sources of variation: mutation recombination. vii.mutations i: changes in chromosome number and...

Sources of Variation:

MutationRecombination

VII. Mutations I: Changes in Chromosome Number and Structure

- Overview:


- Overview:1) A mutation is ….


- Overview:1) A mutation is a change in the genome of a cell.


- Overview:1) A mutation is a change in the genome of a cell.

2) Evolutionarily important mutations are heritable (not somatic). However, the tendency for a gene to mutate in somatic tissue (cancer) as a result of sensitivity to env conditions may be heritable.


- Overview:3) Changes occur at 4 scales (large to small)

- Change in the number of SETS of chromosomes (change in PLOIDY) - Change in the number of chromosomes in a set (ANEUPLOIDY: trisomy, monosomy) - Change in the number/arrangement of genes on a chromosome - Change in the nitrogenous base sequence within a gene

In general, the LARGER the change, the more dramatic (and usually deleterious) the effects. If you have a functioning genome, a big change is going to be MORE LIKELY to disable it than a small change…


A. Polyploidy


A. Polyploidy

1. Mechanisms:


A. Polyploidy

1. Mechanisms:

a. Autopolyploidy: production of a diploid gamete used in reproduction within a species.

Failure of meiosis I or II

2n gamete

Correct meiosis in other parent 1n gamete

3n zygote


A. Polyploidy

1. Mechanisms:


Errors in mitosis can also contribute, in hermaphroditic species

2n

1) Consider a bud cell in the flower bud of a plant.

2n


4n

2) It replicates it’s DNA but fails to divide... Now it is a tetraploid bud cell.

2n


4n


3) A tetraploid flower develops from this tetraploid cell; eventually producing 2n SPERM and 2n EGG

2n


4n


3) A tetraploid flower develops from this tetraploid cell; eventually producing 2n SPERM and 2n EGG

4) If it is self-compatible, it can mate with itself, producing 4n zygotes that develop into a new 4n species.

Why is it a new species?

How do we define ‘species’?

“A group of organisms that reproduce with one another and are reproductively isolated from other such groups”(E. Mayr – ‘biological species concept’)

How do we define ‘species’?

Here, the tetraploid population is even reproductively isolated from its own parent species…So speciation can be an instantaneous genetic event…

4n

2n

Gametes

2n

4n

Zygote

2n

1n

1n

Gametes

Zygote

2n3n

Triploid is a dead-end… so species are separate


A. Polyploidy

1. Mechanisms:


b. Allopolyploidy: fusion of gametes from different species (hybridization). These are usually sterile because the chromosomes are not homologous and can’t pair during gamete formation. BUT… if the chromosomes replicate and separate without cytokinesis, they create their own homologs and sexual reproduction is then possible.

Spartina alterniflora from NA colonized Europe

Spartina maritima native to Europe

X

Spartina anglica – an allopolyploid and a worldwide invasive outcompeting native species

Sterile hybrid – Spartina x townsendii

Allopolyploidy – 1890’s


A. Polyploidy

1. Mechanisms:

2. Frequency:

Polyploidy is common in plants; 50% of angiosperm species may be the product of polyploid speciation events.


A. Polyploidy

1. Mechanisms:

2. Frequency:

Polyploidy is common in plants; 50% of angiosperm species may be the product of polyploid speciation events.

In vertebrates, polyploidy decreases in frequency from fish to amphibians to reptiles, and is undocumented in birds. There is one tetraploid mammal. (Red viscacha rat).


A. Polyploidy

1. Mechanisms:

2. Frequency:

3. The effect of hermaphrodism:


A. Polyploidy

1. Mechanisms:

2. Frequency:


- when the sexes are separate, the rare, random mutation of producing a diploid gamete is UNLIKELY to occur in two parents simultaneously. So, the rare diploid gamete made by one parent (karyokinesis without cytokinesis doubling chromosome number in a cell) will probably fertilize a normal haploid gamete. This produces a TRIPLOID… which may live, but would be incapable of sexual reproduction.

2n

1n3n


A. Polyploidy

1. Mechanisms:

2. Frequency:


- unless…. the new organism could ALSO produce eggs without reduction..clonally… and these are the rare animals that we see – triploid ‘species’ that are composed of females that reproduce asexually. (Some may still mate with their diploid ‘sibling’ species so that the sperm stimulated the egg to develop – but without incorporation of sperm DNA.)

Like this Blue-spotted Salamander A. laterale, which has a triploid sister species, A. tremblayi

C. InornatusC. neomexicanus

C. tigris

Parthenogenetic diploid


A. Polyploidy

1. Mechanisms:

2. Frequency:


- So, in species with separate sexes, polyploidy is probably rare because the typical condition would be TRIPLOIDY, which is usually a sterile condition.


A. Polyploidy

1. Mechanisms:

2. Frequency:


- So, in species with separate sexes, polyploidy is probably rare because the typical condition would be TRIPLOIDY, which is usually a sterile condition.

- But in hermaphroditic organisms (like many plants), a single mutation can affect BOTH male and female gametes.


A. Polyploidy

1. Mechanisms:

2. Frequency:


SO! Polyploidy may be more frequent in plants because they are hermaphroditic more often than animals; especially vertebrates. Most cases of polyploidy in animals is usually where triploid females survive and reproduce asexually.

Also, simpler development in plants means they may tolerate imbalances better.


A. Polyploidy

1. Mechanisms:

2. Frequency:


4. Evolutionary Importance:

- obviously can be an instant speciation event - polyploidy is also a mechanism for “genome doubling” or “whole genome duplication” - this duplication allows for divergence of copied gene function and evolutionary innovation. Eventually, the copies may be so different that they don’t really represent duplicates any more… resulting in “diploidization”.


A. Polyploidy

B. Aneuploidy

B. Aneuploidy

1. Mechanism: Non-disjunction during gamete formationDuring either Meiosis I or II, segregation of (homologs or sister chromatids) does not

occur; both entities are pulled to the same pole.

B. Aneuploidy

1. Mechanism: Non-disjunction during gamete formationDuring either Meiosis I or II, segregation of (homologs or sister chromatids) does not

occur; both entities are pulled to the same pole.

This produces gametes with one more (1n + 1) or one less (1n -1) chromosome than they should have. Subsequent fertilization with a normal haploid (1n) gamete produces a trisomy (2n+1) or monosomy (2n-1).


A. Polyploidy

B. Aneuploidy

C. Changes in Gene Number and Arrangement


A. Polyploidy

B. Aneuploidy


1. Deletions and Additions:


A. Polyploidy

B. Aneuploidy



a. mechanisms:

i. unequal crossing over:

i. Unequal Crossing-Over

a. process:

If homologs line up askew:

A

a b

B

A

a b

B


a. process:

If homologs line up askewAnd a cross-over occurs

A a b

B


a. process:

If homologs line up askewAnd a cross-over occurs Unequal pieces of DNA will be exchanged… the A locus has been duplicated on the lower chromosome and deleted from the upper chromosome


a. process: b. effects:

- can be bad:deletions are usually bad – reveal deleterious recessivesadditions can be bad – change protein concentration




- can be good:more of a single protein could be advantageous (r-RNA genes, melanin genes, etc.)




- can be good:more of a single protein could be advantageous (r-RNA genes, melanin genes, etc.)

source of evolutionary novelty (Ohno hypothesis - 1970)where do new genes (new genetic information) come from?

Gene A Duplicated A

Mutation – may even render the proteinnon-functional

But this organism is not selected against, relative to others in the population that lack the duplication, because it still has the original, functional, gene.

generations

Mutation – may even render the proteinnon-functional

Mutation – other mutations may render the protein functional in a new way

So, now we have a genome that can do all the ‘old stuff’ (with the original gene), but it can now do something NEW. Selection may favor these organisms.

Gene A Duplicated A

generations

If so, then we’d expect many different neighboring genes to have similar sequences. And non-functional pseudogenes (duplicates that had been turned off by mutation).These occur – Gene Families

And, if we can measure the rate of mutation in these genes, then we can determine how much time must have elapsed since the duplication event…

Gene family trees…


A. Polyploidy

B. Aneuploidy



a. mechanisms:

i. unequal crossing over: (both) ii. Intercalary Deletion

A CB


a. mechanisms:


A C

B


a. mechanisms:


A C


a. mechanisms:



a. mechanisms:


-recognized by the formation of a ‘deletion loop’ in homolog during synapsis:


a. mechanisms:

i. unequal crossing over: (both) ii. Intercalary Deletion iii. Transposons (addition)

- transposons are copied (replicated) independent of the S phase of interphase…the copy is inserted elsewhere in the genome. Create homologus regions that lead to unequal crossing over and duplications


a. mechanisms:

i. unequal crossing over: (both) ii. Intercalary Deletion iii. Transposons (addition)

- OR, a transposon can be inserted within a gene, destroying it and functionally ‘deleting’ it.

VI. MutationA.OverviewB.Changes in PloidyC.Changes in ‘Aneuploidy’ (changes in chromosome number)D. Change in Gene Number/Arrangement

1.Deletions and Additions2.Inversion (changes the order of genes on a chromosome)

VII. MutationA. Changes in PloidyB Changes in ‘Aneuploidy’ (changes in chromosome number)C. Change in Gene Number/Arrangement




Chromosomes are no longer homologous along entire length

B-C-D on topd-c-b on bottom



Chromosomes are no longer homologous along entire length

ONE “loops” to get genes across from each other…

And if a cross-over occurs….

The cross-over products are non-functional, with deletions AND duplications



The only functional gametes are those that DID NOT cross over – and preserve the parental combination of alleles



Net effect: stabilizes sets of genes. This allows selection to work on groups of alleles… those that work well TOGETHER are selected for and can be inherited as a ‘co-adapted gene complex’




1.Deletions and Additions2.Inversion (changes the order of genes on a chromosome)3.Translocation

Translocation Downs.

Transfer of a 21 chromosome to a 14 chromosome

Can produce normal, carrier, and Down’s child.

VII. MutationA. Changes in PloidyB Changes in ‘Aneuploidy’ (changes in chromosome number)C. Change in Gene Number/ArrangementD. Change in Gene Structure

1.Mechanism #1: Exon Shuffling

Crossing over WITHIN a gene, in introns, can recombine exons within a gene, producing new alleles.

EXON 1a EXON 2a EXON 3a Allele “a”

EXON 1A EXON 2A EXON 3A Allele “A”


1.Mechanism #1: Exon Shuffling

Crossing over WITHIN a gene, in introns, can recombine exons within a gene, producing new alleles.

EXON 1a EXON 2a EXON 3a Allele “a”

EXON 1A EXON 2A EXON 3A Allele “A”

EXON 2a EXON 3aEXON 1A

EXON 2A EXON 3AEXON 1a

Allele “α”

Allele “ά”

Throws off every 3-base codon from mutation point onward


1. Mechanism #1: Exon Shuffling 2. Mechanism #2: Point Mutations

a. addition/deletion: “frameshift” mutations

…T C C G T A C G T ….

Normal

…A G G C A U G C A …

ARG HIS ALA

Mutant: A inserted

…T C C A G T A C G T ….

…A G G U C A U G C A …

ARG SER CYS

DNA

m-RNA


1. Mechanism #1: Exon Shuffling 2. Mechanism #2: Point Mutations

a. addition/deletion: “frameshift” mutationsb. substitution

At most, only changes one AA (and may not change it…)

… T C C G T A C G T ….

Normal

…A G G C A U G C A …

DNA

m-RNA

ARG HIS ALA

Mutant: A for G

…T C C A T A C G T ….

…A G G U A U G C A …

ARG TYR ALA

SOURCES OF VARIATION

MUTATION

RECOMBINATION??

RECOMBINATION

Independent Assortment

Independent Assortment produces an amazing amount of genetic variation.

Consider an organism, 2n = 4, with two pairs of homologs. They can make 4 different gametes (long Blue, Short Red) (Long Blue, Short Blue), (Long Red, Short Red), (Long Red, Short blue). Gametes carry thousands of genes, so homologous chromosomes will not be identical over their entire length, even though they may be homozygous at particular loci.

Well, the number of gametes can be calculated as 2n

or


Consider an organism with 2n = 6 (AaBbCc) ….There are 2n = 8 different gamete types.

ABC abcAbc abCaBC AbcAbC aBc



And humans, with 2n = 46?

ABC abcAbc abCaBC AbcAbC aBc



And humans, with 2n = 46?

223 = ~ 8 million different types of gametes.

And each can fertilize ONE of the ~ 8 million types of gametes of the mate… for a total 246 = ~70 trillion different chromosomal combinations possible in the offspring. ABC abc

Abc abCaBC AbcAbC aBc

Independent Assortment produces an amazing amount of genetic variation.And each can fertilize ONE of the ~ 8 million types of gametes of the mate… for a total 246 = 70 trillion different chromosomal combinations possible in the offspring.

YOU are 1 of the 70 trillion combinations your own parents could have made. IA creates a huge amount of genetic variation, and that doesn’t include crossing over!!!!

VII. MutationA. Changes in PloidyB Changes in ‘Aneuploidy’ (changes in chromosome number)C. Change in Gene Number/ArrangementD. Change in Gene StructureE. Summary

Sources of Variation Causes of Evolutionary Change

MUTATION: Natural Selection -New Genes: point mutation Mutation (polyploidy can make new exon shuffling species)

RECOMBINATION: - New Genes: crossing over -New Genotypes: crossing over independent assortment

SOURCES OF VARIATION

MUTATION

RECOMBINATIONIndependent AssortmentCrossing Over – New combinations of genes on chromosomes

“Hey!! That solves my dilemma about how new variation is produced each generation!! Too bad I’m dead!

sources of variation: mutation recombination. vii.mutations i: changes in chromosome number and...

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chromosome change

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tetraploid bud cell

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tetraploid cell

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