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) ChlorophytaStreptophyta (green algae) Higher plants Algae other than green algae (e.g. liverworts, mosses, (brown, red, yellow-green) 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 SpeciesCommonPloidy Genome size name level in bp Saccharomyces cerevisiae Yeast1.3 x 10 6 Homo sapiensHuman 23 x 10 9 Arabidopsis thalinaThale cress 21.4 x 10 8 Oryza sativaRice 24.2 x 10 8 Beta vulgarisSugar Beet 27.6 x 10 8 Vicia sativaCommon vetch 21.6 x 10 9 Solanum tuberosumPotato x 10 9 Hordeum vulgareBarley 24.9 x 10 9 Vicia fabaBroad bean 21.2 x Triticum aestivumBreadWheat 61.6 x 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 kb copies per chloroplast ,000 copies per cell Up to 20% of the cells DNA 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 crops- Outbreeding crops- Inbreeding Crops Outbreeding Crops (self-pollinators) (cross-pollinators) WheatMaize BarleyRye OatsBrassicas (cabbage, swedes, rapes) RiceSunflower TomatoPotato Peach Beets- sugar beet Cotton Carrot Peas and beansMango CoffeeRubber PepperBanana

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. Performance (height) P1P1 P2P2 F1F1 F2F2 F3F3 F4F4 F5F5 Increasing homozygousity Selfing

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 Pollen Pollen tube Stigma Ovary Ovule Style Pollination

Female Parent S 1 S 2 S 2 S 3 S 2 S 4 Pollen S 1 S 3 S 1 S 3 S 1 S 3 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

Female Parent S 1 S 2 S 2 S 3 S 2 S 4 Pollen S 1 S 3 S 1 S 3 S 1 S 3 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

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