Diapositiva 1

1
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
2
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 Streptophy
ta (green algae) Higher plants Algae other
than green algae (e.g. liverworts, mosses,
(brown, red, yellow-green) ferns, gymnosperms
and flowering plants)
3
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)
4
Variation in quantity of DNA
Species Common Ploidy Genome size
name level in bp Saccharomyces
cerevisiae Yeast 1.3 x 106 Homo
sapiens Human 2 3 x 109
Arabidopsis thalina Thale cress 2 1.4
x 108 Oryza sativa Rice 2 4.2 x 108
Beta vulgaris Sugar Beet 2 7.6 x 108
Vicia sativa Common vetch 2 1.6 x
109 Solanum tuberosum Potato 4 1.8
x 109 Hordeum vulgare Barley 2 4.9 x
109 Vicia faba Broad bean 2 1.2 x
1010 Triticum aestivum BreadWheat 6 1.6
x 1010
There are probably 30-38,000 functional genes in
plants. Big genomes have more
repetitive DNA.
5
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
6
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.
7
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
8
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
9
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
10
Reproduction Strategies and Plant Breeding Self
crossing is common- (40 of plants) and it is
common in crops Inbreeding crops- Outbreeding
crops- Inbreeding Crops Outbreeding
Crops (self-pollinators) (cross-pollinators)
Wheat Maize Barley Rye
Oats Brassicas (cabbage, swedes, rapes)
Rice Sunflower Tomato Potato Peach
Beets- sugar beet Cotton Carrot Peas
and beans Mango Coffee Rubber
Pepper Banana
11

• 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.

12

• Self-incompatibility-
• A 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

13
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
14
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
15
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.
16

• 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