Mutations at single DNA bases Kinds of mutations

1. Point mutations (change in a single DNA base)

2. Chromosomal mutations • Insertions & deletions • Inversions • duplications • Polyploidizations

Transitions are more frequent than transversions

Synonymous (“silent”) mutations: (karyotype) mutations

Changes in the 3rd base of a codon that do not change the amino acid of the made by • Karyotype: The chromosomal arrangement of an that gene (because of redundancy in the individual (or species) genetic code) • Changes in the karyotype – Deletions and insertions – Inversions Non-synonymous (replacement) mutations: – Gene duplications – Polyploidization Changes in a DNA sequence that do change an amino acid

1 Duplication: Inversion: A doubling of a part of a chromosome, A reversal of the orientation of a of an entire chromosome, or even the part of a chromosome whole genome

Tandem duplication resulting from unequal crossing-over during meiosis

Synteny between human and mouse

MGSC 2002

Human, Chimp, Gorilla, Orangutan: 2N=46 Mouse: 2N=40

Down’s Syndrome Male (21 triploidy) Rice: 2N=24

2 The standard model of genome evolution What is the origin of novelty, and diversity? DNA sequence

The engine : Mutations

Selection (purifying or positive) The steering wheel : Random drift of neutral mutations

DNA sequence (Altered) Or, how do you acquire new function while maintaining the old? Carroll S.B. et al. From DNA to Diversity (2001) Blackwell Science

Evolution by Gene Duplication Susumu Ohno, 1970 A new function is created by duplicating an old gene and modifying the copy.

“natural selection merely modified, while redundancy created”

3 Duplications occur at all genomic scales! A modified model of genome evolution Short indels DNA sequence

y Single nucleotide changes, short insertions or deletions,

c

n inversions, recombinations, domain duplication, gene

e

u

q duplication, cluster duplication, segment duplication,

e r chromosome duplication, genome duplication,

F

t

n

e

r

a

p Selection (purifying or positive) p Domain Mutations A (exon) The steering of wheel : different Random drift of neutral mutations kinds Gene

Gene cluster DNA sequence (Altered) Segment Chromosome Genome

GATCTACCATGAAAGACTTGTGAATCCAGGAAGAGAGACTGACTGGGCAACATGTTATTCAG GTACAAAAAGATTTGGACTGTAACTTAAAAATGATCAAATTATGTTTCCCATGCATCAGGTG Drosophila homeobox as example of gene duplication CAATGGGAAGCTCTTCTGGAGAGTGAGAGAAGCTTCCAGTTAAGGTGACATTGAAGCCAAGT CCTGAAAGATGAGGAAGAGTTGTATGAGAGTGGGGAGGGAAGGGGGAGGTGGAGGGATGG Homeotic mutants GGAATGGGCCGGGATGGGATAGCGCAAACTGCCCGGGAAGGGAAACCAGCACTGTACAGAC Normal fly CTGAACAACGAAGATGGCATATTTTGTTCAGGGAATGGTGAATTAAGTGTGGCAGGAATGCT TTGTAGACACAGTAATTTGCTTGTATGGAATTTTGCCTGAGAGACCTCATTGCAGTTTCTGAT TTTTTGATGTCTTCATCCATCACTGTCCTTGTCAAATAGTTTGGAACAGGTATAATGATCACA ATAACCCCAAGCATAATATTTCGTTAATTCTCACAGAATCACATATAGGTGCCACAGTTATCC CCATTTTATGAATGGAGTEvolutionbyGenomeDuplicationGATGAAAACCTTAGGAATA ATGAATGATTTGCGCAGGCTCACCTGGATATTAAGACTGAGTCAAATGTTGGGTCTGGTCTG ACTTTAATGTTTGCTTTGTTCATGAGCACCACATATTGCCTCTCCTATGCAGTTAAGCAGGTA GGTGACAGAAAAGCCCATGTTTGTCTCTACTCACACACTTCCGACTGAATGTATGTATGGAG Extra set of wings! TTTCTACACCAGATTCTTCAGTGCTCTGGATATTAACTGGGTATCCCATGACTTTATTCTGAC Ubx (Ultrabithorax) gene mutant ACTACCTGGACCTTGTCAAATAGTTTGGACCTTGTCAAATAGTTTGGAGTCCTTGTCAAATAG TTTGGGGTTAGCACAGACCCCACAAGTTAGGGGCTCAGTCCCACGAGGCCATCCTCACTTCA GATGACAATGGCAAGTCCTAAGTTGTCACCATACTTTTGACCAACCTGTTACCAATCGGGGG TTCCCGTAACTGTCTTCTTGGGTTTAATAATTTGCTAGAACAGTTTACGGAACTCAGAAAAAC AGTTTATTTTCTTTTTTTCTGAGAGAGAGGGTCTTATTTTGTTGCCCAGGCTGGTGTGCAATGG TGCAGTCATAGCTCATTGCAGCCTTGATTGTCTGGGTTCCAGTGGTTCTCCCACCTCAGCCTC CCTAGTAGCTGAGACTACATGCCTGCACCACCACATCTGGCTAGTTTCTTTTATTTTTTGTATA Antennae are transformed into legs! GATGGGGTCTTGTTGTGTTGGCCAGGCTGGCCACAAATTCCTGGTCTCAAGTGATCCTCCCAC Antp (Antennapedia) gene mutant CTCAGCCTCTGAAAGTGCTGGGATTACAGATGTGAGCCACCACATCTGGCCAGTTCATTTCCT

ATTACTGGTTCATTGTGAAGGATACATCTCAGAAACAGTCAATGAAAGAGACGTGCATGCTG Carroll S.B. et al. From DNA to Diversity (2001) Blackwell Science GATGCAGTGGCTCATGCCTGTAATCTCAGCACTTTGGGAGGCCAAGGTGGGAGGATCGCTTA AACTCAGGAGTTTGAGACCAGCCTGGGCAACATGGTGAAAACCTGTCTCTATAAAAAATTAA AAAATAATAATAATAACTGGTGTGGTGTTGTGCACCTAGAGTTCCAACTACTAGGGAAGCTG AGATGAGAGGATACCTTGAGCTGGGGACTGGGGAGGCTTAGGTTACAGTAAGCTGAGATTG TGCCACTGCACTCCAGCTTGGACAAAAGAGCCTGATCCTGTCTCAAAAAAAAGAAAGATACC CAGGGTCCACAGGCACAGCTCCATCGTTACAATGGCCTCTTTAGACCCAGCTCCTGCCTCCCA GCCTTCT 4 Eight homeobox (Hox) genes regulate the identity of The Hox genes are paralogs with a highly conserved homeobox domain regions within the adult and embryo.

Multiple alignment of the homeobox domain:

Adult

The Hox genes are transcription regulators and the Homeobox domain is DNA-binding domain. Embryo

Carroll S.B. et al. From DNA to Diversity (2001) Blackwell Science Carroll S.B. et al. From DNA to Diversity (2001) Blackwell Science

At least 7 Hox clusters in zebrafish Evidence 2: Vertebrates have 4 copies of the Hox cluster

Amores et al. (1998) Science 282 1711-1714 Carroll S.B. et al. From DNA to Diversity (2001) Blackwell Science

5 Susumu Ohno and the 2R hypothesis Distribution of chromosome number in fishes is bi-modal

“It is our contention that the ancestors of reptiles, birds, and mammals have experienced at least one tetraploid evolution either at the stage of fish or at the stage of amphibians” (1970)

“A mammalian ancestor might have gone through at least one round of tetraploid evolution at the stage of fish” (1973)

Evolution by gene duplication, Ohno (1970)

Polyploidy events during eukaryote evolution The “one-to-four” rule: One genome duplication at the origin of the The contains up to four paralogs of vertebrates many Drosophila genes.

Another Marine genome invertebrates duplication following divergence with lamprey Genome duplications we’ll talk about

Wolfe, K.H. Nature Reviews Genetics (2001) 2 333-341 Spring J. FEBS Letters 400 (1997) 2-8

6 Things Fall Apart, or How to turn 5 genes into 9 by a 2R model The ‘one-to-four’ rule does not hold up entirely unless we consider independent duplications and deletions

When the human genome was estimated at 80,000 genes the “one-to- four” hypothesis was reasonable but now that the estimate is at 35,000.

Wolfe, K.H. Nature Reviews Genetics (2001) 2 333-341 Makalowski W. Genome Research (2001) 667-670

Nav1.1 Nav1.2 Nav1.3 Nav1.7 Nav1.4 Nav1.5 Nav1.8 Nav1.9 Nav1.6 The Voltage-gated Sodium (Na+) Channels

HC 2 HC 2 HC 2 HC 2 HC 17 HC 3 HC 3 HC 3 HC 12

Single copy in CNS CNS embry- PNS skeletal heart & PNS PNS CNS non-chordate animals PNS onic muscle skeletal PNS CNS muscle

TNa3 TNa4 TNa1 TNa6 TNa2 TNa7 TNa5 TNa8 (1.1/2/3/ (1.1/2/3/ (1.4a) (1.4b) (1.5/8/9b) (1.5/8/9a) (1.6a) (1.6b) 7a 7b) nervous nervous Adult Early and Early and Early CNS and CNS and Family of paralogs in vertebrates; up to 11 genes in mammals system system skeletal adult adult CNS, PNS PNS muscle skeletal heart PNS, and muscle; skeletal electric muscle; organ in adult Great and highly specific functional diversity S.m. heart

7 Eight sodium channel genes 8 genes in fish in teleosts exist on different 10 genes in mammals , and appear to have arisen through chromosomal or whole- How to make comparisons genome duplication between genes? Have to understand difference between In contrast, ten sodium channel genes in mammals exist on only four Gene orthologs chromosomes, and appear to have arisen through Gene paralogs tandem duplications (orthologous genes, vs. paralogous genes)

1-2-4 (2R) Model 1-2-4 (2R) Model

A= Ancestral A= Ancestral Na+ Channel M Na+ Channel

A A M = Muscle N group N = Neural group

8 SCN5 1-2-4 (2R) Model SCN10 HOX 1-2-4 (2R) Model m’ m’ SCN11 A A= Ancestral A= Ancestral M m’’ Na+ Channel M m’’ SCN4 HOXB Na+ Channel

A M = Muscle A M = Muscle N n’ group N n’ SCN8 HOXC group N = Neural group N = Neural group SCN1 SCN2 n’’ n’’ SCN3 HOX SCN7 D SCN9

What does the phylogeny of Na+ channels SCN5 tell us about gene orthology in vertebrates? SCN10 HOX 1-2-4-8 Model m’ SCN11 T2, T7 A Mammalian Teleost Expression SCN 5, T2 and T7 Heart A= Ancestral SCN10, [and PNS in M m’’ SCN4 HOXB Na+ Channel SCN11 mammals] T1, T6 SCN4 T1 and T6 Skeletal Muscle, and A M = Muscle EO in some fish SCN8 T5 and T8 Brain/CNS N n’ SCN8 HOXC group T5, T8 N = Neural group SCN1, SCN2, T3 and T4 CNS/PNS SCN3, SCN7, SCN9 [and heart in SCN1 mammals] SCN2 n’’ SCN3 HOX SCN7 D SCN9 T3, T4

9 Evolution of novel gene function: Ratio of nonsynonymous mutations to Two evolutionarily independent synonymous lineages of weakly-electric fish co- mutations can opted the same sodium channel help detect gene ortholog to “build” a novel different kinds of structure: the electric organ. selection

NS/S < 1 purifying selection

Mammalian Teleost Expression SCN4 T1 and T6 Skeletal NS/S > 1 Muscle, and EO in some fish positive selection

Evolution by Gene Duplication Susumu Ohno, 1970 Purifying selection: selection for an existing allele (DNA sequence), and against other mutations at the same locus. Nonsynonymous mutations selected against, but synonymous ones can accumulate

Positive selection: selection that favors new mutations, to fixation -- especially when the rate of fixations of nonsynonymous mutations exceeds the rate of fixation of neutral (synonymous) mutations “natural selection merely modified, while redundancy created”

10