Saving the Store Banana: A Jigsaw on Biodiversity and Disease

Learning target: I can teach my group evidence about bananas, diseases and wild relatives, and use that evidence to evaluate competing solutions for protecting a food supply while maintaining biodiversity.

Standards connections

MS-LS2-5 (assesses, the individual evaluation of competing solutions in Part C): In Part C each student evaluates three competing solutions for protecting the banana supply (holding the line with one variety, borrowing traits from wild relatives, diversifying varieties) by how well they maintain biodiversity and the provisioning service it supports. The group matrix is practice; the individual recommendation is evidence toward the PE.

MS-ETS1-2 (practice): Home groups score the three solutions against shared criteria in a decision matrix and cite card evidence for each score, a systematic comparison of competing solutions.

Answers with reasoning

The problem

Nearly every banana sold in U.S. stores is one variety, the Cavendish, and a soil fungus is spreading through Cavendish farms around the world. Banana growers, scientists and governments are arguing about what to do. Your team will weigh the evidence and evaluate three competing solutions.

Twelve expert cards (teacher cuts these apart; expert groups A, B, C and D take three cards each)

1 Card A1: One plant, many copiesCavendish bananas, the kind sold in almost every U.S. grocery store, have no working seeds. Farmers start new plants from shoots that sprout at the base of a parent plant, or from tiny pieces of plant tissue grown in a lab. So the Cavendish plants on farms around the world are near-identical genetic copies, called clones.

2 Card A2: Why breeding bananas is slowMost bananas people eat have three sets of chromosomes instead of the usual two. That is why they make few or no seeds: good for eating, bad for breeding. Crossing them with other bananas rarely produces seeds that grow, so developing a new variety by crossing can take many years.

3 Card A3: The banana before CavendishUntil the mid-1900s the main banana sold in the United States and Europe was a variety called Gros Michel. A soil fungus that causes Panama disease, a wilt that kills the plant, spread through Gros Michel plantations in Central America. By the 1960s exporters had switched to Cavendish, which resisted that strain. Gros Michel did not go extinct; small farmers in some countries still grow it.

4 Card B1: A new strain arrivesA newer strain of the same fungus, called Tropical Race 4 (TR4), does attack Cavendish. It blocks the tubes that carry water inside the plant, so the leaves yellow and the plant wilts and dies. TR4 was found in Asia, later spread to Australia, Africa and the Middle East, and reached South America in 2019.

5 Card B2: Hiding in the soilTR4 can survive in soil for decades. It travels in mud on boots, tools and vehicles, in moving water, and in infected planting material. No fungicide cures an infected plant. Farms use boot-washing stations, fences and strict rules for moving plants, which slow the spread but have not stopped it.

6 Card B3: A leaf disease tooBlack Sigatoka, also called black leaf streak, is a fungus that kills patches of banana leaves. With less leaf area, plants make less sugar by photosynthesis and grow smaller bunches. Large Cavendish plantations often spray fungicides many times a year to control it, in some places dozens of times, which is costly and can harm workers and streams if not handled carefully.

7 Card C1: Wild bananasWild bananas grow in forests and forest edges of Southeast Asia and nearby regions. They are packed with hard seeds, so they are not good to eat, but they reproduce from seed and their populations are genetically varied. Some wild bananas carry genes that help them resist TR4 and other diseases. Some wild banana populations are shrinking as forests are cleared.

8 Card C2: A library of living bananasAn international gene bank in Belgium keeps well over a thousand kinds of bananas, wild and cultivated, as living plant tissue in test tubes. Breeders and researchers around the world can request samples to search for useful traits. A gene bank holds only the kinds someone has already collected, so it cannot replace wild populations that keep changing in nature.

9 Card C3: Borrowing a geneResearchers in Australia gave Cavendish plants a disease-resistance gene from a wild Southeast Asian banana. In years of field trials the modified plants resisted TR4, and in 2024 Australian regulators approved the banana, although it was not yet being grown on a large scale. Some shoppers and countries are cautious about genetically modified foods.

10 Card D1: Many kinds on one farmIn parts of East Africa and Southeast Asia, many small farms grow several banana varieties side by side: some for cooking, some for eating fresh, some for brewing. When a disease hits one variety, the others often survive, so the family still has bananas. Many of these varieties bruise more easily or ripen less evenly than Cavendish, so they are harder to ship long distances.

11 Card D2: Built for shippingCavendish bananas are picked green, travel thousands of kilometers by ship without bruising badly, and ripen evenly when they are treated with ethylene, a natural ripening gas. Ships, ripening rooms and supermarkets are set up for this one kind. Switching to other varieties would mean changes for growers, shippers, stores and shoppers.

12 Card D3: Who depends on bananasBananas, including cooking bananas and plantains, are among the world's most important food crops. In some countries of East and Central Africa they are a staple food that many families eat every day. Most bananas grown in the world are eaten in the countries where they are grown rather than shipped abroad.

Part A: Expert round

Teaching model: disease spread in two 1,000-plant plots (teaching dataset)
Simulated teaching dataset: the numbers are invented to be realistic, not measured.
Year	Plot A: plants infected (of 1,000)	Plot B: plants infected (of 1,000)
0	5	5
1	40	12
2	150	30
3	400	65
4	720	110
5	940	160

1.Your expert set (A, B, C or D): ______. For each of your three cards, write the single most important fact in your own words and one sentence explaining how it connects to biodiversity or to disease.

Answer: Set A: Cavendish plants are clones (A1); triploid bananas make few seeds, so breeding is slow (A2); Gros Michel was replaced after Panama disease (A3). Set B: TR4 kills Cavendish (B1); it survives in soil for decades and cannot be cured, only slowed (B2); black Sigatoka forces heavy fungicide use (B3). Set C: wild bananas are genetically varied and some resist TR4 (C1); a gene bank stores over a thousand kinds (C2); a wild gene gave Cavendish resistance (C3). Set D: mixed-variety farms keep some bananas when disease strikes (D1); the trade is built around Cavendish (D2); millions of people rely on bananas as food, mostly grown locally (D3). Each connection should mention variety of traits, uniformity, or disease spread.

Why: Experts must restate facts accurately and link them to biodiversity or disease before teaching.

Also acceptable: Any accurate fact from each card with a sensible link.

Three accurate facts in the student's words
Each linked to biodiversity or disease

Part B: Home group task

2.Group answer: complete the decision matrix.

Answer: One defensible matrix: Solution 1: stores 2 (works for now, but cards B1 and B2 show TR4 spreading anyway), biodiversity 1 (keeps one clone, A1), cost/time 3 (uses existing systems, D2), new disease 1 (every plant shares the same weakness). Total 7. Solution 2: stores 2 (resistant plants exist, C3, but breeding is slow, A2, and approval and acceptance take time), biodiversity 2 to 3 (protects wild populations and gene banks, C1, C2; though the farm crop may still be one variety), cost/time 1 (years of research, A2, C3), new disease 2 (the wild gene pool can supply new traits, C1, but each new disease needs new work). Total 7 to 8. Solution 3: stores 2 (other kinds ship less well, D1, D2), biodiversity 3 (many varieties on farms, D1), cost/time 2 (varieties exist, but the shipping and store system must change, D2), new disease 3 (a new disease is unlikely to hit every variety, D1 and the dataset). Total 10.

Why: Scores are judgments; credit depends on consistency and card evidence, not on matching this matrix.

Also acceptable: Groups may rate Solution 2 highest if they argue that wild genes can protect the shipping system already in place; accept any matrix whose scores are supported by cited cards.

Every cell scored
Totals added correctly
Card numbers cited for the scores
Solution 1 scored low on biodiversity and new-disease resilience

3.Group answer: at year 5, what percent of all plants are infected in Plot A and in Plot B? What percent of the 250 susceptible plants in Plot B are infected? Use the numbers to explain two ways that mixing varieties protected Plot B.

Answer: Plot A: 940 ÷ 1,000 × 100 = 94% infected. Plot B: 160 ÷ 1,000 × 100 = 16% infected. Susceptible plants in Plot B: 160 ÷ 250 × 100 = 64% infected. Two protections: (1) three-quarters of Plot B's plants could not be infected at all, so most of the farm kept producing (84% healthy vs 6% in Plot A); (2) even the susceptible plants in Plot B were infected more slowly (64% vs 94% by year 5) because resistant plants between them slowed the spread from plant to plant.

Why: The model shows both a direct effect of resistant varieties and a slowing effect of spacing susceptible plants apart.

Also acceptable: Accept 6% and 84% healthy as equivalent statements.

94% and 16% computed correctly
64% for susceptible plants
Explains resistant plants protecting the total
Explains slower spread among separated susceptible plants

Part C: Individual accountability (complete alone)

6.Solution 2 depends on wild bananas. Explain why protecting the forests where they grow is a way of maintaining an ecosystem service. Which category of service is it?

Answer: Wild banana forests hold genetic variety, including disease-resistance genes, that breeders and researchers use to develop crops that keep bananas available (C1, C3). That supply of useful genes is a benefit people get from a healthy ecosystem. If the forests are cleared, those genes can be lost before anyone collects them, and a gene bank cannot replace populations that keep changing (C2). It is a provisioning service, because the ecosystem supplies genetic material used to make food.

Why: Genetic resources are counted as a provisioning service; protecting habitat maintains biodiversity and the service.

Also acceptable: Accept 'supporting' if the student argues that biodiversity supports all services, for partial credit.

Forests hold genetic variety or resistance genes
Explains it as a benefit to people
Names provisioning (accept supporting with a reason)

8.Write your own recommendation: one solution or a combination. Use evidence from at least three cards from at least two different expert sets, including one card that argues against your choice, and explain how your choice maintains biodiversity. You may disagree with your group.

Answer: Example: 'I recommend combining Solutions 2 and 3. Card D1 and the dataset show that farms with several varieties keep most of their harvest when a disease strikes (16% infected vs 94%). Card C1 shows wild bananas hold resistance genes, and card C3 shows those genes can protect Cavendish, so protecting wild forests and gene banks (C2) keeps the source of future fixes. Card D2 argues against my plan: the shipping system is built for one variety, so stores may resist new kinds. That is why I would use wild genes to make resistant bananas that ship well while also growing more varieties for local markets. This maintains biodiversity both on farms and in the wild.'

Why: The recommendation must use evidence from several sets, address a counter-card, and explain the link to biodiversity.

Also acceptable: Any solution can earn full credit if the argument meets all criteria; Solution 1 alone must address B1, B2 and A1 to be convincing.

Clear recommendation
Evidence from at least three cards and two sets
Addresses one card that argues against the choice
Explains how biodiversity is maintained
Accurate science

Scoring rubric

Teacher notes

The cards were written to be accurate where the textbook is not. The lesson says Cavendish is safe from Panama disease and that yellow Sigatoka kills Cavendish plants; in fact Cavendish resisted Race 1 of Fusarium wilt (Panama disease) but is susceptible to Tropical Race 4, and Sigatoka diseases are leaf-spot diseases that cut yield rather than killing plants outright (black Sigatoka has largely displaced yellow Sigatoka). Gros Michel was abandoned by the export trade, not driven extinct. The approved genetically modified Cavendish (QCAV-4, approved by Australian regulators in February 2024) carries a resistance gene from the wild banana Musa acuminata ssp. malaccensis. Only the plot dataset is simulated. Home groups of four work best, one expert per set; in a group of three, one student takes two sets, or the teacher briefly presents set D to the whole class. Expert groups gather everyone holding the same set (A, B, C or D). Debrief: most groups land on a combination of Solutions 2 and 3; push them on what each costs and who has to change.

Criterion	4 · Exemplary	3 · Proficient	2 · Developing	1 · Beginning
Expert teaching	Teaches all three cards accurately in own words and links each to biodiversity or disease; answers group questions.	Teaches all three cards accurately, with one weak link.	Reads cards aloud or leaves out a card; links unclear.	Does not teach, or teaches inaccurately.
Decision matrix	All cells scored consistently, totals correct, and card evidence cited for every score.	All cells scored with most evidence cited; one inconsistency.	Matrix partly complete or scores given without evidence.	No systematic comparison.
Use of the teaching dataset	Correct percents (94%, 16%, 64%) and both protective effects of mixing explained.	Correct percents with one effect explained.	Some calculations wrong or no interpretation.	Dataset not used.
Individual recommendation (Part C)	Clear recommendation using three or more cards from two or more sets, addresses a counter-card, and explains how biodiversity and the food supply are maintained.	Clear recommendation with sufficient evidence; counter-card or biodiversity link is thin.	Recommendation with one or two cards and little reasoning.	No recommendation, or one unsupported by the cards.