Transcript Self-incompatibility

Lecture 1 Plant Genetics Overview.
How plants are different from animals?
Variation in quantity of DNA
Polyploidy
Mitochondrial Genome
Chloroplast Genome
Crossing Strategies and Plant Breeding
Cytoplasmic Male Sterility
Plants
From space, the land is green!!
They represent our food source and the basis for a vast array of
products we depend on.
400,000 species of plants
Phylum - Eukaryota
Kingdom- Plantae (Viridiplantae)
Chlorophyta
(green algae)
Algae other than green algae
(brown, red, yellow-green)
Streptophyta
Higher plants
(e.g. liverworts, mosses,
ferns, gymnosperms and
flowering plants)
Flowering Plants- Angiosperms
Evolved about 130 million years ago at the same time as birds
and mammals.
234,000 species
(800,000 insects,
4,600 mammals)
Variation in quantity of DNA
Species
Common
name
Ploidy
level
Genome size
in bp
Saccharomyces cerevisiae
Homo sapiens
Yeast
Human
2
1.3 x 106
3 x 109
Arabidopsis thalina
Oryza sativa
Beta vulgaris
Vicia sativa
Solanum tuberosum
Hordeum vulgare
Vicia faba
Triticum aestivum
Thale cress
Rice
Sugar Beet
Common vetch
Potato
Barley
Broad bean
BreadWheat
2
2
2
2
4
2
2
6
1.4 x 108
4.2 x 108
7.6 x 108
1.6 x 109
1.8 x 109
4.9 x 109
1.2 x 1010
1.6 x 1010
There are probably 30-38,000 functional genes in plants.
Big genomes have more repetitive DNA.
Variation in quantity of DNA
Classes of DNA
Single or Low-Copy sequences -genes including introns (probably 30-38,000)
Repetitive DNA
Multiple copy genes - e.g. ribosomal genes
Telomeres- (CCCTAAA - repeated many times)
Mobile elements
transposons and retrotransposons (which comprise up to 50% of
genome)
Tandemly repeated DNA- short sequences in tandem, being present in
blocks of multiple copies
e.g. Simple sequence repeats or SSRs - short sequences of 1-5bp
tandemly repeated AKA Microsatellites
Polyploidy
Plants are much more diverse in terms of ploidy level than animals.
Almost half of angiosperms (flowering plants) are polyploid.
Diploid gametes can be formed without meiosis, or tetraploid tissue is
formed when cells fail to divide after replication in mitosis.
Triploids (3n) are not uncommon but are generally sterile e.g. commercial
banana.
Tetraploids (4n) are usually healthy and fertile e.g. durum wheat.
Pentaploids (5n) are sterile
Hexaploids (6n) are Ok e.g. bread wheat
.
.
Several hundred ploid (n = 100s) do exist
Polyploidy is very important in evolution.
Commonly, the extra copies of chromosomes are not needed, and undergo
rapid mutations and rearrangements. After several generations, the
tetraploid is more like a diploid with lots of ‘junk’ DNA.
Chloroplast Genome
100-220kb
20-100 copies per chloroplast
500-10,000 copies per cell
Up to 20% of the cells DNA
120-140 Genes
Evolved from Prochloron-like
cyanobacteria
MATERNALLY INHERITED
Mitochondrial Genome
About 60 genes
Mitochondial genome bigger in plants than animals or
yeast, but variable (16kb in animals, 100-2,000kb in plants).
Structure is poorly understood because it appears to be
unstable. It appears to be present as subgenomic
fragments, sometimes linear and sometimes circular. Also
variable amounts within a plant cells.
Trans-splicing
Cytoplasmic male sterility
MATERNALLY INHERITED
Reproduction Strategies and Plant Breeding
Many plants reproduce asexually
Fragmentation (clonal growth, tillering, suckers) e.g. Aspen,
Willow
Apomixis - production of seed identical to mother e.g. Rubus sps.
Most plant species out-cross
Self-incompatibility - mechanisms to prevent selfing
Some plants are monoecious (separate male and female
flowers) e.g. maize
Some plants are dioecious (male and female plants) e.g. holly,
marijuana and these have X and Y chromosomes
like animals
Reproduction Strategies and Plant Breeding
Self crossing is common- (40% of plants) and it is common in crops
Inbreeding cropsOutbreeding cropsInbreeding Crops
(self-pollinators)
Wheat
Barley
Oats
Rice
Tomato
Peach
Cotton
Peas and beans
Coffee
Pepper
Outbreeding Crops
(cross-pollinators)
Maize
Rye
Brassicas (cabbage, swedes, rapes)
Sunflower
Potato
Beets- sugar beet
Carrot
Mango
Rubber
Banana
Inbreeding depression
•Self-crossing is much more common in plants than animals.
•The reason many plants can inbreed may be due the relative importance
of the gametophyte generation.
•The superior performance of an F1 from inbred parents is call Hybrid
Vigour. It is very important in crop production.
Selfing
F1
F2
F3 F4 F5
Performance
(height)
P1 P2
Increasing
homozygousity
Self-incompatibilityA mechanism to prevent selfing
•Genetically controlled by S locus alleles.
•In self-incompatible species there are many S alleles (up to 200)
•These allow the identification of ‘self’ and ‘non-self’
•The male and female have 2 alleles (if they are diploid)
•There are two types of incompatibility•GAMETOPHYTIC and SPOROPHYTIC
Stigma
Pollination
Pollen
Style
Pollen tube
Ovary
Ovule
GAMETOPHYTIC Self-incompatibility
In gametophytic, it is the single S allele of the pollen that
determines pollination. If the S allele of the pollen grain
matches either of the female alleles, there is no germination
Pollen
S1 S3
Female Parent S1S2
S1 S3
S2S3
S1 S3
S2S4
SPOROPHYTIC Self-incompatibility
In sporophytic, it is the combined S alleles of the all pollen that
determines pollination (i.e. it is the alleles of the male plant). If
the S allele of any pollen grains matches the female, no
pollination
Pollen
S1 S3
Female Parent S1S2
S1 S3
S2S3
S1 S3
S2S4
Cytoplasmic male sterility (CMS)
Important in breeding of hybrid seed since the seeds of a male
sterile plant must be hybrids.
CMS is maternally inherited because it is partially dependent on
mitochondrial DNA
A mitochondrial gene disrupts pollen development.
Nuclear genes can restore pollen development. RESTORER genes.
For example, T-type CMS in maize is caused by a constitutive
mitochondrial gene T-urf13 which produces a protein located on
the mitochondrial membranes in all tissues. This protein prevents
pollen development but it is not known how.
Two nuclear restorer genes, Rf1 and Rf2, are needed for male
fertility. RF1 reduced T-urf13 expression by 80%. RF2 codes a
mitochondrial aldehyde of unknown function.
Other important differences between
Plants and Animals
•Totipotency
•Gametophyte generation is very important (in simple plants like
mosses and liverworts it is the dominant generation).
•Inbreeding is common
•Cytosine methylation more common in plants
•Introns generally smaller in plants
•Many plant genes lack the AAUAAA-like polyadenylation signal
•Mitochondria, while similar, are probably of a different origin (a
different symbiotic relationship) to animals