Genetic Diversity in Agroecology

Genetic Diversity/Resources

Introduction

Course Title: EESC 3310 Agroecology
Instructor: Tara Pisani Gareau

Learning Objectives

Understand the value of genetic diversity within agroecosystem sustainability.
Key concepts to grasp:
- Adaptation
- Genotype
- Self vs. Cross Pollination
- Crop Breeding
- Polyploidy (its effects on genetic diversity and function for plants)
Compare different modes of breeding regarding their approaches and results in enhancing genetic diversity.

Scales of Agri-Diversity

Genetic: Total number of genetic characteristics in the genome of a crop species.
Intraspecific Diversity: Genetic variation among varieties (landraces and cultivars) of a crop species.
Interspecific Diversity: Number of crop (and non-crop) species representing spatial diversity.
Temporal Diversity: Changes in species diversity over time, such as crop rotations.
Landscape Diversity: Habitat diversity within an ecological region, such as within a watershed.

Genetic Diversity of Crop Species

Crop Species Cultivars and Numbers:
- Zea mays
- Dent Corn
- Flint Corn
- Sweet Corn
- Flour Corn
- Popcorn
- Total Cultivars: 500+
- Brassica oleracea
- Broccoli
- Brussels Sprouts
- Cauliflower
- Kale
- Kohlrabi
- Total Cultivars: 300+
- Camellia sinensis varieties (tea):
- var. sinensis (small narrow leaf, cool, high elevation, China)
  - Green tea
  - White tea
  - Oolong tea
- var. assamica (large, wide leaves, warm tropical, low elevation, India)
  - Black tea
  - Puerh tea
- Additional Varieties and Cultivars:
  - Var. pubilimba (Vietnam and China)
  - Var. dehungensis (Yunnan, China)
  - 4 varieties; 30+ Chinese cultivars, 8 Taiwanese, 10 Japanese, 4 Vietnamese, 10 Indian.

Decline in Species Diversity

Statistics:
- 382,000 vascular plant species exist.
- 6,000 plant species have been cultivated for food.
- Fewer than 200 plant species contribute to global food output.
- Nine plant species accounted for 66% of production by weight in 2014:
- Maize
- Rice
- Wheat
- Potatoes
- Soybeans
- Sugar Cane
- Oil Palm Fruit
- Sugar Beet
- Cassava

Genetic Erosion within Crop Species

In the U.S., the following statistics indicate genetic erosion:
- Corn: 71% from 6 varieties
- Rice: 65% from 4 varieties
- Wheat: 50% from 9 varieties
- Apples: 6,000 known varieties have gone extinct.

Genetic Vulnerability: A Case Study

Irish Potato Blight (1840s):
- The 'Irish Lumper' potato variety was widely grown due to its performance on poor soils but was very susceptible to blight.
- In 1845, a third of the potato crop was wiped out, and by 1846, the majority of the crop was gone.
- The pathogen responsible was Phytophthora infestans.
- Images: Included scanning electron micrograph of fungus on potato leaf, early and late-stage damages.

Origins and Primary Regions of Diversity of Agricultural Crops

Reference: Khoury et al. (2016).
Primary Regions of Crop Diversity:
- North America
- Caribbean
- Central America and Mexico
- Andes
- Tropical and Temperate South America
- Southeastern and Southwestern Europe
- Various regions in Africa, Asia, Pacific.
Crops Listed Include: Alfalfa, almonds, apples, beans, cabbages, cocoa beans, melons, rice, and more.

Natural Selection

Defined as the process by which individuals better adapted to their environment survive and reproduce.
Key Terms:
- Genotype: The genetic information carried by an individual.
- Phenotype: The physical and behavioral expression of the genotype.

DNA Structure and Genetic Variation

Hierarchy:
- Cell → Chromosome → DNA → Gene
DNA Composition: Nucleotides consisting of base pairs: Guanine, Cytosine, Adenine, and Thymine.

Natural Genetic Variation

Mechanism: During meiosis, DNA replication leads to recombination, shuffling genes, which produces different genetic combinations in each gamete.
Chromosome Number:
- Diploid: 2n
- Haploid: 1n

Polyploidy

Defined as having more than two sets of homologous chromosomes.
Outcome: Usually fatal in animals but common in plants.
Advantages: Increases genetic variation, often results in larger fruit size and enhanced stress tolerance.
Induced Polyploidy: Can be achieved using chemical stimulators.

Genetic Diversity among Crops

Chromosome Counts:
- Barley: $(2N = 14)$
- Sugar beet: $(2N = 18)$
- Maize: $(2N = 20)$
- Watermelon: $(2N = 22)$
- Tomatoes: $(2N = 24)$
- Rice: $(2N = 24)$
- Cassava: $(2N = 36)$
- Soybean: $(2N = 40)$
- Bananas: $(3N = 33)$ (seedless watermelon included)
- Peanut: $(4N = 40)$
- Potatoes: $(4N = 48)$
- Cotton: $(4N = 52)$
- Wheat (bread): $(6N = 42)$
- Strawberry: $(8N = 56)$
- Sugarcane: $(8N = 80)$

Plant Reproduction

Structure of a Hermaphroditic Flower:
- Parts: Anther, stamen, filament, petal, ovules, sepals, stigma, style, pistil, ovary.

Pollination Types

Self Pollinating Crops: (Example includes orange blossom from family Rutaceae).
Cross Pollination: Types include monoecious and dioecious plants.

The Role of Pollinators

Statistics indicate that one-third of food relies on animal (mainly bee) pollinators (McGregor 1976, Klein 2007).
Biotic pollination can increase fruit set by 70% in tropical crops and 85% in European crops.

Pollination Strategies: Trade-offs

Self Pollination vs. Cross Pollination:
- Genetic Variability: Low in self-pollination, high in cross-pollination.
- Plant Vigor: Low in self-pollination, high in cross-pollination.
- Energy Invested in Nectar: Low in self-pollination, high in cross-pollination.
- Ability to Grow in Harsh Environments: High in self-pollinating plants, low in cross-pollinating plants.

Directed Selection in Plants

Mass Selection: Phenotypic selection that led to the development of landraces.
Pure Line Selection: Involves selecting superior self-pollinating plants isolated over years.
Synthetic Varieties: Limit parental genotypes in cross-pollinated plants for greater variation than self-pollinated but less than cross-pollinated.
Hybridization: Involves crossing two parents with significantly distinct characteristics.
Transgenic: Involves insertion of genes from one species into another. Closest to natural selection methods.

Artificial Pollination

Example: Process conducted by Willi Galloway.

Hybridization Case Study

Example of hybrid corn from inbred corn strains B73 and Mo17 (University of Nebraska-Lincoln, 2004).
Hybrid Vigor (Heterosis): Results from crossbreeding.

Transgenic Crops

Definition: Crops that contain artificially inserted genes from different plant species or taxa.
Applications:
- Pest resistance (e.g., Bt corn, papaya).
- Herbicide resistance (e.g., Roundup Ready soybeans, cotton, corn).
- Increased nutritive value (e.g., Golden rice).

US Growth in Transgenic Acreage

Trends: Adoption of genetically engineered crops from 1996 to 2025 across various crops (herbicide-tolerant and insect-resistant varieties).
Visual Data: Percent of planted acres for crops like HT corn, Bt corn, HT cotton, etc.

Benefits of Transgenic Crops

Accelerates the breeding process.
Allows innovative gene combinations unavailable in the plant’s genome.
Theoretical reduction in pesticide use.
Potential increase in yields, especially on marginal land.
Can alleviate malnutrition.

Concerns over Transgenic Crops

Effects on the expression of the genome.
Possible accelerated resistance of pests.
Potential escape of transgenes to wild relatives leading to unforeseen ecological impacts.
Creation of superweeds.
Potential harm to wildlife and beneficial organisms.
Consolidation of agribusiness impacting farmers.
Possibility of introducing harmful toxins or allergens into the food supply.

Conserving Genetic Diversity

Strategies for genetic diversity conservation include:
- Ex situ conservation (off-site conservation measures).
- In situ conservation (on-site conservation measures).
- Policies promoting farmers for planting or raising traditional breeds.
- Research on the role of diversity within agroecosystems.