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

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Crop Improvement

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.

Grafting

• Grafting is common among fruit trees: the stem tissue of one plant is fused to a stem from another plant.
• Commonly done: a hardy rootstock is grafted to the stem of a better fruit variety.
• Another method: attaching a bud form one plant to the stem of another. You can produce an apple tree bearing several different types of apple this way.
• The cambium layers of the two plants being grafted need to be brought into contact. This allows the xylem and phloem to connect to each other
• (^) It is possible to graft a potato rootstock to a tomato top, so both underground potato tubers and aboveground tomato fruits are formed on the same plant.

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

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.

Genetics

• The science of genetics is devoted to understanding the patterns of how traits are inherited during sexual reproduction. It was founded by Gregor Mendel in the 1850's, using pea plants. Despite the obvious differences, humans and peas have very similar inheritance patterns.
• The fundamental observation of genetics: within a species, there are a fixed number of genes, and each gene has a fixed location on one of the chromosomes. - This allows genes to be mapped: a gene's neighbors are always the same. - Most species of higher organism have about 25,000 different genes distributed onto 10-30 different chromosomes.

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.

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).

Independent Assortment

• Much of Mendel’s work involved pairs of genes: how do they affect each other when forming the gametes and combining the gametes to form the next generation?
• Simple answer: in most cases pairs of genes act completely independently of each other. Each gamete gets 1 copy of each gene, chosen randomly.
• Two genes:
• 1. seed shape. Dominant allele S is smooth; recessive allele s is wrinkled.
• 2. seed color. Dominant allele Y is yellow; recessive allele y is green.
• Heterozygous for both has genotype Ss Yy, which is smooth and yellow. Gametes are formed by taking 1 copy of each gene randomly, giving ¼ SY, ¼ Sy, ¼ sY, and ¼ sy.
• These gametes can be put into a Punnett square to show the types of offspring that arise. - Comes out to 9/16 smooth yellow, 3/16 smooth green, 3/16 wrinkled yellow, and 1/16 wrinkled green. - 3/4 are yellow, 1/4 are green, and 3/4 are round, 1/4 are wrinkled

Continuous Variation

• Many traits don’t seem to fall into discrete categories: height, for example. Tall parents usually have tall children. Short parents have short children, and tall x short often gives intermediate height. In all cases, wide variations occur.
• Simple interactions between several genes can give rise to continuous variation. Also: variations caused by environment, and our inability to distinguish fine distinctions lead us to see continuous variation where there actually are discrete classes.

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.

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

Polyploidy

• Normal diploids have 2 copies of every chromosome. Sometimes it is possible to double this number, making a tetraploid, 4 copies of every chromosome. - The drug colchicine does this by causing meiosis to produce diploid gametes instead of the normal haploids. Then, diploid sperm + diploid egg = tetraploid embryo.
• Tetraploids are often bigger, healthier, more nourishing than their diploid parents. - Examples: cotton, durum wheat, potato, daylily
• Tetraploid is a form of polyploid, which means having more than 2 sets of chromosomes (2 sets = diploid).
• There are triploid (e.g. banana and watermelon), hexaploid (bread wheat, chrysanthemum), and octaploid (strawberry, sugar cane) crops - Triploids are sterile

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.