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BISC 219/2009: Mod 1 C. elegans General Information: Difference between revisions
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BISC 219/2009: Mod 1 C. elegans General Information (view source)
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== Nematode Genetics == | == Nematode Genetics == | ||
The small size of ''C. elegans'' (1mm), its short generation time (3 1/2 days at 20°C), | The molecular genetics of ''C. elegans'' is well developed. This nematode holds the distinction of being the first metazoan to have its genome sequenced. In addition, most of the tricks of the molecular geneticist have been employed in ''C. elegans'', including making transgenics (including knockouts) and the technique of RNA interference: RNAi. The small size of ''C. elegans'' (1mm), its short generation time (3 1/2 days at 20°C), as well as the large number of progeny produced per animal (250-350) are also important factors in genetic analysis. Since genetic experiments frequently require the detection of rare events, it is an advantage to grow a large number of individuals in a small space. A petri plate seeded with a single hermaphrodite will contain nearly 10<sup>5</sup> individuals after one week. Confining the animals to the 2-dimensional agar surface permits the observation of rare individuals in large populations with the aid of a dissecting microscope. | ||
Self-fertilization drives populations to homozygosity so that it is easy to isolate isogenic (genetically identical) clones of animals. This greatly facilitates the detection of mutants in a diploid organism. Since chemically induced mutations first appear in the heterozygous condition, recessive mutations cannot be recognized in the F1 progeny of mutagenized animals. In ''C. elegans'' heterozygous hermaphrodites automatically segregate homozygous mutants as one fourth of their progeny, compared to “male/female” organisms where segregation of homozygous mutants requires manual cloning (brother-sister matings) and homozygotes only appear in the third generation after mutagenesis. | Self-fertilization drives populations to homozygosity so that it is easy to isolate isogenic (genetically identical) clones of animals. This greatly facilitates the detection of mutants in a diploid organism. Since chemically induced mutations first appear in the heterozygous condition, recessive mutations cannot be recognized in the F1 progeny of mutagenized animals. In ''C. elegans'', heterozygous hermaphrodites automatically segregate homozygous mutants as one fourth of their progeny, compared to “male/female” organisms where segregation of homozygous mutants requires manual cloning (brother-sister matings) and where homozygotes only appear in the third generation after mutagenesis. Additionally, ''C. elegans'' hermaphrodites may be grown and screened for mutants together, since there is no danger of losing the homozygous form of a mutant by cross-fertilization. It is also advantageous that self-fertilization does not require copulation, allowing severely uncoordinated or deformed mutants to be propagated as homozygotes. Thus, many mutations that would be lethal in ''Drosophila'' or in the mouse are viable in ''C. elegans''. This not only simplifies maintenance of genetic stocks, but it makes possible the growth of large populations of such mutants for biochemical analysis. | ||
Reproduction by self-fertilization, though convenient for mutant isolation and maintenance, does not provide a means to recombine independently isolated mutations. Genetic analysis therefore depends on the existence of males, which are produced spontaneously in hermaphrodite populations by meiotic non-disjunction at a frequency of 0.1%. Males posses 5 pairs of autosomes and one X chromosome, while hermaphrodites posses two X chromosomes in addition to the autosomal complement (the XX vs. XO system). Males are distinguished from hermaphrodites by their smaller size and by the fan-like copulatory bursa at the tip of the male tail. A male culture can by propagated by mating males with hermaphrodites. Half the progeny produced by such cross-fertilization are male. In practice, a culture of wide-type males is constantly maintained and these males are used for mating with mutant hermaphrodites. Such crosses produce heterozygous males that are then used to transfer the mutant marker to other hermaphrodites, so that genetic mapping and complementation tests are possible.<br> | Reproduction by self-fertilization, though convenient for mutant isolation and maintenance, does not provide a means to recombine independently isolated mutations. Genetic analysis therefore depends on the existence of males, which are produced spontaneously in hermaphrodite populations by meiotic non-disjunction at a frequency of 0.1%. Males posses 5 pairs of autosomes and one X chromosome, while hermaphrodites posses two X chromosomes in addition to the autosomal complement (the XX vs. XO system). Males are distinguished from hermaphrodites by their smaller size and by the fan-like copulatory bursa at the tip of the male tail. A male culture can by propagated by mating males with hermaphrodites. Half the progeny produced by such cross-fertilization are male. In practice, a culture of wide-type males is constantly maintained and these males are used for mating with mutant hermaphrodites. Such crosses produce heterozygous males that are then used to transfer the mutant marker to other hermaphrodites, so that genetic mapping and complementation tests are possible.<br> | ||
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[[Image:Caenorhabditis male.jpg]]<br> | [[Image:Caenorhabditis male.jpg]]<br> | ||
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The majority of the mutants characterized | The majority of the mutants characterized to date are non-lethal and display a clearly visible phenotype. A large number of mutants are "uncoordinated" (now representing nearly 100 genes). Uncoordinated phenotypes range from small aberrations in movement to nearly complete paralysis. There are some great movies of hermaphrodite, male and mutant movement at [http://130.15.90.245/c__elegans_movies.htm Dr. Ian Chin-Sang's website]. | ||
Morphological mutants include dumpy, small, long, and blistered animals. Dumpy mutants are shorter than wild-type animals and correspond to twenty different genes dispersed over the six linkage markers and are useful for mapping most other classes of mutants, since the double mutants are easily distinguished. Both morphological and uncoordinated mutants can be used for mapping many types of developmental mutants. | |||
An increasing variety of mutations are now being studied including those affecting drug-resistance, chemotaxis, thermotaxis, male sexual behavior, catabolic pathways, dopamine biosynthesis, muscle assembly, sex determination, development of the ventral nerve cord, and temperature-sensitive lethal mutants affecting embryogenesis and gonadogenesis. A series of translocations and duplications has been characterized as a first step in assembling a collection of "balancers" for recessive lethal mutations. | An increasing variety of mutations are now being studied including those affecting drug-resistance, chemotaxis, thermotaxis, male sexual behavior, catabolic pathways, dopamine biosynthesis, muscle assembly, sex determination, development of the ventral nerve cord, and temperature-sensitive lethal mutants affecting embryogenesis and gonadogenesis. A series of translocations and duplications has been characterized as a first step in assembling a collection of "balancers" for recessive lethal mutations. | ||
Maintaining genetic stocks is less difficult with ''C. elegans'' than with some other organisms used in developmental or behavioral studies, such as ''Drosophila'' or the mouse. Stocks of ''C. elegans'' remain viable when frozen and stored in liquid nitrogen. | Maintaining genetic stocks of mutants and wild type strains is less difficult with ''C. elegans'' than with some other organisms used in developmental or behavioral studies, such as ''Drosophila'' or the mouse. Stocks of ''C. elegans'' remain viable when frozen and stored in liquid nitrogen. | ||
Consult [http://elegans.swmed.edu/ Leon Avery's ''C. elegans'' server] and [http://www.wormbase.org/ Wormbase] for more information about the molecular genetics of the worm. | |||
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