Table 8.3

1
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. 6699, 2005
Table 8.3 Alberts Fig.1.38
2
Genic fraction vs. genome size
Function of non-genic DNA in eukaryotes?
Composition of human genome
Gregory Nature Rev. Genet. 6699, 2005
Fig. 8.15
3
Genic contribution to expansion in genome size
Hartwell Fig. 21.11
4
Scenario showing possible events following whole
genome duplication
26 genes on 2 chromosomes
36 genes on 4 chromosomes
Figure 8.7
5
Evidence for whole genome duplication in ancestor
of yeast
100 million years ago?
Kellis Nature 428617, 2004
see also Fig.8.7
6
Duplication of entire genome much more common in
plant evolution than in animal evolutionary
history
For chromosome number gt12, even numbers much more
common than odd numbers
Frequency distribution of haploid chromosome
numbers in dicot plants
Griffiths 7th ed, Fig. 26-12
7
Evolution of tandem arrays of eukaryotic genes
Over evolutionary time expect independent
mutations to accumulate
but often observe all copies identical (or
nearly so)
- evolve in concert
Fig. 6.25
8
Concerted evolution
- maintenance of homogeneous nt sequences among
multi-gene family members (especially when in
tandem arrays)
- exchange of sequence info so members kept very
similar
- eg. eukaryotic ribosomal RNA gene copies
Fig. 6.26
9
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. 6.27
10
Unequal crossing over
- homologous recombination between misaligned
arrays
- change in number of repeats
Fig. 6.31
11
Example of unequal crossing over in human
b-globin array
Lepore b - thalassemia
misalignment (of sister chromatids during mitosis
in germ cell or homologous chromosomes during
meiosis)
Page Holmes Fig. 3.15
12
Gene conversion
- non-reciprocal recombination
- no change in gene copy number
- can occur in dispersed as well as tandem repeats
- example of yeast mating-type switching
Fig. 6.29
Watson Fig. 10-21
13
Example of concerted evolution in primate b
-globin gene cluster
Exons 1 2
Exon 3
How do you interpret these data?
... and panel 3 of Fig.6.33 ?
Fig. 6.33
14
Resurrection of ribonuclease pseudogene by gene
conversion
PR pancreative ribonuclease
in some bovine species, gene conversion of y SR
with PR gene, so functional again
SR seminal ribonuclease
What is predicted status of SR gene in giraffe?
or sheep?
Fig. 6.33
15
Factors affecting rate of concerted evolution (p.
317-320)
1. Number, arrangement, structure of repeats
- non-coding regions evolve more rapidly, and
if divergent enough may escape homogenization
2. Functional requirement
- selective advantage of high amount of same
gene product vs. diversity
3. Population size
- time for variant to be fixed or eliminated
16
Evolutionary implications of concerted evolution
(p.320-322)
molecular drive
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
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