BIOLOGY 205/SECTION 7 DEVELOPMENT- LILJEGREN Lecture 9 Cell lineage in the nematode C. elegans 1) The worm. a) Phylum Nematoda contains numerous free-living & parasitic species. Among the most numerous and abundant animals on earth. b) Caenorhabditis elegans lives in soil & makes its living eating bacteria. 2) The history. a) In 1968 Sidney Brenner and others decided to choose a simple organism as a model system. In particular they wanted to study the brain. Because of their training, they understood the power of genetics and the advantages of studying simple organisms. So they appreciated the advantages of C. elegans we’ve already talked about. b) Advantages of C. elegans: i) They are small—adults are ~1mm long. ii) They can be grown on agar plates with lawn of bacteria as breakfast, lunch, & dinner. iii) They have a short generation time- 3 days to egg-laying adulthood. 14 hours from fertilization to hatch. iv) They are transparent, so internal anatomy can be easily observed. v) The genome is small- 100 Mb. This is about 3% the size of the human genome (3 Gb). This does not mean it has 3% the number of genes! C. elegans have about 17,500 genes, about half that of humans (30,000-40,000 genes). vi) Most members of the C. elegans community are self-fertilizing hermaphrodites (XX). Actually, they are females that can make sperm for a short time early during their development. Later on the eggs have to pass through the stored sperm on their way to the vulva. Every so often a male (XO) is generated, which allows outcrossing. Since hermaphrodites make both eggs and sperm, they can produce homozygous mutant progeny. Why is this great for genetic analysis? Can get homozygous mutant one generation earlier! Fly: mutant x + Worm: mutant (self-fertilize) + + + F1 mutant x mutant F1: mutant; mutant; + + + mutant + + F2 mutant; mutant; + mutant + + vii) Hermaphrodites produce MANY progeny---from one worm, 10,000 worms! This is particularly good for mosaic analysis c) Research since 1968 has generated other advantages. i) Thousands of genes already identified by mutations. ii) Entire genome sequenced in 1999--first animal to be sequenced! This means that if you know where a mutation maps you can look at the gene sequences in that region for a “candidate” gene. Knowing the sequence allows genes of interest to be easily cloned. iii) Cell lineage(1) However, the most important and incredible advantage is that every animal has the exact same number of somatic cells! (a) Hatching larva=558 cells (b) Males (XO)=1031 somatic cells+ ~1000 sperm (c) Hermaphrodite (XX)= 959 somatic cells+~2000 eggs and sperm. (d) By comparison, we have a billion cells alone in our nervous system. (2) Somatic cells arise by an INVARIANT cell lineage. The divisions are invariant in pattern, timing, & orientation of each division. The lineage was completely determined by John Sulston. This was an enormous one-man effort. He basically went into a darkroom with a microscope for 2 years and followed every single cell through embryonic and postembryonic development. That means that for every cell, we know exactly what cells gave rise to that cell, when those divisions occurred, and in what orientation they occurred. (3) In contrast, it would be amazing if 100 years from now we understood the mammalian brain in this way. (4) Knowing the lineage has made it possible to determine each cell's exact position in body. (a) For example, neuroscientists have determined the inputs and outputs for each of the 302 neurons by cutting the whole worm up from head to tail and looking at each thin section using electron microscopy. In other words, WE KNOW THE EXACT WIRING of the entire nervous system of this animal. We are not even close to this level of structural characterization for any other organism (for example there are trillions of connections in the human brain). With these 302 neurons, the worms eat, defecate, sense their environment, find mates, have sex, avoid danger, and move their muscles, all thing we’d like to understand about ourselves. (5) What are the interesting things learned? Where specific tissues come from. Lineages do not strictly consist of single tissue types. (a) In other words, most organs are made with cells from different sub-lineages. For example, while intestine & germ-line come from single founder cells, the muscle, nervous system & skin cells each arise from multiple lineages. This means, for example, that muscles do not have one “founder” cell early in development (6) Also learned that cell death can be a genetically programmed cell fate. (a) 131 cells have death as their normal fate. Cells born then commit suicide and their corpses are eaten by neighboring cells. Found mutants where no cells died: Mutations in genes ced-3 and ced-4 eliminate all cell deaths! (b) Regulated programmed cell death=apoptosis. This occurs in our bodies as well. Ie. we make many more neurons than actually use. Those that don’t get connected commit suicide. (c) Cell death also allows organisms to get rid of cells that have gone wrong. Ie. when DNA has become damaged. If don’t, cells will accumulate DNA damage. If enough damage accumulates, will lead to cancer. (d) In Parkinson’s and Alzheimer’s there is excess cell death. 3) Asymmetric segregation of determinants is one mechanism through which cell lineage controls cell fate. a) One mechanism that we’ve discussed before is asymmetric segregation of determinants such as P-granules. i) P-granules are small mRNA/protein particles that are uniformly distributed in unfertilized eggii) But they are inherited only by 1 of first 2 daughter cells—the posterior cells. iii) P-granules are also partitioned at the next three divisions, such that only germline progenitor P4 cell inherits them. iv) P-granules contain "determinants" that gives rise to the germline fate. v) How does it work? Actin and microtubules are impoartant b) Genetics revealed mutations exist that affect segregation of particular determinants i) For example, worms mutant for par (partition) genes have defects in segregating determinants to all daughter cells. P-granules found in all daughter cells. 4) Genetic control of cell lineage. a) Many genes regulate the pattern and timing of cell divisions
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