• Definition• Selective breeding is also known as artificial selection. It is humans selecting desirable traits in a species and choosing which individuals breed in order to increase these traits in the species.• All crops and domesticated animals today are a product of selective breeding.
Selective breeding v’s natural selection• Natural selection is certain individuals of a species being “fitter”, possessing alleles which make them more successful in their environment. These individuals survive longer and produce more offspring. Therefore more of their alleles are passed on to the next generation and over time these alleles become more common in the gene pool• In selective breeding humans determine which phenotypes, and therefore genotypes are desirable in the species. They then increase the alleles for these traits through breeding programmes. Often the species produced would be unable to survive in the wild
Selective breeding – nothing new• Agriculture began 10,000yrs ago. Humans selected wild varieties of plants and animals with the traits they desired and began selective breeding to increase these desired traits; e.g. to produce bigger, easier to harvest grains with a greater yield, to breed animals that were the most docile and easy to handle.
Selective breeding animals• Belgium Blue cattle – cattle bred to produce a huge amount of muscle (meat). Produced by selecting individuals with a mutation in myostatin gene which results in the production of an increased number of muscle fibres (hyperplasia)• Excess muscle growth begins in utero so calves have to be born by caesarean section.• Fertilisation is almost always by artificial insemination, meaning that sperm can be shipped across continents and only the ‘best’ bulls are selected to breed.
Selective breeding -methods• Inbreeding is reproduction from the mating of parents who are closely related genetically.• Livestock breeders often practice controlled breeding to eliminate undesirable characteristics within a population, which is also coupled with culling of what is considered unfit offspring, especially when trying to establish a new and desirable trait in the stock.• Repeated test crosses are often used in order to produce pure breeding individuals
Modern corn Ancient corn from Peru (~4000 yrs old)Choosing only the best corn plants for seeds resultsin better crops over a long time.
Polyploidy• Polyploidy is a mutation that occurs during meiosis and results in multiple sets of chromosomes (3n/4n etc)• Polyploidy is usually fatal in animal species, but frequently occurs in plants.• Polyploid plants have bigger fruits and grains and infertile polyploid are seedless. These traits are selected for in selective breeding programmes• Polyploidy can be induced in plants using a chemical called colchine. This is used to produce bigger, stronger polyploid plants and to make fertile polyploid plants.
How To Make A Fertile Polyploid HybridTo produce a tetraploid plant, the alkaloid colchicine is applied to the terminal bud ofa branch. All the cells in the developing branch will be tetraploid (4n) with four sets ofchromosomes. This includes cells of the stem, leaves, flowers and fruit. Gametes (eggand sperm) produced by a flower on this tetraploid branch will be diploid (2n) withtwo sets of chromosomes. A flower on the normal diploid (2n) branch will producehaploid (n) gametes containing one set of chromosomes. http://waynesword.palomar.edu/hybrids1.htm
How Colchicine induces polyploidyThe original mother cell is diploid (2n). During anaphase the chromatids separate andmove to opposite ends of the cell. Colchicine causes the dissolution (depolymerization)of protein microtubules which make up the mitotic spindle in dividing cells. This leavesthe cell with twice as many single chromosomes (four sets rather than two). When thiscell divides, each of the two daughter cells will have fours sets of chromosomes, a totalof eight chomosomes per cell. [Note: Spindle poisons such as colchicine are used toprevent tumor cells from dividing in certain chemotherapy treatments.] http://waynesword.palomar.edu/hybrids1.htm
Genome analysis• Genome analysis is determining the locus (position on the chromosome) and base sequence of all an organisms genes.• Chromosome mapping determines on which chromosome and at which locus a gene occurs.• DNA sequencing determines the exact base sequence of each gene, it can be used to distinguish between different alleles.• Genome analysis is used in selective breeding to determine if an individual has a specific, desired allele and to select individuals for breeding programmes based on their alleles
Genome analysis and selective breeding - examples• Genome analysis of kiwifruit is being used to selectively breed new, trademarked varieties of fruit with characteristics such as disease resistance.Source: http://www.plantandfood.co.nz/page/our-research/breeding- genomics/• Sheep in NZ are being selectively bred to be immune to facial eczema, a fungal disease that can destroy whole flocks. Genome analysis of sheep was carried out and individuals immune to the disease were selected for a breeding programme. Source:Ag research NZ
Other Applications of Selective breeding• Breeding programmes for endangered species, may involve genome analysis and selection of the least genetically related individuals to breed (to maintain genetic diversity in the species).• Selective breeding programmes have resulted in higher yields and better disease resistance in aquaculture species, such as salmon.
Implications of selective breeding• We are concerned with the biological implications of selective breeding that may impact on :1. Ecosystems2. Genetic biodiversity3. Health or survival of individuals4. Survival of populations5. Evolution of populationsBrainstorm some possible (general) implications of selective breeding for each of these.