lineage is normal in egg laying mutants Normal Precocious Retarded Retarded Germ line Normal Distinctive cell division pattern in L1 vs L2 Precocious Actually skipped over the L1 cell division pattern Retarded Retarded Repeats the L1 cell division pattern over and over Repeats the L1 cell division pattern over and over Heterochronic Mutants Misregulation of another lineage These mutants have been called HETEROCHRONIC mutants they affect the relative timing of major developmental events The existence of such mutants indicates that there is coordination of the temporal sequence of many cell fates an animal develops This temporal coordination is genetically regulated Lineages H G T Q V K P W G T Q V M and Gonadal G lin 4 and lin 14 mutants affect postembryonic lineages H1 H2 M V1 V6 as well as the T lineage Other Heterochronic mutants also identified Mutant Phenotype lin 4 lf Retarded lin 14 lf Retarded lin 14 gf Precocious lin 28 lf Precocious lin 29 lf Retarded Double Mutants How would you go about determining the order in which lin 4 lin 14 lin 28 and lin 29 operate to control the timing of development lin 4 lf lin 14 gf lin 14 lf lin 28 lf lin 29 lf Single mutants lin 14 gf lin 14 lf lin 28 lf lin 29 lf Retarded Retarded Precocious Precocious Retarded Retarded mixed Retarded Precocious Precocious Retarded Precocious Retarded Retarded Larval Program lin 4 lin 14 lin 28 lin 29 Larval Program lin 4 Adult Program lin 14 lin 28 lin 29 lin 4 encodes small RNAs 61nt and 21 nt lin 14 encodes a nuclear protein lin 28 encodes another protein lin 29 encodes a Transcription factor Adult Program MicroRNAs control gene expression by hybridizing to specific mRNA species Larval Program lin 4 Normal Precocious Retarded Retarded Temporal expression of the lin 4 RNA is crucial for the orchestration of lineage specific cell division lin 4 RNA RNA or protein level hatch L1 L2 lin 14 or lin 28 protein lin 14 lin 28 lin 29 Adult Program Larval Program lin 4 lin 14 lin 28 lin 29 Adult Program This was the first microRNA regulatory molecule to be discovered microRNAs are widespread in nature including humans and are very often involved in development and differentiation
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