Variation in Chromosome Number and Arrangement

1. Variation in Chromosome Number and Arrangement Timothy G. Standish, Ph. D.

2. The Modern Synthesis • Charles Darwin recognized that variation existed in populations and suggested natural selection as a mechanism for choosing some variants over others resulting in survival of the fittest and gradual changes in populations of organisms. • Without a mechanism for generation of new variation, populations would be selected into a corner where only one variation would survive and new species could never arise. • The Modern Synthesis combines the mechanism of mutation in DNA to generate variation with natural selection to produce new species.

3. Mutation • Mutation = Change • Biologists use the term “mutation” when talking about any change in the genetic material. Not all result in a change in phenotype. • There are two major types of mutations: • Macromutations - Also called macrolesions and chromosomal aberrations. Involve changes in large amounts of DNA. • Micromutations - Commonly called point mutations and microlesions.

4. Macromutations • Six major types of macromutations in two categories are recognized: • Change within or between chromosomes 1 Deletions - Loss of chromosome sections 2 Duplications - Duplication of chromosome sections 3 Inversions - Flipping of parts of chromosomes • Translocations - Movement of one part of a chromosome to another part • Changes in Chromosome number • Polyploidy - Addition of whole sets of chromosomes • Aneuploidy - Addition or subtraction of chromosomes, but not whole sets

5. Chromosome Centromere Genes A B C D E F G H A B C D G H E F 1 Deletion

6. A B C D E F G H A B C D G H Deletions Can Be Seen During Pairing of Homologous Giant Chromosomes

7. Compensation loop E F A B C D G H A B C D G H Deletions Can Be Seen During Pairing of Homologous Giant Chromosomes

8. A B C D E F G H A B C D E F G H A B C D E F A B C D G H F E A B C D G H Production of Deletions • Two types of deletions occur: • Terminal deletions • Intercalary deletions

9. Impact of Deletions • Large deletions are generally lethal • In a very general way, the larger the deletion, the larger the impact • Deletion of genes essential for survival is always lethal • Pseudodominance, in which typically recessive alleles behave in a dominant fashion may result from deletions due to short stretches of chromosomes which become hemizygous due to a deletion in one homolog • The Notch mutation, deletion of a short stretch of the Drosophila X chromosome, provides an example of this • When the notch deletion is on one X chromosome, white, facet and split alleles on the other X chromosome behave in a dominant fashion

10. Cri-du-Chat SyndromeThe cry of the cat • Cri-du-chat syndrome results from deletion of approximately 1/2 the human chromosome 5 p arm • It is one of very few human deletions known to survive to live birth • The larger the deletion the more severe the symptoms which include: • Gastrointestinal and cardiac malformation • Mental retardation • Abnormal development of the glottis and larynx resulting in a cry resembling that of a mewing cat • Incidence of about 1/50,000 live births

11. Chromosome Centromere Genes A B C D E F G H A B C D E FE F G H E F Duplication 2 Duplication

12. A B C D E F G H A B C D E FE F G H Duplication Also Produces Compensation Loops

13. A B C D E F G H A B C D E F G H E F Compensation loop Duplication Also Produces Compensation Loops

14. A B C D E F G H A B C D E F G H A B C D E F G H A B C D E FE F G H A B C D E F G H A B C D G H Production of Duplications • Duplications are thought to be the result of unequal crossing over

15. Impact of Duplications • More copies of a gene results in more of the gene product of the gene • This can result in dosage problems • Position effects may also have an influence on the way gene duplication impacts organisms

16. Chromosome Centromere Genes A B C D EF G H A B C D F E G H Inversion 3 Inversion

17. Chromosome Centromere Genes A B C D E F G H A B E F C D G H 4 Translocation

18. Does Speciation Occur? • Yes • There are a number of well -documented examples of speciation • A good example is the London planetree:

19. Normal Chromosome Numbers In Selected Species Organism Chromosomes Homo sapiens 46 Cat 38 Rat 42 Hen 78 Drosophila 8 Hydra 30 Corn 20 Peas 14 E. coli 1

20. American Planetree or Sycamore Platanus occidentalis

21. Oxford Botanical Gardens c1670 Platanus acerifolia North America Europe >5,000 km Platanus occidentalis Platanus orientalis Atlantic Africa The London Planetree 50,000,000 Years • Speciation in the production of P. acerifolia was the result of hybridization, not mutation and selection • If they can interbreed, are P. occidentalis and P. orientalis really separate species?

22. In His Own Words "I may, of course, be egregiously wrong; but I cannot persuade myself that a theory which explains several large classes of facts can be wholly wrong...." Charles Darwin, November 13, 1859, in letter to L. Jenyns

23. Has Natural Selection Been Shown To Produce New Species? • No • Darwin emphasized the slow gradual change that is necessary if natural selection is to work on changing a population gradually from one species to another • “Many large groups of facts are intelligible only on the principle that species have been evolved by very small steps.” • The Origin of Species Chapter VII under “Reasons for disbelieving in great and abrupt modifications”

24. The End