LECTURE 34- origin of crop plants and domestication

what is agriculture

the caring for, or cultivation of plants

what is domestication

genetic alteration of plants of plants or animals through human activities

what are the different ways genetic alterations happen

1. natural mutations

2. artificial selection

3. hybridization

4. polyploidy

what are natural mutations

‘Mistakes’ in DNA

Has to be heritable and transmitted to the next generation

what is artificial selection

Improving or enhancing a trait artificially

Selecting plants with a favourable trait and using them for the next generation, to shift the distribution of the trait for the next generation

what is hybridization

Crossing two different species to create a hybrid species

Hybrid species- combined genomes of different species at the diploid level (AB)

• Hybrid will be a diploid with 2 different genomes

• AA diploid x BB diploid = AB diploid offspring

what is important to note with hybrid plants

hybrid plants are often sterile- chromosomes have trouble matching up

what is polyploidy

when an organism having more than 2 sets of chromosomes

what is an autopolyploid

(AAAA): form within a species

• autotetraploid with 4 copies of chromosomes

• chromosomes double within a species

what is an allopolyploid

(AABB): form between species (involves hybridization)

• allotetraploid with 4 copies of chromosomes from 2 different species

•  hybridization between species and chromosomes doubling

what are the 2 options for polyploid formation

1. sterile f1 hybrid forms with unreduced gametes (AB) and breeds with another hybrid to make an allotetraploid (AABB)

2. original parents immediately produce unreduced gametes (AABB) in offspring under stress

when were crops domesticated

most crops were domesticated around the same time (10,000-5,000 years ago)

what are the major centres of crop domestication

Americas, Fertile Crescent (near east), and Asia

• grains and legumes domesticated here

what plants were domesticated in the near east

HERBS: sage, thyme, oregano, parsley

GRAINS: wheat, barley, oats

OTHERS: olives, lettuce, lentils

what plants were domesticated in the americas

HERBS/SPICES: chili peppers, paprika, vanilla

GRAINS: wild rice, corn/maize

OTHERS: tomatoes, potatoes, peanuts, avocado, pineapple

what plants were domesticated in the asia

HERBS/SPICES: black pepper, garlic, ginger

GRAINS: rice

OTHERS: citrus/rose family: onions, lemons, oranges, peach, soybeans, cherries, hemp, almonds tea

why did plant domestication occur

• changes in plants (garbage heap hypothesis)

• changes in hunter-gatherer behaviour (climate change + growing population)

• devoping technology to collect, process and store food

what did plant/animal domestication lead to

plant /animal domestication was a prerequisite for the rise of civilisation

what are the 2 modes of domestication

1. unconscious domestication

2. deliberate selection

Unconscious selection

• Not deliberate

• A loss of natural dispersal/dehiscence mechanisms

• Reduction in seed dormancy

• Altered response to photoperiod

deliberate selection

• Loss of toxic compounds

• Reduced seed content in some fruits

• A few, large, easily harvested heads

wild wheat characteristics

• DIPLOID (14 chromosomes)

• Small seeds

• Shattering (pikelets naturally fall apart at maturity)

• Hulled (had to bash glumes off of grains to obtain seeds)

domesticated wheat characteristics

• DIPLOID(14 chromosomes)

• Larger seeds

• Non-shattering (spikelets remain on plant)

• Free-threshing (don’t have to bash them to obtain seed)

einkorn wheat characteristics

• DIPLOID(14 chromosomes)

• Arose from natural mutation that suppressed shattering

• Small seeds

• Not free-threshing

Emmer wheat characteristics

• ALLOTETRAPLOID (28 chromosomes)

• Pasta and bead lineage

• Hybridization and allopolyploidy

• Main trait changed was size

• larger seeds and non-shattering

Durum wheat characteristics

• ALLOTETRAPLOID (28 chromosomes)

• Further selected from emmer wheat

• Selected for free-threshing grain

• Thin glumes to easily get grain (naked grain)

bread wheat characteristics

• HEXAPLOID (6 copies of chromosomes)

• Arose from a cross between emmer wheat and another wild species (D genome species with 14 chromosomes)

• HEXAPLOID (6 copies of chromosomes)

• Higher gluten content good for making bread

what are the issues around crop domestication

• Loss of genetic diversity (genetic erosion)

• Ownership of genetic resources (stealing resources)

• Landscape changes (clearing landscape for agriculture)

• Interaction with environment (crops interbreeding with wild plants)