PLB143 - Lecture 08
What is a crop?
The domestication syndrome
© Paul Gepts 2009
PLB143: Readings - Lecture 08
- Required:
- Additional readings:
- Smartt J (1978) Evolution of pulse crops. Econ. Bot. 32:185-198.
- Davies MS, Hillman GC (1992) Domestication of cereals. In: Chapman GP (ed) Grass evolution and domestication. Cambridge Univ Press, Cambridge, UK: pp. 199-224
- Doebley J, Stec A, Wendel J, Edwards M. 1990. Genetic and morphological analysis of a maize-teosinte F2 population: implications for the origin of maize. Proc. Nat. Acad. Sci. USA 87:9888-9892
- Presentation slides:
- Pdf version of Lecture 08 (version 2009)
What is Domestication?
Harlan 1992; Davies and Hillman 1992
- Process which:
- Occurs under cultivation in populations of early wild-type crops (sown from seed gathered from wild stands)
- Selectively advantages rare mutant plants lacking features necessary for survival in the wild (or having features necessary for survival in the field)
- Continues until the mutant phenotypes dominate the crop population
- Important aspects:
- Cultivation (necessary but not sufficient condition); selection --> allele frequency changes; evolutionary process: gradation within and between species
Domesticated plants: taxonomic distribution
Harlan 1992
- Number of species:
- Leguminosae: 41 (bean, pea, lentil)
- Gramineae: 29: maize, wheat, barley, rice,
sorghum (from U. of Wisconsin Botany image collection ) - Solanaceae: 18 (tomato, pepper, potato)
- Cruciferae: 13 (canola , radish)
- Cucurbitaceae: 13 (cucumber, melon , squash)
- Rosaceae: 11 (apple, Prunus, strawberry)
- Liliaceae: 11 ( onion )
- Umbelliferae: 9 (carrots, fennel, dill)
- Araceae: 8 (taro ,
,
from Nation of Hawai`i's Canoe Plants ) - and other families such as Compositae
(lettuce, sunflower , from Texas A & M)
- Large number of vicarious domestications:
- Old vs. New World: cotton, lupin, amaranth, Solanum
- Mesoamerica vs. South America: Phaseolus, Capsicum
- Trend: species actually contributing to nutrition of
humanity
- small and declining number
- increased emphasis on cereals
- Sorghum
- Canola
- Onion
- Carrot
- Taro
- Sunflower
Some related sites of interest
Major crops in the world
World's leading food crops in terms of production
Harlan 1992
The
predominance of cereals over other crops Selection pressures exercised during domestication
Selection was exercised at different stages of the growth cycle. Each of these are underlined in the next section, starting with harvesting
.
Harvesting: Increase in seed yield (I)
- Increase in seed recovered:
- elimination or reduction of seed dispersal
- non-shattering: e.g., cereals: teosinte (wild maize)
- non-dehiscence
- additional traits affecting seed dispersal: example of Triticum monococcum:
- more compact growth habit
- reduction in branching: e.g., teosinte
- synchronous tillering
- climbing --> bush
- elimination or reduction of seed dispersal
- Seed dispersal in teosinte, the ancestor of maize:
- Additional traits affecting seed dispersal in cereals:
(from Davies and Hillman 1992, © Cambridge University Press)
- Reduction in branching:
(from
Doebley et al. 1990)
Recovery rate of seeds under different harvesting methods
Davies and Hillman 1992: For further explanations, see also Lecture 16: Experiments of Hillman and Davies
|
Harvest method |
Fragile rachis (wild) |
Tough rachis (domesticated) |
|
Beating |
44-84 % (single - multiple pass) |
5 % |
|
Sickle reaping |
40 % |
100 % |
Evolution from climbing to bush growth habit in legumes
Smartt 1978
- Wild beans are vines, i.e. plants that climb on top of other plants used as physical support
- Hence, they have long branches: long internodes and many nodes; many branches; "twining" ability;
- Domesticated beans:
- Show a a wide range of growth habits from climbing to bush
- The most "advanced" or modified, compared to the
original climbing habit, is the bush
habit , which combines the following traits:
- Reduction in internode length
- Reduction in the number of nodes
- Suppression of twining response
- Determinacy
- Reduced branching
- Conversion from diageotropic branching to negative geotropic branching + change in angle of branching
- Climbing show only part of these traits or a reduction in intensity of their expression
- Beans in their native habitat:
- Bush growth habit in domesticated beans
An overall view (road to Huigra, Ecuador)
A closer look ...
Harvesting: Increase in seed yield (II)
- Increase in number of seeds produced
- Reduction in daylength sensitivity:
- Adaptive value of sensitivity
- Reversal of reduction to sterility
- Example: barley: 2-ranked vs. 6-ranked
- Increase in inflorescence size:
- Examples: maize, sorghum, pearl millet, foxtail millet (from B. Smith, the Emergence of Agriculture, © Scientific American Library 1995)
- Increase in the number of inflorescences:
- Examples: wheat, barley, rye
- Reduction in daylength sensitivity:
- Foxtail millet
Evolution of Zea mays female inflorescence or "ear"
Harlan 1992
- From teosinte to domesticated maize ear. How?
- Teosinte: axil of several leaves: several small, two-ranked (two rows) fragile ears, each in a husk
- Early maize: axil of several leaves: small, four-ranked (8 rows), non-fragile ears, each in a husk
- Modern maize: 1 (2) large many-ranked non-fragile ear surrounded by several husks
- Teosinte "ear" (inflorescence):
- Maize ear:
- Evolution of ear from teosinte to maize
(from Doebley et al. 1990)
Planting: Increase in seedling vigor
- Larger seeds:
- Increase reserves for germination
- Conflict with number of seeds
- How? carbohydrates!
- Non-dormant seeds
- Adaptive value of dormancy
- Example: Wild oat, einkorn, emmer: 2 seeds/spikelet:
- larger, without dormancy
- smaller, dormancy > 1 yr.
- Conflict: premature germination
- Correlated response: reduced chaff
Reproductive system
- Outcrossing --> selfing
- Reasons?
- Reduction or absence of sexual reproduction
- Examples: bananas, plantains; navel orange
- Vegetatively reproduced crops:
- Vegetative propagation --> instant domestication
- Grafting technique: extend advantages of vegetative propagation to crops propagated by seed
- Navel orange
from Clonal , University of California, Riverside
- Banana
from Edible Botany, Hort 201H, Texas A & M
Human environment: Adaptation to human utilization and tastes
- Farmer:
- Growth habit & cropping system
- Phenology
- Consumer:
- Color (e.g., common bean), flavor, texture, storage quality, cooking time, etc
- Reduction in toxic or unpleasant compounds:
- cyanogenic glucoside: e.g., cassava, lima bean
- bitterness: e.g., squash: wild vs. cultivated
- Other examples:
- maize: popping, boiling, off-the-cob, flour (from National Geographic Society)
- rice: glutinous vs. non-glutinous, long-grained vs. short-grained, aromatic, etc.
- barley: food, livestock feed, beer
- Common bean seeds
© Paul Gepts - Wild squash
© Paul Gepts
Domesticated squash
© Paul Gepts
(a farmer's harvest in Oaxaca, Mexico) - Maize types
Summary of the major differences between wild and cultivated common-bean (Phaseolus vulgaris)
________________________________________________________________________________
General Wild Domesticated
attribute Trait (G12873) cv. Midas)
_____________________________________________________________________________
Seed Pod suture fibers Present Absent
dispersal Pod wall fibers Present Absent
Seed Germination 70.5% 100%
dormancy
Growth habit Determinacy Indeterminate Determinate Twining Twining Non-twining Number of nodes on the main stem 22.5 7.5 Number of pods 43.2 13.9 Internode length 1.6 cm 2.9 cm Gigantism Pod length 5.7 cm 9.8 cm
One-hundred-seed
weight 3.5 g 19.5 g
Earliness Number of days to
flowering under
12 h days 69 46
Number of days to
maturity 107 80
Photoperiod Delay in flowering
sensitivity under 16h days >90 days 0 days
Harvest Seed yield/biomass 0.42 0.62
index
Seed Presence vs. Present Absent
pigmentation absence
_________________________________________________________________
Less than full domestication
Example: Oil palm (Elaeis guineensis)- fruit: 50% oil in mesocarp; cooking, soap
- thrives at forest edges; favored by slash-and-burn and by religious beliefs
- three types: durra (traditional, thick shell, single homozygous dominant gene); tenera (thinner shell, heterozygous); pisifera (no shell, homozygous recessive, female sterile)
- tenera and pisifera are preferred for oil production
Some related sites of interest
Oil palm tree
from Centre for Remote Imaging, Sensing and Processing (CRISP)
- Oil palm fruit bunch
from H. Wilson, Botany 301, Texas A & M
- Fruit cross-section
from http://www.oils-vegetable.com/palmoil.htm
- Tenera fruit cross-section
from CIRAD, France
"Trial-and-error" during domestication
- Example 1: Mesoamerica (Callen 1967)
- Setaria (foxtail) seeds increased in size; suggested domestication
- abandoned when maize was introduced
- Example 2: China (Li 1969)
- Malva sylvestris: most important green vegetable in Ancient China replaced by Brassica chinensis (Chinese cabbage)
- Example 3: Near East
- Medicago sativa : abundant in earliest sites in Near East, disappears afterward upon introduction of other legumes (pea, lentil, etc.)
- Alfalfa
(from Southwest School of Botanical Medecine, Michael Moore, Medicinal Plant Images)
The domestication syndrome: conclusion
- Fully domesticated plants are characterized by a similar set of traits that confer adaptation to the human environment
- The two most important traits of the syndrome are loss of seed dispersal and seed dormancy
- The specific details for each trait will depend for each crop
- Fully domesticated plants have most of the traits of the syndrome but generally not all of them; within a crop, differences in the degree of domestication can be observed