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Biology 164 Laboratory Artificial Selection In Biology 164 Laboratory Artificial Selection in Brassica, Part I (Based on a lab exercise originally developed by Bruce Fall, Univ. of Minnesota and revised by Tim Christensen, Colby College) I. Objectives 1. Understand the process of artificial selection. 2. Become familiar with some plant cultivars in the genus Brassica, which are the products of artificial selection. 3. Participate in an on-going artificial selection study involving three related cultivars of Brassica rapa in which you will: a. quantify the variability of a specific trait in the three cultivars, b. measure how the expression of the specific trait has responded to varying numbers of rounds of artificial selection, c. subject one of the cultivars to another round of artificial selection, and d. determine if there is evidence for an upper limit having been reached in the expression of the specific trait as additional rounds of artificial selection were employed. II. Introduction A visit to a farm, supermarket, pet store or plant nursery will offer many examples of selective breeding of plants and animals by humans. Over hundreds and even thousands of years, humans have altered various species of plants and animals for our own use by selecting individuals for breeding that possessed certain desirable traits. This selective breeding process was continued for generation after generation. Often the products of such selective breeding are remarkable. Quite diverse domestic dog breeds, from chihuahuas and miniature poodles to Newfoundlands and Irish wolfhounds are all related to a common wild ancestor, the wolf (Canis lupus). Domestic chickens are all derived from the wild Jungle Fowl (Gallus gallus). The success of plant and animal breeders in dramatically changing the appearance of various lineages of organisms in a relatively short period of time is an obvious yet profound fact. This fact did not escape Charles Darwin as the first chapter of The Origin of Species concerns artificial selection by humans (“Variation under Domestication”). Darwin used many examples of artificial selection by humans to help support the case for his proposed mechanism for the evolution of natural populations – natural selection. For example, Darwin was particularly taken with the number of pigeon varieties such as tumblers, pouters, fantails and many others. All of these pigeon varieties were derived from wild Rock Doves (Columba livia) over the past 5,000 years. One can find similar examples of selective breeding among plants, including those humans have bred for food. To gain a better understanding of selection and inheritance, biologists have experimented with artificial selection involving a variety of traits in many different species of bacteria, yeast, plants and animals. The results, obtained in a relatively short period of time, are often impressive. A general finding of these studies is that most variable traits in organisms respond to artificial selection. In other words, it is usually possible to increase or decrease the frequency or average value of a trait in a lineage through careful selective breeding. In our lab study you will have a chance to examine whether or not Colby students have been successful plant breeders using artificial selection to alter the genotype of a plant. Artificial Selection in Brassica, Part I Page 1 How artificial selection differs from natural selection In contrast to natural selection, artificial selection 1) favors traits that for some reason are preferred by humans; 2) has a goal or direction toward which the selection process is directed; and 3) generally is much faster than natural selection because the next generation can be absolutely restricted to offspring of only those parents that meet the desired criteria (natural selection is rarely so absolute). In artificial selection, humans perform the selecting, intentionally restricting breeding to individuals with certain characteristics. In natural selection, the environment accomplishes the selecting – individuals that survive and reproduce better in a given environment, are “naturally selected”. The ‘environment’ includes the myriad of factors such as predators, food supply, weather, etc. that determine which traits make it into the next generation. Some important plant cultivars that have arisen by artificial selection A specific plant type or variety that is cultivated for human use is known as a cultivar (cultivated variety). The genus Brassica of the mustard family (Brassicaceae) includes many cultivars that are important in agriculture. A number of nutritious and tasty vegetable cultivars have originated from three species of Brassica (Brassica rapa, B. oleracea and B. juncea). Some cultivars have been bred for root production, others for leaves, flower buds or oil production. You may be surprised to learn that these familiar vegetables, though very different in appearance, are actually descendents of the same ancestral species. Centuries of artificial selection have produced greatly divergent cultivars within these species (Fig. 1). The following cultivars originate from the three wild species indicated in italics: Brassica oleracea – kale, cauliflower, broccoli, cabbage, Brussels sprouts, kohlrabi, collards Brassica juncea – leaf mustard, root mustard, head mustard and lots of other mustard varieties Brassica rapa – turnip, Chinese cabbage, pak choi, rapid-cycling turnip Chinese cabbage pak choi rapid cycling Fig. 1: Examples of four cultivars of the species, Brassica rapa. The rapid-cycling cultivar of Brassica rapa was developed at the University of Wisconsin and has been used extensively in plant research and education. This cultivar was developed for its ability to rapidly flower and mature, produce lots of seeds, have a short stature, and thrive under artificial light. The generation time (from seed to maturation to fertilization to mature seed) is 6-7 weeks, short enough to allow us to study one complete generation this semester. The life cycle of these so-called Wisconsin Fast Plants is shown in Fig. 2. Artificial Selection in Brassica, Part I Page 2 Figure 2. Life cycle of rapid-cycling cultivars of Brassica rapa. Variable traits: continuous or discreet? There are a number of fairly obvious variable traits that one can observe in a large population of Fast Plants. A brief list might include: number of flowers, color of hypocotyl, number of leaves, type of petiole, size of first true leaf, and plant height. In contrast, other traits usually don’t vary at all: number of petals per flower (four), petal color (yellow), number of cotyledons (two), and leaf color (green). Refer to Fig. 2 if you are unfamiliar with any of these terms. Artificial Selection in Brassica, Part I Page 3 Your lab instructor will give each student a container with five plants. Treat these plants gently; they are young and tender, and easily damaged! Examine the small sample of plants that you and your lab partners have at your table and note the variability you can see among the plants. Remember, the plants are the same age so differences you see (such as height or leaf number) are not due to differences in age. Note how some traits vary continuously over a range of possibilities, while others fall into discreet (either/or) categories. For the traits below indicate whether they vary continuously or discreetly. Number of flowers: Color of hypocotyl: Number of leaves: Type of petiole: Size of first true leaf: Plant height: One variable that you may have overlooked is “hairiness” of leaves, petioles and stems. Look at some plants more closely using a hand lens, and see if you can observe these hairs, also known as trichomes. Does the hairiness trait vary continuously or discreetly? Trichomes have been demonstrated to have specific functions, which differ from species to species. In the space below, hypothesize what these functions might be. What do you speculate are some functions of trichomes? III. Experimental Materials and Methods A. The Three Cultivars Involved in the Study You and the other members of your lab section will participate in an artificial selection study that began two years ago and involves subjecting a base population of a rapid-cycling cultivar of Brassica rapa to a number of rounds of artificial selection. You will complete the first portion of the study today but will not complete the second portion of the study for several weeks, when you will be able to observe the expression of traits in plants that you have subjected to a further round of artificial selection. The three different cultivars used in this year’s study include a) the base population (BP), b) a population that has undergone one round of artificial selection (AS1), and b) a population that has undergone two rounds of artificial selection (AS2). Note that these cultivars are related to one another, representing three generations of plants that were successively selected for the expression of an extreme form of a specific trait. Each round of artificial selection involved producing offspring from only the top 10% of individuals within each population that expressed the most extreme form of the trait. We established populations of the three cultivars by planting seeds approximately 17 days prior to the lab. All plants were grown identically, at 23°C, using commercial potting soil, a constant feeding regime, and continual light of 200µE. brightness. As you begin this study, these plants will have begun flowering (see Figure 2 above). Artificial Selection in Brassica, Part I Page 4 B. Experimental Design and Purpose
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