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LAB 09: DOMESTICATION and PLANT MORPHOLOGY Introduction The domestication of plants for human use as crops is an excellent example of artificial selection*, which results in morphological change analogous to that caused by natural selection, except that the agent of change is human preference and changes typically occur over shorter time intervals. Charles Darwin’s observations and experiments on artificial selection (pigeons, orchids) helped him articulate his theory of evolution by natural selection. A specific plant type or variety that is cultivated for human use is known as a cultivar (cultivated variety). Long before Darwin and Wallace, farmers and breeders were using the idea of selection to cause major changes in the features of their plants and animals over the course of decades. They allowed only the plants and animals with desirable characteristics to reproduce, causing the evolution of farm stock. This process is called artificial selection because people (instead of nature) select which organisms get to reproduce1. In this exercise, you will study examples of morphological change associated with the domestication of corn, bananas, Brassica oleraceae and peppers. You will also work in groups to research a crop trait and its genetics using the posted article by Meyer & Purugganan (2013). 1- Corn and Teosinte Zea Mays (corn) was domesticated from teosinte. Observe fresh plants of corn and fresh or dried plants of teosinte (UW medicinal garden and greenhouse). Illustrate a whole plant and female inflorescence (“ear”) for each below. 104 Fill in the following comparative table: Zea mays Overall plant architecture Apical dominance (presence/absence) Tassel (male inflorescence) location, overall appearance “Ear” size (female inflorescence/fruits) Kernel toughness (hard, soft) Number of kernels in an ear Teosinte 2- Bananas Almost all modern edible bananas come from two wild species: Musa acuminate and M. balbisiana. Illustrate a wild banana fruit whole and in cross section below; include a scale. Compare commercial and wild banana fruits (UW greenhouse) in terms of: Fruit size _____________________________________________________________________ Seed presence/size______________________________________________________________ Skin thickness/toughness _________________________________________________________ Amount of flesh (and sweetenes, if you dare try…)_____________________________________ 3- Domestication of Brassica species 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 descendants 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 (known as Wisconsin Fast Plants®, it resembles the wild species and is used extensively in plant research and education). 105 Wild B. rapa Selection on turnip Chinese cabbage pak choi __________________ __________________ ____________________ Fig. 1: Examples of four cultivars of the species Brassica rapa (modified from lab. exercise by Bruce Fall, Univ. of Minnesota and Tim Christensen, Colby College) Brassica oleraceae All of the crops illustrated below and in demonstration in lab are derived from domestication of the single species Brassica oleraceae, known as wild cabbage. In each case, humans have selected random mutations affecting a different plant structure Observe the available crops and the wild cabbage ancestor. Indicate which part of the plant has been modified through selection in each case. Modified Trait _______________________________________________________________ _______________________________________________________________ 106 4- Cultivated and wild peppers2 Capsicum originated in the New World, where at least 20 species have been reported. Domestication might have taken place 10,000 to 12,000 years ago. Therefore, pepper was one of the first plants domesticated in the Americas. Columbus introduced chiles to Europe, where they were quickly incorporated to existing recipes. From there, they quickly expanded to Africa and Asia and eventually to North America. There are 5 domesticated species of Capsicum. Wild peppers can be distinguished from the cultivated ones by several characteristics outlined below. Compare these traits in wild and cultivated fruits available in the lab. Wild Cultivated Small fruit Vary in sizes Red Vary in colors Pungent Sweet or pungent Soft flesh Hard flesh, few soft Upright fruits Mostly pendant Light fruit load Mostly heavy Cross pollinated Mostly selfed (short style) Vocabulary *Artificial selection: A process in which humans consciously select for or against particular features in organisms. For example, the human may allow only organisms with the desired feature to reproduce or may provide more resources to the organisms with the desired feature. This process causes evolutionary change in the organism and is analogous to natural selection, only with humans, not nature, doing the selecting. Natural selection: Differential survival or reproduction of different genotypes in a population leading to changes in the gene frequencies of a population. The conditions required for the operation of evolution by natural selection include variation, a system of heredity, differential reproduction, and time. 5-Working within a group, use the table in Meyer and Purugganan (2013)3 to choose one crop that has not yet been discussed in lecture or lab. Follow the references to investigate the genetic basis for an agronomic trait within your crop, answer the questionnaire posted and present your results to the class. All materials are posted in the Canvas course websiteb under Files>Labs>Lab 09 discussion materials. Sources: 1 evolution.berkeley.edu http://www.plantsciences.ucdavis.edu/vc221/pepper/PEPPERrd.htm 3 Meyer, R.S., and Purugganan, M.D. (2013). Evolution of crop species: genetics of domestication and diversification. Nat Rev Genet 14, 840–852. 2 107