Mutation is ultimate source of all genetic variation

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Mutation is ultimate source of all genetic variation

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1. Mutation is ultimate source of all genetic variation

2. Other ways populations generate or acquire genetic variation? Gene flow (migration) Recombination Hybridization Horizontal gene transfer

3. Mutations in animals Germ line mutations Somatic mutations Which are heritable?

4. Mutations: single base-pair mutations

5. Other ‘silent’ mutations single-base mutations, deletions, or insertions within introns mutations within other non-coding regions such as repetitive DNA and pseudogenes

6. However, some noncoding sequences have essential functions promoters enhancers transcription termination signals intron splice junctions Mutations in these noncoding regions will have phenotypic effects.

7. Mutation rates

8. Best data from C. elegans Accumulated mutations in 72 inbred lines for 400 generations Sequenced ~62,000 bp from each line 30 mutations 13 insertions (1-500 bp) 4 deletions (1-66 bp) 13 base substitutions (8 transitions; 5 transversions) 2.1 mutations per haploid genome per generation Denver et al. (2004) Nature 430: 679-683.

9. Effects of mutations

10. Transposable elements translocate from one location to another can cause deletions, insertions, inversions, rearrangements

11. Hybrid Dysgenesis Sterility and gonad abnormalities in offspring of some crosses between different populations of Drosophila melanogaster. But only in one direction of the cross! Mv x Pƒ Pv x Mƒ Pv x Pƒ Mv x Mƒ dysgenesis normal

12. Hybrid dysgenesis is due to TEs called “P elements” Flies that can produce dysgenesis have these P elements, but dysgenesis only occurs when males that have these elements are crossed to females that don’t have them. Eggs from “P” females suppresses transposition. P elements spread through natural populations starting abut 50 years ago. Suppressor of transposition also evolved in these pops. But many “old” laboratory populations don’t have P elements b/c they were established from natural populations more than 50 years ago.

13. Hybrid Dysgenesis Also observed in other organisms: Mammals (marsupials) Plants (rice) Probably occurs in other organisms too.

14. Are mutations random? With respect to selective advantage? That is, do mutations arise spontaneously without regard to whether or not they are advantageous in the current environment? Or, is there a tendency for mutations to occur that are advantageous in the current environment?

15. Lederberg & Lederberg (1952) Asked whether adaptation arose from a process of random mutation followed by selection in a new environment, or whether environmental stressors directly induce mutations that confer an adaptive. Knew that if a population of E. coli was exposed to antibiotic, most of the bacterial cells would die. But, if the original population was large enough, it would soon be repopulated with bacteria, all of which were resistant to the antibiotic. Does antiobiotic select for rare pre-existing cells that have the resistance mutation, or does the antibiotic itself induce the cells to produce new mutations that confer resistance?

17. Conclusions Mutations arise randomly with respect to selective advantage in the current environment.

18. Are mutations random? With respect to position in the genome? No, mutational ‘hotspots’ exist Positions in DNA that mutate more frequently than expected (more often than other positions) Due to unusual character of those sites (e.g. repeated sequences, methylated bases)

19. Chromosomal mutations are the source of variation for chromosomal evolution

20. Gene duplications are a source of evolutionary novelty Ribosomal DNA Globins (alpha and beta) Homeobox genes

21. Inversions

23. Inversions Suppress Recombination! Organisms carrying an inversion tend to undergo little crossing over in the inversion region in both inversion and non-inversion chromosomes.

24. Figure: 07-15a Caption: (a) Possible origin of a reciprocal translocation. (b) Synaptic configuration formed during meiosis in an individual that is heterozygous for the translocation. (c) Two possible segregation patterns, one of which leads to a normal and a balanced gamete (called alternate segregation) and one that leads to gametes containing duplications and deficiencies (called adjacent segregation). Figure: 07-15a Caption: (a) Possible origin of a reciprocal translocation. (b) Synaptic configuration formed during meiosis in an individual that is heterozygous for the translocation. (c) Two possible segregation patterns, one of which leads to a normal and a balanced gamete (called alternate segregation) and one that leads to gametes containing duplications and deficiencies (called adjacent segregation).

28. Hybridization: introgression

29. Horizontal gene transfer Species that cannot interbreed should not be able to exchange genes But, a few cases of transfer of genetic material between distantly related species Cross-infection by retroviruses (which can incorporate host DNA into their genomes) is suspected route of gene transfer.

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