Plant Reproduction and Crop Improvement: Sexual and Asexual Methods and Genetics, Slides of Biology

Download Plant Reproduction and Crop Improvement: Sexual and Asexual Methods and Genetics and more Slides Biology in PDF only on Docsity! Crop Improvement Docsity.com Sexual and Asexual Reproduction • Long term survival requires reproduction. Even the longest-lived organisms are less than 10,000 years old. – Cellular machinery wears out, or gets clogged with waste products. – Environmental conditions change • Plants often reproduce asexually, through cuttings or runners or buds (e.g. potatoes). The resulting plants are clones: they are genetically identical to the parent. – Used to preserve good combinations of traits. • Sexual reproduction is also found in plants, and in all animals. Sexual reproduction means combining genes from two different parents, resulting in new combinations of genes. Each parent contributes a randomly-chosen half of their genes to the offspring. – This can be a good thing, because some new combinations will survive better than the old ones. – It can also be bad: lack of uniformity in the offspring. Docsity.com Tissue Culture • A more modern way of propagating plants vegetatively is through tissue culture. This is often called “micropropagation”. – Useful for genetically engineered plants, for plants that don’t set viable seeds, and for rare and valuable plants. • Unlike animals, many plant cells, especially in the meristems, are totipotent: they can generate an entire plant under the proper conditions. • Pieces of the plant are cut out and placed on an agar medium under sterile conditions. • Manipulating plant hormones is the key: an excess of auxin produces roots, and excess of cytokinin produces shoots, and a balanced mixture allows the cells to multiply as an undifferentiated mass of cells called a callus. • Pieces of the callus can be cut out and propagated indefinitely. Docsity.com Sexual Reproduction • Diploid: having 2 copies of each chromosome, one set from each parent. – Humans have 46 chromosomes, 23 from each parent. – Almost any organisms you can see: plant, animal, fungus, is diploid. • Haploid: having only 1 copy of each chromosome. – Sperm and eggs (=gametes) are haploid – moss, a primitive plant, is haploid for most of its life • Plants, animals, and other eukaryotes alternate between haploid and diploid phases. This is called alternation of generations. Docsity.com Life Cycle • Diploid organism generates haploid gametes using the process of meiosis. The gametes combine during the process of fertilization to form a new diploid organism. • In animals, the haploid phase is just one cell generation, the gametes, which immediately do fertilization to produce a diploid zygote, the first cell of the new individual. • In plants, the haploid phase is several cell generations at least. – Lower plants are mostly haploid – Higher plants are haploid for only a few cell generations • The diploid plant is called the sporophyte, and the haploid plant is called the gametophyte. Docsity.com Genetics • In many plants, you can self-pollinate: cross the male parts of a plant with the female parts of the same plant. – In this case, both copies of any given gene are identical. This is called homozygous. The plants are homozygotes, either PP (purple) or pp (white). – The closest cross you can do in animals is brother x sister. • Hybrids. If you cross two true-breeding lines with each other and examine some trait where the parents had different alleles, you produce a heterozygote: the two copies of the gene are different. – Surprisingly, you often find that the heterozygote looks just like one of the parents. The Pp heterozygote is purple, just like its PP parent. – This is the F1 generation in the diagram. Docsity.com Genetics • Dominant and recessive. If a heterozygote is identical to one parent, the allele from that parent is dominant. The allele from the other parent is recessive. That is, the heterozygote looks like the dominant parent. – This is why we say purple is dominant to white, and give purple the capital letter P. • Phenotype and genotype. Phenotype is the physical appearance, and genotype is the genetic constitution. – The heterozygote in the previous paragraph has the same phenotype as the homozygous dominant parent (i.e. purple flowers), but a different genotype (the heterozygote is Pp and the parent is PP). Docsity.com Genetics • Now we want to move to the next generation, by self-pollinating the heterozygotes. • When a heterozygote undergoes meiosis to produce the haploid gametes, half are P and half are p. – These gametes combine randomly, producing 1/4 PP, 1/2 Pp, and 1/4 pp offspring. • Since PP and Pp have the same phenotype, 3/4 of the offspring are purple and 1/4 are white. Docsity.com Linkage • Most pairs of genes assort independently. • However, if two genes are close together on the same chromosome, they are said to be linked, which means the genes don’t do into the gametes independently of each other. • The closer two genes are, the more the parental combination of alleles stays together. This relationship can be used to make maps of genes on chromosomes. Docsity.com Methods of Crop Improvement • The idea that we can improve the inherited characteristics of crop species is fundamental. Very few of the plants we use are unmodified wild plants: most of them have been modified to make them easier to grow and harvest, and to increase the quality and quantity of the desired product. • We will see many examples of crop improvement this semester. Here are some of the basic methods used. Docsity.com Single Gene Traits and Mutation • Single gene traits. Many useful traits are controlled by a single gene. Spontaneous mutations can lead to important, abrupt changes – A good example: sweet corn. The recessive mutation su (sugary) produces kernels that are 5-10% sugar. But, only when homozygous: the non-sugary allele (Su) is dominant. • Single gene mutations occur rarely, but often enough so that observant people notice and propagate them. – Sweet corn was recognized and propagated by several Native American tribes. The Iroquois introduced it to European settlers. – Mutation rate: 1 in 10,000 to 1 in 1,000,000 plants. – Artificially-induced mutation occasionally works, but most are spontaneous. • Single gene traits are inherited in a Mendelian fashion: – each individual carries one copy of the gene from each parent, – the relationship between phenotype (sweet vs. starchy corn) and genotype (homozygous or heterozygous) is determined by dominance vs. recessiveness. Genotype Phenotype Su Su Starchy Su su Starchy su su Sweet Docsity.com Hybridization • Plants are not as rigid in maintaining species boundaries as animals are. It is often possible to produce hybrids between two different, but closely related species. – Members of the same genus will often hybridize • The resulting plants often have characteristics different from both parents – Often sterile, but many plants can be propagated vegetatively • The grapefruit is a naturally-occurring hybrid between a pomelo (native to Indonesia) and a sweet orange (native to Asia).. It was discovered in Barbados in 1750, then brought to Florida and propagated. • Hybrids have an “x” in their species name: Citrus x paradisi • Sometimes, a hybrid will spontaneously double its chromosomes, so you end up with a tetraploid . These interspecies tetraploids are usually fertile, and they benefit from the general effect of tetraploidy: bigger, healthier plants. Docsity.com Genetic Engineering • In the last 30 years it has become possible to take a gene out of one organism and put it into the DNA of another organism. This process is called genetic engineering. The resulting organisms are genetically modified organisms (GMOs) and the gene that has been transplanted is a transgene. • There are no real interspecies barriers here: all organisms use the same genetic code, so genes from bacteria (for example) will produce the correct protein in a corn plant. – However, some modifications must be made to the signals that control gene expression, since these are more species-specific. • A few examples: – Bt corn. Bacillus thuringiensis, a soil bacterium, produces a protein that kills many insect pests, especially the corn earworm. The gene for this protein has been transplanted into much of the US corn crop. – Roundup Ready soybeans (plus other crops). Roundup is the Monsanto brand name for the herbicide glyphosate. A bacterial gene that confers resistance to this herbicide has been transplanted to many crops. The farmer can then spray the fields with glyphosate and kill virtually all the weeds without harming the crop. About 87% of the US soybean crop is now Roundup Ready transgenic plants. • Some cultural issues here: are GMOs safe to eat? Docsity.com Molecular Cloning • The first step in genetic engineering is molecular cloning. • Molecular cloning means taking a gene, a piece of DNA, out of the genome and growing it in bacteria. The bacteria (usually E. coli) produce large amounts of this particular gene. • The cloned gene can then be used for further research, or to produce large amounts of protein, or to be inserted into cells of another species (to confer a useful trait). • The basic tools: • 1. plasmid vector: small circle of DNA that grows inside the bacteria. It carries the gene being cloned • 2. Restriction enzymes: cut the DNA at specific spots, allowing the isolation of specific genes. • 3. DNA ligase, an enzyme that attached pieces of DNA together. • 4. transformation. Putting the DNA back into living cells and having it function. Docsity.com Centers of Domestication • Primary theory came from Nikolay Vavilov, – Vavilov was a Russian who came to a bad end in one of Stalin’s prison camps in Siberia. He believed in Mendelian genetics, which was considered “bourgeois” and thus evil by the Communist Party. (Lysenko) • “Centers of domestication”. The idea is that a plant was probably first domesticated where there are many wild relatives living and where there is a lot of variation in the domesticated plant. Lots of diversity near a domestication center. • Eight major centers: – Southern Mexico and Central America: maize, beans, cotton, pepper, sweet potato – South America (mostly Peru): potato, common bean, tomato, cocoa, tobacco – Mediterranean: pea, mustard, flax, cabbage, asparagus, clover, olive – Middle East (Turkey and eastward): wheat, alfalfa, rye, lentil, melon, fig – Ethiopia: barley, millet, coffee, indigo, sorghum – Central Asia: onion, apple, carrot, almond, grape – India: sugar cane, yam, cucumber, chickpea, orange, coconut, banana, pepper – China: soybean, buckwheat, peach, opium poppy, tea Docsity.com Centers of Origin cy European Siberian poi hocothet es, © »Nikolay Ivanovich Vavilov « Centers of origin for cultivated plants. Docsity.com More Domestication • More recently, Jack Harlan (from U of Illinois) examined genetic data and found that many crops were domesticated multiple times in multiple locations. Also, some were domesticated over very wide areas that don’t seem much like “centers”. • Nevertheless, our current crops come from many different areas of the world. We will look at the origins of specific crops as we study them. Docsity.com