Download Lab 09 Domestication

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.
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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).
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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 _______________________________________________________________
_______________________________________________________________
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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
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