Variation. Evolution Lectures 3

Transcription

1 Variation Evolution Lectures 3

2 What is variation? Inspiration of Darwin was individuals are different and these are the steps of evolution. Today we think the variation at the population level is the source for evolution. In other words: variability refers to the tendency or propensity to exhibit variation rather than a particular instance of variation. We would agree that genetic variation leads to evolution and we are going to learn some of the mechanisms leading to variation.

3 Sources of variation Chromosomal Aneuploidy Polyploidy Rearrangement Gene regulation Mutation Point Recombination Transposon Horizontal gene transfer Selfish DNA

4 Aneuploidy Unbalanced chromosomal distribution Chromosomes are added or subtracted Abnormal meiotic disjunction Plants have more

5 Extra sets of chromosomes More common in plants Interbreeding Polyploidy Triploid crops: apple, banana, citrus, ginger, watermelon Tetraploid crops: apple, cotton, potato, cabbage, Hexaploid crops: wheat, oat, kiwifruit Octaploid crops: strawberry, sugar cane

6 Love terminologies?

7 Evolution of wheat Natural hybrid of a wild einkhorn and a goat grass; both had 14 chromosomes and therefore hybrid, wild emmer, 28 chromosomes. Fertile crecent, years. Human selection through cultivation created new form of emmer, 9500 years Emmer formed another hybrid (42 chromosome) with another goat grass from southwest of Caspian sea; 9000 years. A lucky mutation changed the nature of the ear/spike, producing a shell allowing thrashing; 8500 years. That means this new species/hybrid (T. aestivum) needs human to propagate them. Since then we are selecting this species for varieties based on our need.

8 Laboratory created speciation Cross between Nicotiana tabacum (n=24) and N. glutinosa (n=12) created sterile hybrid The hybrid kept alive by vegetative propagation Eventually chromosome-doubling event happened New species N. digluta (n=36)

9 Chromosomal rearrangement Deletions or Deficiencies Loss of chromosomal material

10 Chromosomal rearrangement Duplication Prevalent cause is unequal crossing over

11 Chromosomal rearrangement Inversion Lowers the recombination frequency

12 Chromosomal rearrangement Translocation Transfer of material from one chromosome to a non-homologous chromosome

13 Example of translocation

14 Gene regulation

15 Gene regulation Prokaryote

16 Gene regulation Eukaryote Special regulatory proteins binds to designated regulatory sites next to the gene cis regulation Special sites on DNA sequence like CAAT & TATA boxes Changes in DNA double helical form from right hand to the left hand The regulatory proteins trans regulation These are called transcription factors Post-transcriptional regulation RNAi: RNA interference, dsrna, sirna mirna: micro RNA

17 RNAi

18 mirna

19 How gene regulation works Bicoid Hunchback Krüppel Knirps Giant Drosophila gastrulation Fushi tarazu

20 Mutation Point

21 Mutation Recombination

22 Mutation Transposon

23 Mutation Horizontal gene transfer When DNA is transferred between individuals of the same generation Transposons can be a tool At least 2/3 of all the gene families have been subjected to gene transfer Selfish DNA Repeated DNA sequences (like transposable elements) contribute very little to the host function They gets replicated nonetheless Can occasionally provide platform for adaptive mutation

24 Estimating mutation rate Mutation rates vary On an average 10-6 to 10-5 mutations per gamete per generation Average mutation rate per base pair to (prokaryotes) 10-9 (eukaryotes) 4.8 X 10-9 (human)

25 Spontaneous mutation rate Human genome has 3.2X10-9 Drake 1998 bp Mutation rate is 4.8 X 10-9 That is 317 mutations per generation 2.5% of the genome is functional That is 7 potential mutations Lynch et al 1999

26 Mutation is random Although we can predict what is the mutation rate we can not predict where Mutation is not influenced by the environment Lederberg (Joshua and Luria) expt

27 Genetic polymorphism Recessive (both neutral and deleterious) can accumulate in a population Provides a pool of genetic variability Organism Number of species examined Average loci studies / species Proportion of polymorphic loci Heterozygo sity per locus Drosophila Reptiles Human

28 What if the characters are not product of a single gene? These are genes with small phenotypic effect Polygenes & Quantitative trait loci Threespine stickleback 1 QLT for variation in plate number 2 QTL for variation in gill rakers number

29 Continuous variation

30 Strickberger s evolution (4 th ) Chapter 10 Some images and data from Evolution by Futuyama (2005)