Learning Objectives

After completing this laboratory topic, students should be able to:

  1. Better understand the genetic component of plant growth and development.
  2. Recognize the generic diversity of plants, in particular edible members of Brassica.
  3. Relate the concept of a biological species ro crop varieties, ability to interbreed, and range of heritable traits.
  4. Understand the life cycle of a tast-growing plant from seed to seed.
  5. Relate the process of human-guided selection to changes in populations of Brassica rapa over several generations.
  6. Connect human-guided selection to plant domestication and crop improvement.

EXERCISE A: Genetic Diversity of Crops

The produce section of a supermarket displays a vast div er-sin* of fresh fruits and vegetables—more, in fact, than most of us have personally sampled. But u fruits" and "vegetables* arc only one wa\ of classifying diese plant products. What other ways can we make some sense out of all the variety observed? A produce manager might prefer to group die types according to the care they require—for example, fleshy varieties may need to be sprayed occasionally with water to keep them from wilting, while dry varieties such as potatoes, onions, and nuts can easily be placed in bins. On the other hand, you as a consumer may find it more convenient if all die sweet fruit is grouped together.

Now think like a biologist. How might a biologist divide the plants products? Scientists usually try to group organisms that are related. The most basic of these groups is the species. A species is a population of organisms that have many characteristics in common and produce fertile offspring through the exchange of generic information. In seed plants, genetic exchange and reproduction are accomplished through pollination and fertilization.

In this exercise, you will look carefully at the variation among common vegetables and try to group these organisms into one or more biological species. We will then test your groupings by determining whether the plants within them can produce fertile offspring from controlled crosses.

Materials Needed for Exercise A

Edible plant products purchased from a local market, depending on availability (e.g., turnip, rutabaga, red cabbage, green cabbagc, kohlrabi, broccoli, cauliflower, col lards, bok choy (pak choi brussel sprouts, Chinese cabbage, rapini, mustard greens, root mustard, kale, savoy cabbage)

Wisconsin Fast Plants (Rbr) (grown from seed at least I week i

Procedure for Exercise A

1. Diversity of brassicas. Examine all the vegetables available in the lab. Their names should be provided in case you do nor recognize them. First concentrate on how each one differs from the others. Flow can you tell them apart? List as many distinctive features as vou can on worksheet 8-1 at the end of this lab-oratory topic.

Now switch your attention to the similarities among all the vegetables. What characteristics do they all share or at least some of them share?

By comparing the similarities with the differences, classify the plants into groups that you think reflect their biological rclatcdncss. Try to assign each vegetable to a group that you think best represents a species.

Now compare your groups with the classification in table 8.1. These are the species recognized by most experts on the brassicas. How does your classification comparc with theirs? Were you surprised? What experiment could vou conduct to test whether vou or the • »

experts are more correct?

All plants represented in this activity are members of the genus Brassica. Brassicas are members of a large family of plants called crucifers. The name refers to die characteristic cross-shaped pattern of the four petals of the flower. The brassicas include many important food and oil crops (fig. 8.1).

Six of the most important Brassica species arc closely interrelated. Each species has a different number of chromosomes. The number of chromosomes and the ability of each species to interbreed wfith other species in the group help botanists justify the boundaries of each species. The relationship between the six important Bras-sica species can be represented as a triangle, with the three diploid species forming the points of the triangle and the amphidiploid species (crosses between die first three) on die sides (fig. 8.2). The diploid species have the follow ing characteristics:

  • Brassica nigra is a common weed. It often grows in disturbed habitats, such as plowed fields. If the haploid (X) set of chromosomes for diis species is designated as a, then die diploid can be designated as aa (or 2N = 16).
  • Brassica olcracca is the species of most of the vegetables displayed in this lab. The haploid N) set of chromosomes has 9 chromosomes and can be designated as b. The diploid is thus bb (or 2N = 18). Included in diis spccics are cabbage, kale, broccoli, cauliflower, collards, brussels sprouts, and kohlrabi. Why do vou think botanists classify all these crops in the same species?
  • Brassica rapa (2N - 20) includes turnips, Chinese cabbage, and Rbr. Chinese cabbagc is consumed by millions of people around the world, especially throughout .Asia. Since it can be stored during the winter, it provides a rich source of vitamins, especially vitamin C, even in rimes when most fields are dormant. There are 10 chromosomes in the haploid set, designated as c. The diploid (cc) has 20 chromosomes.

The other three species form from crosses between pairs of the above diploid species. The resultant hybrid spccics arc tetraploids and have four sets of chromosomes (two sets from each parcnr).

• Brassica carinara (genome = bbcc, 4 .V = 34» results from a cross between B. nigra and li. olcracclt. It is a tall, leafy plant found in Ethiopia. The seeds are pressed as a source of edible oil.

Genetic Diversity of Our Food 99

TABLE 8.1 ECONOMICALLY IMPORTANT BRASSICAS.

Ispecfes (Genome)*

Subspecies or Variety

Cultivar Croup or Common Name

Brass<a rttgro (genome = bb. 2N = 16)

—•

BJack mustard

Brossico oleráceo (genome = cc. 2N = 18)

ocephcla

Kales

atboglabro

Chinese kale, kailan

botrytis

Cauliflower, heading broccoli

CGpitOtG

Cabbage

costata

Portuguese cabbage, tronchuda

gemmifero

Brussels sprouts

gongykxJes

Kohlrabi

aaHca

Broccoli, calabrese

medutosa

Marrow stem kale

palmifolia

Tree cabbage, Jersey kale

ramosa

Thousand-head kale

scbaodo

Savoy cabbage

sobell<a

Collards

selcnsta

Borecole

Brossko ropa (genome = aa, 2N=20)

chínense

Pak choi. bok choy

narinosa

Tsatsai

ntpponsinka

Mizuna. mibuna

oleífero

Turnip rape, toria

porachinensis

Saichin. choy sum

pekJnensis

Chinese cabbage

pervirtáis

Tendergreen. komatsuna

ropifera

Turnip

trilocuiaria

Yellow sarson

utHis

Broccoletto. broccoli raab

Brossxo connota (genome = bbcc. 4N = 34)

Ethiopian mustard

8ross>co juncco (genome = aabb. 4N = 36)

copitaia

Head mustard

crispifoSo

Cut leaf mustard

fodlifhra

Broccoli mustard

lopitoza

Large petiole mustard

mulUCtps

Multishoot mustard

oleífero

Indian mustard, raya

ropifera

Root mustard

rugoso

Leaf mustard

spiceo

Mustard

tSO-tSOi

&g stem mustard

Brossico nepus (genome = aacc, 4N = 38)

Fodder rape

oleifera

Oil rape

ropifera

Swede, rutabaga

"The haploid complement of chromosomes is a = 10. b = 8. and c = 9.

Data taken from Wisconsin Fast Plants Program. 1987. Around the worid with brassicas WFP02I097. University of Wisconsin—Madison

"The haploid complement of chromosomes is a = 10. b = 8. and c = 9.

Data taken from Wisconsin Fast Plants Program. 1987. Around the worid with brassicas WFP02I097. University of Wisconsin—Madison

• Rutabagas and rape arc members of Brassica napus (genome = aacc, 4N = 38), a cross between B. oleraua and B. rapa. Because of the high carbohydrate content of rutabaga roots, they are grown more as food for livestock than for human consumption. The seeds of rape arc pressed to produce canola oil. This vegetable oil is third behind soybean and peanut oil in production. • Brassica jun ceci genome = aabb, 4 N - 36), or mustard, is a cross between B. rapa and B. nigra. All brassicas produce glucosinolates, the distinctive flavor oÇ mustard, with B. juncea producing the highest levels.

Cauliflower Production Guide

Cauliflower

Cabbage

Wild type

Brussels sprouts

Broccoli

Kohlrabi

EXERCISE B: They Look So Different; Do They Really Belong to the Same Species

As stated in the introduction to this laboratory topic, members within a species often share characteristics and can interbreed. In Exercise A. you learned that Chinese cabbage, turnips, and Rbr arc all members of die same species even though the mature vegetative stages of diese cultivars appear to have little in common. If, however, you plant a few seeds of turnip or Chinese cabbage along with the Rbr, you will find that the seedlings and first few leaves of all the young plants look similar. They even taste similar. Go ahead and trv some!

Only when the vegetative parts grow further do the plants become different. These differences arc a result of centuries of domestication. Over 3,000 years ago, farmers in the Mediterranean region sclcctcd for plants with large roots that could be stored and fed to animals. At about the same time in China, farmers selected for plants with leaves that curled into a ball-shaped head. The formers carried out this selection process by saving the seeds from the best plants in every generation. Over time, this process resulted in two distinct forms—the turnip and the Chinese cabbage. Was the divergence great enough to create two new species? The only sure way to test this is to see if they can still interbreed and produce fertile offspring.

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