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Parasexuality - genetic analysis of nuclear genes without meiosis BIO341

Parasexuality - genetic analysis of nuclear genes without meiosis BIO341. Meiotic analysis limited to those organisms that can be crossed sexually. Organism of interest may not reproduce sexually. Organisms of interest may be too distantly related to cross.

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Parasexuality - genetic analysis of nuclear genes without meiosis BIO341

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  1. Parasexuality - genetic analysis of nuclear genes without meiosisBIO341 • Meiotic analysis limited to those organisms that can be crossed sexually. • Organism of interest may not reproduce sexually. • Organisms of interest may be too distantly related to cross. • Your concern may be with somatic cells.

  2. Stages of the parasexual cycle in Aspergillus • 1. heterokaryosis • 2. nuclear fusion - diploidy (1/106 to 1/107) • 3. propagation of diploid • 4. mitotic crossing over ( 2/103) • 5. haploidization (1/103) Note: steps 2, 4, and 5 occur at low frequency. Also, steps 4 and 5 are completely independent of one another and occur by completely different mechanisms.

  3. Example in Aspergillus nidulansw pro x y ad paba

  4. Example in Aspergillusw pro x y ad paba • Pair strains on complex medium. • Transfer from confluent zone of colonies to minimal medium. • Yellow and white clusters of conidia mixed together are characteristic of heterkaryons. • Each cluster is either yellow or white (indicating what?). • Harvest conidia, plate on MM to find diploids. Conidia are uninucleate. Why is this important in this scheme? • Diploids have green conidia. • During growth colored sectors may be produced. The ploidy of these sectors can be inferred from conidial size. • Segregants in following selected by color (From Pontecorvo and Kafer).

  5. Wild-type is green White and yellow sectors

  6. Diploid formation A B a b C c A B a b c C

  7. Diploids (post chromatid replication) A B a b c C

  8. Diploid - metaphase plate c A B a b C a b c A B C

  9. Normal chromosome segregation to form daughter diploids a c A B b C A B C a b c

  10. Abnormal chromosome segregation to form an aneuploid a c A B b C a b A B C c

  11. Aneuploid - metaphase plate c A B C c A B C

  12. Haploid, recombinant for A/C and for B/C, but not for A/B Another abnormal chromosome segregation to form a haploid C c A B C A B c

  13. Summary of haploidization

  14. Mitotic crossing over: A B A B 1 b A B 2 b a 3 a B a a b 4 b 50% 1,3 and 2,4: heterozygosity at A, homozygosity at B 50% 1,4 and 2,3: heterozygosity at both A and B

  15. Mapping chromosomes by mitotic crossing over between a selectable marker and the centromere. Plate on MM containing histidine, proline, biotin and arginine, but no adenine. Recover ade-suppressed, assay other markers. bio pro arg Ade+segregants: 50 Prototrophic 200 Arg- 100 Arg-Bio- 150 Arg-Bio-Pro- What is map for Arg, Bio, Pro, and the centromere? ade his su + ade + + + +

  16. Parasexual cross: w pro x y ad pb Below are the phenotypes and ploidy of segregants from this parasexual cross. Diploid: 32 Prototrophic, Yellow 78 Paba-, Yellow 56 Prototrophic, White Haploid 19 Paba-, Ad-, Yellow 25 Pro-, White 25 Paba-, Ad-, White

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