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Table 8.3 & Alberts Fig.1.38
EVOLUTION OF GENOMES
C-value paradox: - in certain cases, lack of correlation between morphological complexity and genome size
“[For] some commonly cited extreme values for amoebae... considerable uncertainty about the accuracy of these measurements and the ploidy level of the species...” Gregory Nature Rev. Genet. 6:699, 2005
Fig. 8.15
Genic fraction vs. genome size
Function of non-genic DNA in eukaryotes?
Gregory Nature Rev. Genet. 6:699, 2005
Composition of human genome
Hartwell Fig. 21.11
Genic contribution to expansion in genome size
Figure 8.7
Scenario showing possible events following whole genome duplication
26 genes on 2 chromosomes
36 genes on 4 chromosomes
Kellis Nature 428:617, 2004
Evidence for whole genome duplication in ancestor of yeast
see also Fig.8.7
~ 100 million years ago?
Frequency distribution of haploid chromosome numbers in dicot plants
For chromosome number >12, even numbers much more common than odd numbers
Griffiths 7th ed, Fig. 26-12
Duplication of entire genome much more common in plant evolution than in animal evolutionary history
Fig. 6.25
Over evolutionary timeexpect independent mutationsto accumulate
Evolution of tandem arrays of eukaryotic genes
… but often observe all copiesidentical (or nearly so)
- evolve “in concert”
Concerted evolution
- maintenance of homogeneous nt sequences among multi-gene family members (especially when in tandem arrays)
- eg. eukaryotic ribosomal RNA gene copies
- exchange of sequence info so members kept very similar
Fig. 6.26
Fig. 6.27
Possible evolutionary scenarios resulting in “homogenized” tandem array
1. Beneficial mutations fixed by positive selection
-but spacers with no known function show concerted evolution
2. Recent amplification
3. Mutation in one repeat “spreads” to others
Fig. 6.31
Unequal crossing over
- homologous recombination between misaligned arrays
- change in number of repeats
Example of unequal crossing over in human globin array
misalignment (of sister chromatidsduring mitosis in germ cell orhomologous chromosomes during meiosis…)
Page & Holmes Fig. 3.15
“Lepore” thalassemia
Gene conversion
- non-reciprocal recombination
- no change in gene copy number
- can occur in dispersed as well as tandem repeats
Fig. 6.29 Watson Fig. 10-21
- example of yeast mating-type switching
Fig. 6.33
Exon 3 Exons 1 & 2
How do you interpret these data?
Example of concerted evolution in primate globin gene cluster
... and panel 3 of Fig.6.33 ?
Fig. 6.33
PR pancreative ribonuclease
SR seminal ribonuclease
Resurrection of ribonuclease pseudogene by gene conversion
What is predicted status of SR gene in giraffe? or sheep?
… in some bovine species, gene conversion of SR with PR gene, so functional again
Factors affecting rate of concerted evolution (p. 317-320)
1. Number, arrangement, structure of repeats
2. Functional requirement
- selective advantage of high amount of same geneproduct vs. diversity
3. Population size
- non-coding regions evolve more rapidly, and ifdivergent enough may “escape” homogenization
- time for variant to be fixed or eliminated
Evolutionary implications of concerted evolution (p.320-322)
1. Spread of advantageous mutations (or removal of deleterious ones)
2. Retards paralogous gene divergence (preventing redundant copy from becoming non-functional)
3. Generates increased genetic variation at a particular locus within a population
“molecular drive”
Methodological implications
- degree of sequence divergence of paralogous genes undergoing concerted evolution is not correlated with evolutionary time
so gene duplications can appear younger than they really are…