L11 Polyploidy Flashcards
C value paradox
the amount of DNA does not correlate with perceived complexity on the phylogenetic tree
Complexity
number of cell types
metabolic complexity
behavioural complexity
The fraction that is coding vs Intergenic changes
- for prokaryotes and viruses, about 10% of the genome is intergenic
- for animals and plants, less than 10% may encode for proteins
What explains genome size variation?
- Gene duplication
- blocks of duplication
- satellites
- relative rate of insertions and deletions
- Transposable elements; 40% of human genome is TE
- Polyploidy
Polyploid
Having more than two complete sets of chromosomes e.g. 3n, 4n,5n
Euploid
Having a complete complement of chromosomes
Aneuploid
Having an incomplete complement of chromosomes e.g. missing/having an extra chromosome, trisomy 21
Autopolyploidy
Duplication of the genome with a species e.g. potato, banana
Allopolyploidy
Genome duplication deriving from hybridisation between two parental species e.g. wheat, cotton
The gametes are 2n
Distinct from a diploid hybrid
Somatic doubling
mitotic doubling then failure of cell division- vegetative propagation in plants
Polyspermy
multiple sperm fertilising an egg - 1-3% of human conceptions (not viable)
Gametic non-reduction
See OneNote diagram
failure of cell division during meiosis producing 2n gametes - unreduced sperm uncommon in animals, common step in pathway to allopolyploidy in plants
How common is polyploidy?
- very common in plants, 70% lineages of angiosperms and led to 15% angiosperm speciation events
- fairly common in fish, amphibians, insects
- rare in mammals
Red Vischacha rat
See OneNote
Wheat varieties
See OneNote
- hexaploid
- homeologues
Homeologues
Homologs derived from polyploidization event
Diploidization
- most loci now behaving/segregating like diploids
What evidence is there for ancient polyploidy events?
See OneNote
- paleo-polyploidy
1. evolutionary jumps in c-value e.g. red verschacha rat
2. evolutionary jumps in chromosome number
3. Chromosomal pairing- quadrivalents?
4. Gene number
5. Gene arrangement
6. Gene tree topology
7. Age of gene duplication events (molecular clock)
C-value
C-value is the amount, in picograms, of DNA contained within a haploid nucleus (e.g. a gamete) or one half the amount in a diploid somatic cell of a eukaryotic organism.
Ancient genome duplication
See OneNote
- gene loss after duplication
Multiple sub-genomic duplications
See OneNote
- not whole genome duplication
- more than 2 copies of some genes
Yeast - a paleo-polyploid?
See OneNote
Vertebrate 2R hypothesis
See OneNote
- Susumu Ohno proposed that there had been two rounds of polyploidy in the vertebrate lineage
- C-value
- Isozymes
- Identified two pairs of duplicated genes on chromosome 11 and 12 delineating duplicated chromosomal segment
Found consistent data in mice homeobox
Evidence for 2R hypothesis
See OneNote
2R in seed plants?
See OneNote
- point of divergence should match
- 2 distinct peaks
Proximate causation
- mechanistic explanation e.g. failure of gamete reduction
Ultimate causation
- the why question
Possible selective advantages
- increased DNA content
- increased heterozygosity
- able to retain genetic diversity even with small census size
- multiple gene copies
- higher mutation rate(could also be a cost)
Increased DNA content
See OneNote
4 points
Increased heterozygosity
See OneNote
2 points
Able to retain genetic diversity even with small census size
See OneNote
2 points
Multiple gene copies
- neofunctionalisation
- sub-functionalisation
Possible costs
- chromosomal
- developmental
- physiological
- population
Chromosomal costs
See OneNote
2 points
Developmental costs
See OneNote
3 points
Physiological costs
See OneNote
1 point
Population costs
See OneNote
3 points
Correlated with asexual reproduction
- in plants asexual reproduction may be a consequence of self-incompatibility systems breaking down
- polyploid insects and reptiles probably arose from parthenogenic animals
Parthenogenesis
reproduction from an ovum without fertilization, especially as a normal process in some invertebrates and lower plants.
Asexual reproduction
Asexual reproduction is a type of reproduction by which offspring arise from a single organism, and inherit the genes of that parent only; it does not involve the fusion of gametes, and almost never changes the number of chromosomes
Reconciling costs and benefits
Recently formed polyploids have: 1. higher extinction rate than diploids 2. lower speciation rate 3. lower diversification rate YET polyploidy appears associated with major evolutionary diversification events
