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Drosophila As a Genetic Model Drosophila as a Genetic Model Praveen V Prasad Asst Prof, Dept. of Botany Sree Narayana College Nattika PG 2nd Sem Drosophila as a model organism • Easy to breed • Tolerant to diverse conditions • short generation time (so several generations can be studied within a few weeks) • It has a high fecundity • Genome is sequenced • Large number genetic mutants are available • Free exchange of research material What is a model organism? • A model organism is a non-human species that is extensively studied to understand particular biological phenomenon, with the expectation that discoveries made in the organism model will provide insight into the workings of other organisms. What makes Model Organisms possible? Common ancestry of all organisms resulting conservation of major aspects of biology Transposon: Jumping genes DNA sequences that move from one location to another location in the genome Discovered by Barbara Mc Clintock, who was studying pigmentation of the kernel in maize Structure of Transposon Transposase gene 31 np inverted terminal repeat How does a transposon get inserted in the genome? Transposase binds to both ends of the transposon, which consist of inverted repeats It also binds to a sequence of DNA that makes up the target site. Some transposases require a specific sequence as their target site; others can insert the transposon anywhere in the genome. Transposons can mutate genes Hybrid Dysgenesis Hybrid dysgenesis refers to the high rate of mutation in germ line cells of Drosophila strains resulting from a cross of males with autonomous P elements (P Strain/P cytotype) and females that lack P elements (M Strain/M cytotype) Transposition only occurs in germ-line cells, because a splicing event needed to make transposase mRNA doesn’t occur in somatic cells Why don’t we see hybrid dysgenesis if we use females of P strain?? The eggs of P strain females contain high amounts of a repressor protein that prevents transcription of the transposase gene. The eggs of M strain mothers, which do not contain the repressor protein, allow for transposition of P elements from the sperm of fathers. How do the fly genetist use this? Transposase gene Gene of Interest 31 np inverted terminal repeat Implications in Drosophila Research 1.To introduce foreign DNA 2.Generate mutations 3.Study the spatial expression of gene Transformation: the Introduction of Cloned DNA into Flies • P elements used as vectors • Insert fly DNA into intact P element and then into plasmid • Inject into syncytial embryos from M strain mothers • Cross to P males • Mimicking hybrid dysgenesis Fig. D.8a Ref: Tata Mc Graw hill + P[w+, ry+], Cy ry + + Ki D2-3, ry+ + Sco ry Y + + + P[w+, ry+], Cy ry + + Ki D2-3, ry+ + Sco ry YX + + + P[w+, ry+], Cy Ki D2-3, r XX + ry Y + ry Y + ry (phenotype=Cy, Ki, ry+) + P[w+, ry+], Cy ry + + Ki D2-3, ry+ + Sco ry YX + + ^ + P[w+, ry+], Cy Ki D2-3, ry+ XX + ry Y + ry Y X + ry (phenotype Cy, Ki, ry+) ^ + P[w+, ry+], Cy ry + + Ki D2-3, ry+ + Sco ry YX + + ^ + P[w+, ry+], Cy Ki D2-3, ry+ XX + ry Y + ry Y X + ry (Cy, Ki, ry+) ^ +P? +P? ryP? XX + ry Y + ry Y + ry (not Cy, not Ki. If carrying a jumped P, will be ry+) + P[w+, ry+], Cy ry + + Ki D2-3, ry+ + Sco ry YX + + ^ + P[w+, ry+], Cy Ki D2-3, ry+ XX + ry Y + ry Y X + ry + (Cy, Ki, ry ) P? P? P? ^ + + ry XX + ry Y + ry X Y + ry (not Cy, not Ki. If carrying + a jumped P, will be ry ) Stain F2 for lacZ Look for desired expression pattern Implications in Drosophila Research 1.To introduce foreign DNA 2.Generate mutations 3.Study the spatial expression of gene Enhancer trap Fig. 20.18 Illustration of the use of P elements to introduce genes into the Drosophila genome 台大農藝系 遺傳學 601 20000 Chapter 20 slide 20 Peter J. Russell, iGenetics: Copyright © Pearson Education, Inc., publishing as Benjamin Cummings. Complementation Two strains of an organism with different homozygous recessive mutations that produce the same phenotype (for example, a change in wing structure in flies) produce offspring with the wild-type phenotype when mated or crossed. Complementation tests Q: When do you use this? Note: requires recessive mutations Complementation: the production of a mut-1 WT phenotype wild-type phenotype when two different mutations are combined in trans. mut-3 The two mutations are probably in different genes. mut-1 Noncomplementation Mut phenotype or failure to complement mut-2 between two mutations usually indicates that the two mutations are in the same gene. Complementation tests are always performed with mutations that give recessive phenotypes. Mutations that give dominant phenotype will, by definition, always fail to complement any other mutation that they are paired with. To do a complementation test, we generate the transheterozygote for two mutations. That is, we cross homozygotes for one mutation with homozygotes for another mutation. The cross progeny will therefore contain one allele from each mutant – these are called transheterozygotes. If the two mutations being studied are in different genes (like mut-1 and mut-3 above), then there will be a wild-type allele for each gene, and they will thus complement (give the wild-type phenotype). By contrast, if the two mutations are in the same gene (like mut-1 and mut-2 above), then both alleles for that gene will be mutated in the transheterozygote. Without a wild-type allele for that gene, the transheterozygote will show the mutant phenotype (i.e., there will have been a failure to complement). Uses of complementation In genetics, test for determining whether two mutations associated with a specific phenotype represent two different forms of the same gene (alleles) or are variations of two different genes. The complementation test is relevant for recessive traits When two mutations occur in different genes, they are said to be complementary, because the heterozygote condition rescues the function otherwise lost in the homozygous recessive state. Deficiency mapping!! Deficiency: In genetics, a deletion in which a part of a chromosome or a sequence of DNA is missing How does it work? Def 1 Def 2 Def 3 Def 4 Def 5 Pedigree Analysis A pedigree is a diagram of family relationships that uses symbols to represent people and lines to represent genetic relationships. Pedigrees are often used to determine the mode of inheritance (dominant, recessive, etc.) of genetic diseases. A sample pedigree chart Pedigree Chart Squares represent males and circles represent females. Shade in the symbol represent affected individual Horizontal lines connecting a male and female represent mating. Vertical lines extending downward from a couple represent their children. .
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