PLB143 - Lecture 09

What is a weed?

© Paul Gepts 2012

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PLB143: Readings - Lecture 09

Why study weeds?

  • Many crops are believed to have been weeds originally. What is a weed?
  • The role of hybridizations:
    • D x D, D x W, W x W
    • Outcome ?

Definitions of weeds

  • Two types of definitions:
    • Usefulness to humans: "not wanted", "obnoxious", "harmful", "objectionable", "nuisance", "out of place"
    • Adapted to disturbances: '
      • "pioneers of secondary succession (i.e. after disturbance): e.g., arable field"
      • "introduced plant species which take possession of cultivated or fallow fields and pastures"
  • Is Homo sapiens the weediest species of all?
    • creates and profits from disturbances in environment
    • no conscious genetic improvement
    • unwantedness: population explosion

Importance of disturbances

  • Intensity of disturbances
  • Natural disturbances: river cliffs, sea shores, active dunes, steep cliffs, active volcanoes, active glaciers
  • Before agriculture:
    • Most widespread disturbance: glaciers in N. America and Europe: advance and retreat
  • Agriculture:
    • Disturbances: plowing, naked soil, fallow
    • Vaster: whole continents or regions: e.g., California Central Valley
    • More rapid

The "dump-heap" theory of Andersen (1952)

  • " middens of sedentary fisherfolk would be a natural place where aggressive plants from riverbanks might find a home, where seeds brought back from up the hill ... might sometimes sprout ..."
  • "Species which had never intermingled, might do so there... the open habitat of the dump heap would be a new habitat in which strange new mongrels could survive..."
  • "... when man first took to growing plants, these dump-heap mongrels would be among the most likely candidates..."

Crops (and weeds) as colonizers of disturbed areas

  • Certain combinations of life history characters appear to ccur more often or less often than expected by chance <==> correlated sets of life history characters = strategy ("lifestyle")
  • MacArthur and Wilson (1967) :
    • r-strategists: dispersal + colonization, reproduction in expanding populations, unstable environments
    • K-strategists: persistence, reproduction in stable populations, stable environments
  • Logistic model of population growth (Silvertown 1992)
  • Lower population density: r strategists reproduce faster
    Higher population density: K strategists reproduce faster
r vs. K

Comparison of r- and K-strategists

r-strategist K-strategist
Environment unstable stable
Development fast slow
Age 1st reprod. early late
Reprod. allocation large small
No. of seed crops 1 >1
Size of seed crops large small
Size of seeds small large
Adult longevity short long

An illustration of the r- vs. K-strategies in dandelions

  • (from Silvertown J (1982) Plant population ecology. Longman, London)

Role of hybridizations in crop evolution

  • Bad: Aggressive weeds: e.g., sorghum - shattercane
  • Benign: Transient hybrid populations
  • Better: new domesticates, recombinants, or introgressants
Sorghum - shattercane
  • Origin:
    • In certain parts of Africa, appearance of a weedy sorghum race:
      • juvenile plant 
      • mature plants in field of grain sorghum 
    • Massive infestations of cultivated sorghum fields
    • Difficult to control
    • Hybridizes with cultivated sorghum: geographic parallel in morphology: 
  • Differences in the shattering reaction:
    • Wild sorghum: abscission layer --> suppressed during domestication
    • Shattercane: breakage of inflorescence slightly below normal place for abscission layer --> shattered spikelets have short branch fragments
  • Four categories of sorghum:
    • Truly wild races; somewhat weedy
    • Shattercanes derived from W x D crosses; serious pests
    • Shattercanes derived from cultivated; serious pests
    • Semi-domesticated to fully-domesticated
  • Other examples of crop mimicry: harvested with crop:
    • flax: Camelina sativa subsp. linicola: resembles flax varieties in stature, maturity, and seed size
    • flax: evolution of seed size in Spergula, a weed associated with flax: 
    • rice: Echinochloa crus-galli var. oryzicola : barnyardgrass: resembles rice plant closely from seedling to flowering time --> no weeding:
      •   juvenile plant
      •   inflorescence
  • Other adaptations:
    • phenotypic plasticity
    • annuals: large no. of seed produced, often small, strong dormancy
    • perennials: longevity through rhizomes, deep taproots with lateral buds, rootsprouters, thorns

  • Shattercane
    • juvenile plant

    • adult plant
       (photos from University of Illinois Crop Sciences Extension)
  • Distribution of domesticated races of sorghum in Africa
    (from J. Hancock)
  • Evolution of seed size in Spergula
    (from Schwanitz 1966, © Harvard Univ. Press)
  • Echinochloa crus-galli var. oryzicola:
    • juvenile plant

    • inflorescence

      (photos from University of Illinois Crop Sciences Extension)

  • Definition:
    • introduction of genes through hybridization with a distinct race or species
  • Difficulty in studying introgression: detection
    • Morphological traits: gene flow, common ancestry, or convergence?
    • Molecular markers: neutral: gene flow or common ancestry?
  • Solution:
    • If a marker typical of a domesticate appears at low frequency in a sympatric wild ancestor that typically lacks that marker, this would constitute evidence for gene flow from domesticate to wild ancestor (and conversely)

Example 1:
  • Local introgression into Lycopersicon esculentum from L. pimpinellifolium

Region n Standard isozyme profile (%) Rare alleles (%)
Ecuador 14 29 93
Peru 32 31 19
Other South America 12 100 0
Central America 27 89 12
Mexico 22 82 14
Europe 45 81 18
U.S. 26 81 19

  • Example 2:
    • Potential wild x cultivated introgressants in Mesoamerican common bean (Singh et al. 1991)
Bean PCA
  • Example 3:
    • Phaseolus (bean) species resulting from hybridization

  • Example 4:
    • Allopolyploidy: e.g., wheat (AABBDD), Brassicas: Triangle of U

What is a weed?

  • There are two features of weeds that are relevant to crop plants
    • adaptation to disturbed habitats
    • hybridization leading to mimicry of the crop

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